ALBERT

Description: We present a methodology for infrasonic remote sensing of winds in the stratosphere that does not require discrete ground-truth events. Our method uses measured time delays between arrays of sensors to provide group velocities (referred to here as celerities) and then minimizes the difference between observed and predicted celerities by perturbing an initial atmospheric specification. Because we focus on interarray propagation effects, it is not necessary to simulate the full propagation path from source to receiver. This feature allows us to use a relatively simple forward model that is applicable over short-regional distances. By focusing on stratospheric returns, we show that our non-linear inversion scheme converges much better if the starting model contains a strong stratospheric duct. Using the Horizontal Wind Model (HWM)/Mass Spectrometer Incoherent Scatter (MSISE) empirical climatology as a starting model, we demonstrate that the inversion scheme is robust to large uncertainties in backazimuth, but that uncertainties in the measured trace velocity and celerity require the use of prior constraints to ensure suitable convergence. The inversion of synthetic data, using realistic estimates of measurement error, shows that our scheme will nevertheless improve upon a starting model under most scenarios. The inversion scheme is applied to infrasound data recorded from a large event on 2010 December 25, which is presumed to be a bolide, using data from a nine-element infrasound network in Utah. We show that our recorded data require a stronger zonal wind speed in the stratosphere than is present in the HWM profile, and are more consistent with the Ground-to-Space (G2S) profile.

Description: Coda- Q is a stochastic parameter reflecting the heterogeneities of medium that seismic waves travel through. We confirmed that coda- Q would vary with the stress loaded to an elastic medium using numerical simulations of seismic wave propagation. When the stress is loaded, cracks in the crust could either close or newly open. The closure and opening of the cracks are not random but depending on the magnitude and the direction of the stress and the crack aspect ratio. The cracks in the medium after loading stress could be aligned in a specific orientation, and elastic wave velocity field would become anisotropic due to the alignment of specific crack orientations. Elastic wave velocity is in general faster along the direction corresponding with the crack orientation while slower along the perpendicular direction. In the numerical simulation, the effect of anisotropy in elastic wave velocity field due to the selective closure and opening of the cracks is calculated using a 2-D finite difference method assuming elastic wave velocity to be a function of the magnitude of loaded stress. The coda- Q calculated from seismic waves simulated for a model varies when the averaged normal stress changes. Our simulation indicated that the sensitivity of coda- Q –1 , that is the reciprocal of the coda- Q , would be 1.0 10 –2 (1.0 MPa –1 ) against the magnitude of the confining pressure and 1.0 10 –3 (1.0 deg –1 ) against the direction of principal stress. We would like to conclude that coda- Q , a stochastic parameter reflecting heterogeneities of subsurface medium, could become a quantitative state indicator of the stress field of the medium where seismic waves propagate through. Spatiotemporal variation of coda- Q reflects change in the stress field in the crust.

Description: How do body-wave traveltimes constrain the Earth's radial (1-D) seismic structure? Existing 1-D seismological models underpin 3-D seismic tomography and earthquake location algorithms. It is therefore crucial to assess the quality of such 1-D models, yet quantifying uncertainties in seismological models is challenging and thus often ignored. Ideally, quality assessment should be an integral part of the inverse method. Our aim in this study is twofold: (i) we show how to solve a general Bayesian non-linear inverse problem and quantify model uncertainties, and (ii) we investigate the constraint on spherically symmetric P -wave velocity ( V P ) structure provided by body-wave traveltimes from the EHB bulletin (phases Pn , P , PP and PKP ). Our approach is based on artificial neural networks, which are very common in pattern recognition problems and can be used to approximate an arbitrary function. We use a Mixture Density Network to obtain 1-D marginal posterior probability density functions (pdfs), which provide a quantitative description of our knowledge on the individual Earth parameters. No linearization or model damping is required, which allows us to infer a model which is constrained purely by the data. We present 1-D marginal posterior pdfs for the 22 V P parameters and seven discontinuity depths in our model. P -wave velocities in the inner core, outer core and lower mantle are resolved well, with standard deviations of ~0.2 to 1 per cent with respect to the mean of the posterior pdfs. The maximum likelihoods of V P are in general similar to the corresponding ak135 values, which lie within one or two standard deviations from the posterior means, thus providing an independent validation of ak135 in this part of the radial model. Conversely, the data contain little or no information on P -wave velocity in the D '' layer, the upper mantle and the homogeneous crustal layers. Further, the data do not constrain the depth of the discontinuities in our model. Using additional phases available in the ISC bulletin, such as PcP , PKKP and the converted phases SP and ScP , may enhance the resolvability of these parameters. Finally, we show how the method can be extended to obtain a posterior pdf for a multidimensional model space. This enables us to investigate correlations between model parameters.

Description: We have developed a network optimization method for regional-scale microseismic monitoring networks and applied it to optimize the densification of the existing seismic network in northeastern Switzerland. The new network will build the backbone of a 10-yr study on the neotectonic activity of this area that will help to better constrain the seismic hazard imposed on nuclear power plants and waste repository sites. This task defined the requirements regarding location precision (0.5 km in epicentre and 2 km in source depth) and detection capability [magnitude of completeness M c  = 1.0 ( M L )]. The goal of the optimization was to find the geometry and size of the network that met these requirements. Existing stations in Switzerland, Germany and Austria were considered in the optimization procedure. We based the optimization on the simulated annealing approach proposed by Hardt & Scherbaum, which aims to minimize the volume of the error ellipsoid of the linearized earthquake location problem ( D -criterion). We have extended their algorithm to: calculate traveltimes of seismic body waves using a finite difference ray tracer and the 3-D velocity model of Switzerland, calculate seismic body-wave amplitudes at arbitrary stations assuming the Brune source model and using scaling and attenuation relations recently derived for Switzerland, and estimate the noise level at arbitrary locations within Switzerland using a first-order ambient seismic noise model based on 14 land-use classes defined by the EU-project CORINE and open GIS data. We calculated optimized geometries for networks with 10–35 added stations and tested the stability of the optimization result by repeated runs with changing initial conditions. Further, we estimated the attainable magnitude of completeness ( M c ) for the different sized optimal networks using the Bayesian Magnitude of Completeness (BMC) method introduced by Mignan et al. The algorithm developed in this study is also applicable to smaller optimization problems, for example, small local monitoring networks. Possible applications are volcano monitoring, the surveillance of induced seismicity associated with geotechnical operations and many more. Our algorithm is especially useful to optimize networks in populated areas with heterogeneous noise conditions and if complex velocity structures or existing stations have to be considered.

Description: Fluid injection in and withdrawal from wells are basic procedures in mining activities and deep resources exploitation, such as oil and gas extraction, permeability enhancement for geothermal exploitation and waste fluid disposal. All of these activities have the potential to induce seismicity, as exemplified by the 2006 Basel earthquake ( M L 3.4). Despite several decades of experience, the mechanisms of induced seismicity are not known in detail, which prevents effective risk assessment and/or mitigation. In this study, we provide an interpretation of induced seismicity based on computation of Coulomb stress changes that result from fluid injection/withdrawal at depth, mainly focused on the interpretation of induced seismicity due to stimulation of a geothermal reservoir. Seismicity is, theoretically, more likely where Coulomb stress changes are larger. For modeling purposes, we simulate the thermodynamic evolution of a system after fluid injection/withdrawal. The associated changes in pressure and temperature are subsequently considered as sources of incremental stress changes, which are then converted to Coulomb stress changes on favourably oriented faults, taking into account the background regional stress. Numerical results are applied to the water injection that was performed to create the fractured reservoir at the enhanced-geothermal-system site, Soultz-sous-Forets (France). Our approach describes well the observed seismicity, and provides an explanation for the different behaviors of a system when fluids are injected or withdrawn.

Description: We report on a broad-band high-resolution attenuation model for the North China Craton and surrounding regions based on regional Lg -wave data. Vertical broad-band waveforms recorded at 39 stations from 176 crustal earthquakes are collected to extract the Lg -wave amplitude spectra between 0.05 and 10.0 Hz. We use the dual-station method to generate a preliminary Q Lg model and use it as the initial model. Then, we combine the dual- and single-station data together to jointly invert the Q Lg distribution and Lg source excitation functions. These inversions are conducted independently at individual frequencies without using any a priori assumption about the frequency dependences in Q Lg and source terms. The maximum spatial resolution is approximately 1° x 1° in well-covered areas for frequencies between 0.05 and 2.0 Hz. The Q Lg image is then used to determine the relationship between the attenuation and different geological structures. Results show an average Q 0 (1 Hz Q Lg ) of 374 for the entire North China Craton with an increasing trend from east to west. Average Q 0 values are 337, 361 and 421 for the east, central and west blocks, respectively. For the surrounding regions, the Eastern Tibetan plateau has a very low Q 0 of 188, while the Northeast China Plate and the Tianshan–Xingmeng fold belts are characterized by high Q 0 values of 506 and 424, respectively. We also investigate regional variations of the Lg attenuation in low-frequency band between 0.2 and 1.0 Hz.

Description: We present a systematic study on the influence of pressure (0.1–600 MPa), temperature (750–1200 °C), carbon dioxide fugacity (log f CO 2  = –4.41 to 3.60) and time (2–12 hr) on the chemical and physical properties of carbonate rock. Our experiments aim to reproduce the conditions at the periphery of magma chamber where carbonate host rock is influenced by, but not readily assimilated by, magma. This permits the investigation of the natural conditions at which circulating fluids/gases promote infiltration reactions typical of metasomatic skarns that can involve large volumes of subvolcanic carbonate basements. Results show that, providing that carbon dioxide is retained in the pore space, decarbonation does not proceed at any magmatic pressure and temperature. However, when the carbon dioxide is free to escape, decarbonation can occur rapidly and is not hindered by a low initial porosity or permeability. Together with carbon dioxide and lime, portlandite, a mineral commonly found in voluminous metasomatic skarns, readily forms during carbonate decomposition. Post-experimental analyses highlight that thermal microcracking, a result of the highly anisotropic thermal expansion of calcite, exerts a greater influence on rock physical properties (porosity, ultrasonic wave velocities and elastic moduli) than decarbonation. Our data suggest that this will be especially true at the margins of dykes or magma bodies, where temperatures can reach up to 1200 °C. However, rock compressive strength is significantly reduced by both thermal cracking and decarbonation, explained by the relative weakness of lime + portlandite compared to calcite, and an increase in grain size with increasing temperature. Metasomatic skarns, whose petrogenetic reactions may involve a few tens of cubic kilometres, could therefore represent an important source of volcanic instability.

Description: The scattering of plane SH waves incident on a circular sectorial canyon is considered. An accurate region-matching technique is applied to derive a rigorous series solution. Appropriate wavefunctions are employed to describe antiplane motions. Judicious basis functions, involving Gegenbauer polynomials, are well utilized to correctly capture the singular behaviour in stress fields near the canyon bottom. The enforcement of matching conditions on the auxiliary boundary leads to the determination of unknown coefficients. Plotted results demonstrate the influence of pertinent parameters on surface and subsurface motions. Both steady-state and transient results are included. The solution technique proposed achieves a considerable reduction in the computational effort, facilitating benchmark computations. The derived series solution enriches the limited list of series solutions presently known for canyon problems related to SH -wave scattering.

Description: We present 3-D models of the P- and S -wave velocity distributions in the crust and uppermost mantle beneath Sicily, Calabria (Southern Italy), and surrounding submerged areas, obtained by tomographic inversion of traveltimes of regional body waves phases. Our method combines double-difference tomographic inversion with a post-processing procedure [Weighted Average Model method (WAM)]. This procedure was applied to a set of models consistent with the experimental data. We tested the ability of the WAM procedure to mitigate the uncertainty associated with the arbitrary nature of the many input parameters required for each inversion. The local reliability and resolution of the obtained models have been assessed through: synthetic tests, experimental tests carried out with independent data sets and unconventional tests based on the analysis of the internal consistency of the P - and S -velocity models. The tomographic images provide a detailed sketch of P- and S- wave velocity anomalies. These clearly show the shape of the Sicilian-Maghrebian belt beneath Sicily and Calabrian Arc at different depths. Low V P and Vs bodies are imaged beneath Stromboli and Marsili volcanoes in the southern Tyrrhenian, whereas high and low seismic velocities alternate beneath the Etna giving inferences on the possible depth of the mantle melting feeding the volcano. In the upper crust, the main sedimentary basins and tectonic features are also well imaged. Finally, tomographic cross sections show the trend of the Moho in the study area, where its depth ranges between 35 and 40 km beneath the Sicilian belt and between 15 and 22 km in the southern Tyrrhenian basin and Ionian Sea.

Description: We used two tracks of ALOS PALSAR images to investigate the focal mechanism and slip distribution of the 2011 March 24, M W 6.8 Burma strike-slip earthquake. Three different SAR techniques, namely conventional interferometry, SAR pixel offsets (SPO) and multiple-aperture InSAR (MAI), were employed to obtain the coseismic surface deformation fields along the ~30 km length of the fault rupture. Along-track measurements from SPO and MAI techniques show a high correlation, and were subsequently used to precisely determine the location and extent of the surface fault trace. The best-fitting fault model geometry derived from an iterative inversion technique suggests that the rupture occurred on a near-vertical sinistral strike-slip fault west of the Nam Ma fault with a strike of 70°. A maximum slip of 4.2 m occurs at a depth of 2.5 km, with significant slip constrained only to the upper 10 km of the crust.

Description: Knowledge of the mantle reflectivity structure is highly dependent on our ability to efficiently extract, and properly interpret, small seismic arrivals. Among the various data types and techniques, long-period SS/PP precursors and high-frequency receiver functions are routinely utilized to increase the confidence of the recovered mantle stratifications at distinct spatial scales. However, low resolution and a complex Fresnel zone are glaring weaknesses of SS precursors, while over-reliance on receiver distribution is a formidable challenge for the analysis of converted waves from oceanic regions. A promising high frequency alternative to receiver functions is P ' P ' precursors, which are capable of resolving mantle structures at vertical and lateral resolution of ~5 and ~200 km, respectively, owing to their spectral content, shallow angle of incidence and near-symmetric Fresnel zones. This study presents a novel processing method for both SS (or PP) and P ' P ' precursors based on deconvolution, stacking, Radon transform and depth migration. A suite of synthetic tests is performed to quantify the fidelity and stability of this method under different data conditions. Our multiresolution survey of the mantle at targeted areas near Nazca-South America subduction zone reveal both olivine and garnet related transitions at depths below 400 km. We attribute a depressed 660 to thermal variations, whereas compositional variations atop the upper-mantle transition zone are needed to explain the diminished or highly complex reflected/scattered signals from the 410 km discontinuity. We also observe prominent P ' P ' reflections within the transition zone, and the anomalous amplitudes near the plate boundary zone indicate a sharp (~10 km thick) transition that likely resonates with the frequency content of P ' P ' precursors. The migration of SS precursors in this study shows no evidence of split 660 reflections, but potential majorite–ilmenite (590–640 km) and ilmenite–perovskite transitions (740–750 km) are identified based on similarly processed high-frequency P ' P ' precursors. Additional findings of severely scattered energy in the lithosphere and distinct lower mantle reflections at ~800 km could be potentially important but require further verifications. Overall, our improved imaging methods and the strong sensitivity of P ' P ' precursors to the existence, depth, sharpness and strength of reflective structures offer significant future promise for the understanding of mantle mineralogy and dynamics.

Description: Time-dependent probabilistic seismic hazard assessment requires a stochastic description of earthquake occurrences. While short-term seismicity models are well-constrained by observations, the recurrences of characteristic on-fault earthquakes are only derived from theoretical considerations, uncertain palaeo-events or proxy data. Despite the involved uncertainties and complexity, simple statistical models for a quasi-period recurrence of on-fault events are implemented in seismic hazard assessments. To test the applicability of statistical models, such as the Brownian relaxation oscillator or the stress release model, we perform a systematic comparison with deterministic simulations based on rate- and state-dependent friction, high-resolution representations of fault systems and quasi-dynamic rupture propagation. For the specific fault network of the Lower Rhine Embayment, Germany, we run both stochastic and deterministic model simulations based on the same fault geometries and stress interactions. Our results indicate that the stochastic simulators are able to reproduce the first-order characteristics of the major earthquakes on isolated faults as well as for coupled faults with moderate stress interactions. However, we find that all tested statistical models fail to reproduce the characteristics of strongly coupled faults, because multisegment rupturing resulting from a spatiotemporally correlated stress field is underestimated in the stochastic simulators. Our results suggest that stochastic models have to be extended by multirupture probability distributions to provide more reliable results.

Description: Stress waves, known as acoustic emissions (AEs), are released by localized inelastic deformation events during the progressive failure of brittle rocks. Although several numerical models have been developed to simulate the deformation and damage processes of rocks, such as non-linear stress–strain behaviour and localization of failure, only a limited number have been capable of providing quantitative information regarding the associated seismicity. Moreover, the majority of these studies have adopted a pseudo-static approach based on elastic strain energy dissipation that completely disregards elastodynamic effects. This paper describes a new AE modelling technique based on the combined finite-discrete element method (FEM/DEM), a numerical tool that simulates material failure by explicitly considering fracture nucleation and propagation in the modelling domain. Given the explicit time integration scheme of the solver, stress wave propagation and the effect of radiated seismic energy can be directly captured. Quasi-dynamic seismic information is extracted from a FEM/DEM model with a newly developed algorithm based on the monitoring of internal variables (e.g. relative displacements and kinetic energy) in proximity to propagating cracks. The AE of a wing crack propagation model based on this algorithm are cross-analysed by traveltime inversion and energy estimation from seismic recordings. Results indicate a good correlation of AE initiation times and locations, and scaling of energies, independently calculated with the two methods. Finally, the modelling technique is validated by simulating a laboratory compression test on a granite sample. The micromechanical parameters of the heterogeneous model are first calibrated to reproduce the macroscopic stress–strain response measured during standard laboratory tests. Subsequently, AE frequency–magnitude statistics, spatial clustering of source locations and the evolution of AE rate are investigated. The distribution of event magnitude tends to decay as power law while the spatial distribution of sources exhibits a fractal character, in agreement with experimental observations. Moreover, the model can capture the decrease of seismic b value associated with the macrorupture of the rock sample and the transition of AE spatial distribution from diffuse, in the pre-peak stage, to strongly localized at the peak and post-peak stages, as reported in a number of published laboratory studies. In future studies, the validated FEM/DEM-AE modelling technique will be used to obtain further insights into the micromechanics of rock failure with potential applications ranging from laboratory-scale microcracking to engineering-scale processes (e.g. excavations within mines, tunnels and caverns, petroleum and geothermal reservoirs) to tectonic earthquakes triggering.

Description: We used strong-motion records from the 2012 May 20 and 29 Emilia-Romagna earthquakes ( M w 6.1 and 5.9, respectively) and four aftershocks with magnitudes ranging between 4.9 and 5.5 to analyse the S -wave spectral amplitude decay with distance and estimate acceleration source functions and site effects. The data set consists of six earthquakes, 44 stations and 248 records with hypocentral distances in the range 10 〈 r  〈 100 km. We rotated the accelerograms to calculate transverse and radial components of the acceleration spectrum. We found non-parametric attenuation functions that describe the spectral amplitude decay of SH and SV waves with distance at 60 different frequencies between 0.1 and 40 Hz. These attenuation functions provide an estimate of the quality factor Q at each frequency analysed. Assuming that geometrical spreading is 1/ r for r  ≤ r x and 1/( r x r ) 0.5 for r  〉 r x with r x  = 60 km and normalizing at 15 km (the recording distance where the attenuation functions start to decay), we find that the average Q for SH waves can be approximated by Q SH  = 82 ± 1 f  1.2±0.02 and by Q SV  = 79 ± 1 f  1.24±0.03 for SV waves in the frequency range 0.10 ≤ f  ≤ 10.7 Hz. At higher frequencies, 11.8 ≤ f  ≤ 40 Hz, the frequency dependence of Q weakens and is approximated by Q SH  = 301 ± 1 f   0.36±0.04 and Q SV  = 384 ± 1 f  0.28±0.04 . These results indicate that the S -wave attenuation is radially isotropic at local distances in the epicentral area. Nevertheless, we used these attenuation parameters separately to correct the radial (with Q SV ) and transverse (with Q SH ) components of the acceleration spectra and to separate source and site effects using a non-parametric spectral inversion scheme. We found that the source function of the main event and the bigger aftershocks show enhanced low frequency radiation between 0.4 and 3.0 Hz. We converted the source functions into far-field source acceleration spectra and interpreted the resulting source spectra in terms of Brune's model. The stress drops obtained range between approximately 0.9 and 2.9 MPa. Although all the recording stations used are located in the Po Plain, the site functions obtained from the spectral inversion show important amplification variability between the sites. We compared these site functions with the average horizontal to vertical spectral ratios calculated for each station, and we found consistent results for most stations.

Description: The dihedral angle formed at junctions between two plagioclase grains and a grain of augite is only very rarely in textural equilibrium in gabbros from kilometre-scale crustal layered intrusions. The median of a population of these disequilibrium angles, cpp , varies systematically within a single layered intrusion, remaining constant over large stretches of stratigraphy with significant increases and decreases associated with the addition or reduction respectively of the number of phases on the liquidus of the bulk magma. The stepwise changes in cpp are present in the Upper Zone of the Bushveld Complex, the Megacyclic Unit I of the Sept Iles Intrusion, and the Layered Series of the Skaergaard intrusion. The plagioclase-bearing cumulates of Rum have a bimodal distribution of cpp , dependent on whether the cumulus assemblage includes clinopyroxene. The presence of the stepwise changes is independent of the order of arrival of cumulus phases and of the composition of either the cumulus phases or the inferred composition of the interstitial liquid. The only parameter that behaves in an exactly analogous manner to cpp is the rate of change in enthalpy with temperature ( H / T ) during crystallization. Both H / T and cpp increase with the addition of a liquidus phase, and decrease with the removal of a liquidus phase. The replacement of one phase by another has little effect on H / T and no discernible effect on cpp . An increase of H / T results in an increase in the fraction of the total enthalpy budget that is the latent heat of crystallization (the fractional latent heat). It also increases the mass crystallized in each incremental temperature drop (the crystal productivity). These increases of both fractional latent heat and crystal productivity are likely to cause an increase in the time taken to form three-grain junctions in the mush via thermal buffering of a thickened mushy layer. We suggest these are the underlying causes of stepwise increases in cpp . Stepwise changes in the geometry of three-grain junctions in fully solidified gabbros thus provide a clear microstructural marker for the progress of fractionation down the liquid line of descent in layered intrusions.

Description: High-resolution sampling in monogenetic fields has the potential to reveal fine-scale heterogeneity of the mantle, a feature that may be overwhelmed by larger fluxes of magma, or missed by under-sampling. The Quaternary Auckland Volcanic Field (AVF) in northern New Zealand is a basaltic field of 51 small-volume volcanic centres, and is one of the best-sampled examples of a monogenetic volcanic field. We present data for 12 centres in the volcanic field. These show the large compositional variations between volcanoes as well as through single eruptive sequences. Whole-rock compositions range from subalkaline basalt in the larger centres, through alkali basalt to nephelinite in the smallest centres. Fractional crystallization has had a limited effect in many of the centres, but high-pressure clinopyroxene crystallization may have occurred in others. Three end-members are observed in Pb isotope space, indicating that distinct mantle source components are involved in the petrogenesis of the magmas. Whole-rock multi-element patterns show that the larger centres have prominent positive Sr anomalies and lack K anomalies, whereas the smaller centres have prominent negative K anomalies and lack Sr anomalies. The melting parameters and compositions of the sources involved are modelled using trace element ratios and multi-element patterns, and three components are characterized: (1) fertile peridotite with a Pb-isotope composition similar to Pacific mid-ocean ridge basalt; (2) eclogite domains with a HIMU-like isotope composition dispersed within the fertile peridotite; (3) slightly depleted subduction-metasomatized peridotitic lithospheric mantle (containing c . 3% subduction fluids). Modelling shows that melting in the AVF begins in garnet-bearing fertile asthenosphere (with preferential melting of eclogite domains) and that melts are variably diluted by melts of the lithospheric source. The U–Th isotope compositions of the end-members in the AVF show 230 Th excess [( 230 Th/ 232 Th) ratios of 1·11–1·38], with the samples of lower ( 230 Th/ 232 Th) exhibiting higher ( 238 U/ 232 Th), which we attribute to the dilution effect of the melts from the lithospheric mantle source. Modelling reveals a correlation between melting in the asthenosphere, the degree of melting and incorporation of the metasomatized lithospheric mantle source, and the resultant size of the volcanic centre. This suggests that the scale of the eruption may essentially be controlled by asthenospheric mantle dynamics.

Description: The origin of mafic and ultramafic sills exhibiting different whole-rock compositional profiles (e.g. I-, C-, D-, M- and S-shaped profiles) remains controversial. We have addressed this issue by revisiting three ~100 m thick Siberian dolerite sills (Vavukansky, Kuz’movsky and Vilyuysky) that display remarkable internal differentiation. The Vavukansky sill has an M-shaped profile with prominent basal and top reversals showing inward increases in whole-rock MgO, Mg-number [100Mg/(Mg + Fe)] and normative An content [100An/(An + Ab)], followed by the Layered and Upper Border Series with inward decreases in these indices. The Kuz’movsky and Vilyuysky sills both show S-shaped profiles similar to the Vavukansky sill, but lack a top reversal. These whole-rock M- and S-shaped profiles are accompanied by similar profiles in mineral compositions. Plagioclase and, to a lesser extent, olivine show systematic inward increases in An content and Mg-number, respectively, across basal and top reversals. These compositional trends are followed by inward decreases in these ratios in the interiors of the Vavukansky and Kuz’movsky sills. Currently accepted models attribute whole-rock M- and S-shaped compositional profiles to crystal settling, compositional convection or compaction operating in closed systems. Our observations challenge these traditional interpretations because variations in mineral compositions observed in marginal reversals cannot result from closed-system fractionation. We suggest instead that initially the sills evolved as open systems that were slowly inflated by magmas that became gradually more primitive with time. The inflation was accompanied by in situ crystallization that preserved the preceding fractionation history of the injected magmas by forming basal and top reversals with minerals becoming more primitive inwards. This process culminated with rapid inflation of the sills to their current size owing to a major influx of primitive magma. Subsequently, magma flow through the sills ceased and they evolved as closed systems by fractional crystallization. This resulted in the Layered and Upper Border Series with minerals becoming more evolved inwards. This model can be extended to explain other compositional profiles and petrological features in mafic and ultramafic sills.

Description: Resistive forces along convergent plate boundaries have a major impact on surface deformation, most visibly at collisional plate boundaries. Although quantification of these forces is key to understanding the evolution and present state of mountain belts, they remain highly uncertain due to the complexity of plate boundary structures and rheologies. In previous studies of the Eurasian Plate, we have analysed the balance of plate boundary forces, tractions resulting from lithosphere–mantle coupling, and intraplate variations in topography and density structure. This yielded a range of acceptable force distributions. In this study, we investigate to which extent the observed present-day stress field provides further constraints on the distribution of forces. We address the dynamics of the Eurasian Plate as a whole. This enables us to base our analysis on mechanical equilibrium of a tectonic plate and to evaluate all forces as part of an internally consistent set of forces driving and deforming Eurasia. We incorporate tractions from convective mantle flow modelling in a lithospheric model in which edge and lithospheric body forces are modelled explicitly and compute resulting stresses in a homogeneous elastic thin shell. Intraplate stress observations used are from the World Stress Map project. Eurasia's stress field turns out to be particularly sensitive to the distribution of collision forces on the plate's southern margin and, to a much lesser extent, to lithospheric density structure and tractions from mantle flow. Stress observations require collision forces on the India–Eurasia boundary of 7.0–10.5 TN m –1 and on the Arabia–Eurasia boundary of 1.3–2.7 TN m –1 . Implication of mechanical equilibrium of the plate is that forces on the contacts with the African and Australian plates amount to 1.0–2.5 and 0–1.3 TN m –1 , respectively. We use our results to assess the validity of the classical view that the mean elevation of an orogenic plateau can be taken as a measure of the magnitude of the compressive (in this case: collision-related) forces involved. For both the Tibetan and the Iranian plateaus, two plateaus with significantly different average elevations, we find that the horizontal force derived from the excess gravitational potential energy (collapse force) is in balance with the collision force.

Description: Magma movement and fault slip alter the magnitude and orientation of the stress in the surrounding crust. Observations of a sequence of events clustered in space and time provide information about the triggering mechanism and stress interactions between magma intrusion, earthquakes and eruptions. We investigate the syn- and post-intrusion stress changes associated with the 2007 Gelei dyke intrusion episode and subsequent eruption of nearby Oldonyo Lengai. Previous studies produced a kinematic model of the 2007 June–August sequence involving ~1 m slip on a normal fault followed by the intrusion of the 7–10-km long Gelei dyke, collapse of a shallow graben and the deflation of the Gelei magma chamber. Immediately following this, the volcano Oldoinyo Lengai (〈10 km away) experienced a new phase of explosive activity lasting for several months. Here, we present new geodetic observations covering Gelei and Oldoinyo Lengai in 2008 September–2010. We show continued slip on graben-bounding faults above the Gelei dyke. The eruption of Oldoinyo Lengai was accompanied by the intrusion of a 4 km-long E–W-trending dyke followed by deflation of a shallow source directly below the summit of the volcano. Next, we use stress calculations to investigate a number of hypotheses linking these events. (1) Before the onset of surface deformation, a dyke sufficiently deep and narrow to be geodetically undetectable could still have produced sufficient stress changes to trigger slip on the normal fault (i.e. the sequence could have been magmatically driven). (2) Stresses at the dyke tip would have been sufficient to overcome the effect of continued slip on the normal fault, allowing the dyke to propagate upwards into a region of clamping. (3) The Gelei sequence would have produced a significant stress change on the chamber beneath Oldoinyo Lengai. These static stress calculations allow us to discuss the roles played by dynamic stress, deeper magmatic changes and background stresses throughout the sequence with implications for the stress triggering of both seismic and volcanic hazards.

Description: The relationships between magnetic susceptibility and pedogenic development are different in various regions of the world. For example, loess magnetic susceptibility shows a positive correlation with pedogenic development in Chinese Loess Plateau (CLP), while it displays a negative correlation with pedogenesis in Alaska and Siberia. To better understand the relationship between magnetic properties and pedogenic development, detailed sampling of Dashing Rocks loess section at Timaru, South Island, New Zealand, was carried out. Multiproxy magnetic parameters such as magnetic susceptibility, anhysteretic remanent magnetization, magnetic hysteresis loops, M s – T curves and – T curves were measured. The results show that the types of magnetic minerals are similar to CLP: magnetite, maghemite, goethite and hematite. However, great differences are found in their concentration: most minerals in the Dashing Rocks section are hard magnetic, such as goethite, the content of paramagnetic minerals is rather high, while the soft-magnetic mineral content is very low. Hard-magnetic and paramagnetic minerals increase with depth, but soft-magnetic minerals decrease with depth, and are absent in the lower part of the profile. Gammate soil structures and Fe/Mn nodules (or pans) are commonly observed in the section, indicating that high susceptibility magnetite and maghemite have been converted to goethite and migrated downward to enrich certain horizons during chemical weathering. This process leads to lower magnetic susceptibility values, possibly related to the source and the transformation of soft-magnetic minerals in a high soil moisture environment. The relationship between magnetic susceptibility and pedogenic development in Dashing Rocks loess section is therefore different from the simple positive and negative relationships in CLP and Siberia, respectively. The more complex relationships between magnetic properties and pedogenic development in New Zealand loess may be related to differing degrees of magnetic mineral transformation at different depths and at different times.

Description: Vehicle traffic is at present one of the major sources of environmental pollution in urban areas. Magnetic parameters are successfully applied in environmental studies to obtain detailed information about concentrations and quality of iron-bearing minerals. A general aim of this research was to investigate the magnetic, microstructural and mineralogical properties of dust extracted from the roadside snowpack accumulated on the side of an urban highway, northern Helsinki. Vertical snow profiles were taken at different distances (5, 10 and 15 m) from the road edge, during winter season 2010–2011. The temporal distribution of mass magnetic susceptibility () of the road dust shows that the concentration of magnetic particles increases in the snowpack during winter. Roadside snowpack preserves a large fraction of the magnetic particulate until the late stages of melting and this could be considered as one of the main factors responsible for the resuspension phenomenon observed in Nordic countries. The vertical distribution of and SIRM (saturation isothermal remanent magnetization)/ ratio may indicate the migration of magnetic particles down in the snowpack during melting conditions. Ultrafine to coarse-grained (superparamagnetic to multidomain) magnetite was identified as the primary magnetic mineral in all the studied road dust samples. The examined road dust contains significant amount of dia/paramagnetic minerals (e.g. quartz, albite, biotite) and the content of magnetite is relatively low (below 1 weight percent, wt%). The roadside snowpack is enriched in anthropogenic particles such as angular and spherical iron-oxides, tungsten-rich particles and sodium chloride. This study demonstrates the suitability of snow as an efficient collecting medium of magnetic particulates generated by anthropogenic activities.

Description: We explore the use of on-land GPS observations to detect deformation due to tsunami propagation near source regions of large interplate earthquakes. Here, we focus on the M w 9 Tohoku-oki earthquake, which occurred around 14:46 (JST) on 2011 March 11. We consider GPS data in the time span 14:54–15:22 (JST) along the Sanriku coast, where the tsunami had the largest amplitude. The displacement data shows the signatures of large aftershocks as well as post-seismic fault slip (afterslip). These effects are particularly evident in the east component. From the horizontal displacement vectors, we construct a simple fault model for the early phase of the afterslip. Mean slip velocity of the early afterslip reaches 0.1 mm s –1 . By compiling the early afterslip velocity of recent interplate earthquakes around that region, we find its increasing trend with the main shock magnitude. This scaling relation may reflect higher stressing rates at edges of larger main shock faults. Separately, we forward calculate land deformation due to tsunami height changes based on a tsunami simulation. Tsunami-induced deformation is only evident in the vertical direction at coastal GPS stations. The predicted subsidence amounts at some coastal stations can account for a large portion of the residuals between the observation and the modelled deformation due to the fault slip.

Description: Observations of the Earth's nutations provide constraints on the mechanical coupling at the core–mantle and inner core boundaries. An important physical mechanism that could be responsible for the observed dissipation is the electromagnetic (EM) coupling, to which this paper is devoted. Previous studies assumed that the main feature of the magnetic field that affects the EM coupling is its overall strength, its morphology being considered unimportant. In particular, these studies rely on the hypothesis that the contribution to the torque from all the non-dipolar components of the field can be approximated by the contribution that a uniform radial field with the same strength would have. In this study, we compute the EM torque for more realistic configurations of the magnetic field at the core boundaries and thereby assess the role of its spatial distribution on the strength of the EM torque. For field strengths typical of the core–mantle boundary (CMB), we show that the spatial distribution affects weakly the strength of the torque, with the approximation by a uniform field leading to an overestimation of the torque magnitude by ~15–20 per cent. However, for field strengths typical of the inner core boundary (ICB), the morphology of the field has a more significant influence on the EM torque and the approximation by a uniform field overestimates the torque by ~30–40 per cent. Assuming that EM coupling is responsible for the observed dissipation, we infer constraints on the strength of the radial magnetic field at both the CMB and ICB. We show how the unknown morphology of the magnetic field induces uncertainties on the estimated field strength at the ICB, which can take values anywhere in the range of ~9–16 mT. These very large values suggest that EM coupling at the ICB cannot be the only mechanism responsible for the observed dissipation.

Description: Variations in the degree-2 Stokes coefficients C 20 , C 21 and S 21 can be used to understand long- and short-term climate forcing. Here we derive changes in these coefficients for the period 2003 January–2012 April using Earth rotation data. Earth rotation data contain contributions from motion terms (the effects of winds and currents) and contributions from the effects of mass redistribution. We remove the effects of tides, atmospheric winds and oceanic currents from our data. We compare two different models of atmospheric and oceanic angular momentum for removing the effects of winds and currents: (1) using products from the National Centers for Environmental Prediction and (2) using data from the European Centre for Medium-range Weather Forecasts (ECMWF). We assess the quality of these motion models by comparing the two resulting sets of degree-2 Stokes coefficients to independent degree-2 estimates from satellite laser ranging (SLR), GRACE and a geophysical loading model. We find a good agreement between the coefficients from Earth rotation and the coefficients from other sources. In general, the agreement is better for the coefficients we obtain by removing winds and currents effects using the ECMWF model. In this case, we find higher correlations with the independent models and smaller scatters in differences. This fact holds in particular for C 20 and C 21 , whereas we cannot observe a significant difference for S 21 . At the annual and semiannual periods, our Earth rotation derived coefficients agree well with the estimates from the other sources, particularly for C 21 and S 21 . The slight discrepancies we obtain for C 20 can probably be explained by errors in the atmospheric models and are most likely the result of an over-/underestimation of the annual and semiannual contributions of atmospheric winds to the length-of-day excitation.

Description: Subduction zones are complex 3-D features in which one tectonic plate sinks underneath another into the deep mantle. During subduction the overriding plate (OP) remains in physical contact with the subducting plate and stresses generated at the subduction zone interface and by mantle flow force the OP to deform. We present results of 3-D dynamic laboratory models of subduction that include an OP. We introduce new interplate materials comprising homogeneous mixtures of petrolatum and paraffin oil to achieve progressive subduction. The rheology of these mixtures is characterized by measurements using a strain rate controlled rheometer. The results show that the strength of the mixture increases with petrolatum content, which can be used as a proxy for the degree of mechanical coupling along the subduction interface. Results of subduction experiments are presented with different degrees of mechanical coupling and the influence this has on the dynamics and kinematics of subduction. The modelling results show that variations in the degree of mechanical coupling between the plates have a major impact on subduction velocities, slab geometry and the rate of OP deformation. In all experiments the OP is displaced following trench migration and experiences overall extension localized in the plate interior. This suggests that OP deformation is driven primarily by the toroidal component of subduction-related mantle return flow. The subduction rate is always very slow in experiments with medium mechanical coupling, and subduction stops prematurely in experiments with very high coupling. This implies that the shear forces along the plate interface in natural subduction zone systems must be relatively low and do not vary significantly. Otherwise a higher variability in natural subduction velocities should be observed for mature, non-perturbed subduction zones. The required low shear force is likely controlled by the rheology of highly hydrated sedimentary and basaltic rocks.

Description: The Frank-Kamenetskii (FK) approximation is a common method to represent the Arrhenius-type viscosity of planetary mantles because it reduces the viscosity contrast in the lithosphere to save computational resources and prevent numerical errors. In some cases, this approximation does not lead to satisfying results; for example, it can lead to a mobile-lid regime, whereas use of the Arrhenius law shows a thin stagnant lid. We therefore derive a new, more accurate approximation called ‘damped FK approximation’ for a temperature- and pressure-dependent viscosity. This damped FK approximation is a mixture between the standard first-order FK approximation and an approximation of second-order accuracy controlled by a damping parameter. Furthermore, the FK parameters are determined self-consistently at every time step. This study shows that the damped FK approximation represents the mantle flow of an Arrhenius-type viscosity for a much larger parameter space than for the standard first-order approximation. It can also be used to simulate terrestrial planets, such as super-Earths, with high pressure dependence of the viscosity, if the surface temperature does not exceed a specific threshold value and if a high enough damping parameter is used. We also test the FK approximation for plate tectonics simulations. The second-order FK approximation best represents the Arrhenius flow in the investigated parameter range. In particular, the dependence of the critical yield stress, at which the transition from the plate tectonics regime to the stagnant-lid regime can be observed, on the Rayleigh number can differ from the Arrhenius case (and the second-order FK approximation) when using a first-order FK approximation or rheology parameters in the Arrhenius law that differ from laboratory values to yield small viscosity contrasts. This finding may have strong implications for the prediction of plate tectonics on terrestrial planets.

Description: We propose new scaling laws for the properties of planetary dynamos. In particular, the Rossby number, the magnetic Reynolds number, the ratio of magnetic to kinetic energy, the Ohmic dissipation timescale and the characteristic aspect ratio of the columnar convection cells are all predicted to be power-law functions of two observable quantities: the magnetic dipole moment and the planetary rotation rate. The resulting scaling laws constitute a somewhat modified version of the scalings proposed by Christensen and Aubert. The main difference is that, in view of the small value of the Rossby number in planetary cores, we insist that the non-linear inertial term, ${\boldsymbol u} \cdot \nabla {\boldsymbol u}$ , is negligible. This changes the exponents in the power-laws which relate the various properties of the fluid dynamo to the planetary dipole moment and rotation rate. Our scaling laws are consistent with the available numerical evidence.

Description: Earlier studies at the large Vredefort impact structure since 1960 have shown that values of natural remanent magnetizations (NRMs) and, hence, Koenigsberger's Q values (ratio of remanent over induced magnetization), for different rock lithologies are elevated compared to the values for similar rock types around the world. Three origins for the high Q values have been suggested, namely shock by meteorite impact, enhanced plasma field and lightning strikes. We have studied whether laboratory lightning experiments can produce enhanced NRMs in the Vredefort target rocks. For comparison, we also included rocks from the Johannesburg dome, which is not a meteorite impact site. The results revealed increased NRMs, susceptibility and Q values of the rocks from both Vredefort and Johannesburg domes. Rock magnetic measurements and scanning electron microscope analyses of lightning pulsed and unpulsed samples showed that the lightning included changes in magnetic properties of the rocks. We suggest that in some samples lightning have changed magnetic mineralogy by oxidizing magnetite to maghemite. Indication of this oxidation came from the low-temperature variation of the remanent magnetization where we observed several hallmarks of maghemitization in samples treated by lightning strikes. Further indications of mineralogical changes include increased Curie points above the magnetite's Curie point (580 °C) and appearance of pronounced lower temperature (200–400 °C) phases in susceptibility versus temperature curves. These changes are interpreted to indicate partially oxidized magnetite (maghemitization) coupled with grain fragmentations and by this way grain size reduction. High-temperature hysteresis and REM (= NRM/saturation isothermal remanent magnetization) studies support these conclusions. Our results were analogous with the ones for lodestones and protolodestones where partially oxidized magnetite is thought to make magnetization more intense.

Description: Regeneration of the Earth's magnetic field by convection in the liquid core produces a broad spectrum of time variation. Relative palaeointensity measurements in marine sediments provide a detailed record over the past 2 Myr, but an explicit reconstruction of the underlying dynamics is not feasible. A more practical alternative is to construct a stochastic model from estimates of the virtual axial dipole moment. The deterministic part of the model (drift term) describes time-averaged behaviour, whereas the random part (diffusion term) characterizes complex interactions over convective timescales. We recover estimates of the drift and diffusion terms from the SINT2000 model of Valet et al. and the PADM2M model of Ziegler et al. The results are used in numerical solutions of the Fokker–Planck equation to predict statistical properties of the palaeomagnetic field, including the average rates of magnetic reversals and excursions. A physical interpretation of the stochastic model suggests that the timescale for adjustments in the axial dipole moment is set by the dipole decay time d . We obtain d = 29 kyr from the stochastic models, which falls within the expected range for the Earth's core. We also predict the amplitude of convective fluctuations in the core, and establish a physical connection to the rates of magnetic reversals and excursions. Chrons lasting longer than 10 Myr are unlikely under present-day conditions. However, long chrons become more likely if the diffusion term is reduced by a factor of 2. Such a change is accomplished by reducing the velocity fluctuations in the core by a factor of 2, which could be attributed to a shift in the spatial pattern of heat flux from the core or a reduction in the total core heat flow.

Description: Since King presented the ‘plates and spheres’ model in an attempt to investigate the origin of the inclination error in sediments, no one to date has conducted specific experiments designed to separate the individual contribution of platy and spherical particles to depositional remanent magnetizations (DRMs). It is commonly accepted that it is the flattening of plates, rather than the rolling of spheres that is the main source of inclination error in sediments. Recently, however, Bilardello et al. have shown that spheres alone may lead to significant amounts of shallowing. A comparison of experiments run in parallel using synthetic platy and spherical particles is presented. Experiments of the duration of 24 hr were run in 100 μT field intensity ( μ 0 H ) and varying field inclinations ( I F ) from vertical to horizontal. A systematic dependence of the magnetization on field inclination is apparent. Results indicate that magnetic moment measurements are more repeatable for spherical particles than for plates, yielding smaller uncertainties. Inclination measurements, however, are more repeatable for platy particles, with a more linear relationship of inclination error to applied field inclination. Moreover, plates yield smaller inclination error than spheres. A clear field inclination dependency of the inclination error also exists, with the error decreasing through field inclinations of 30°, 60° and 90°. A continuous acquisition experiment involving plates was also run up to 10 d of deposition in μ 0 H = 100 μT and I F = 60°. The acquisition curves for moment, inclination and thickness of depositing sediment are compared to the mean curves measured for spheres by Bilardello et al. under the same field conditions. No unequivocal evidence of compaction of the platy particles is observed, while the inclination error is acquired virtually instantaneously for all particles. These preliminary results contradict the widespread understanding that inclination shallowing is more prominent for platy particles (e.g. hematite) than it is for more spherical particles (e.g. magnetite). It is true that larger amounts of shallowing have been commonly observed in natural hematite-bearing rocks, but the overall ranges of shallowing are also larger. The particles used in these experiments may not be a reliable proxy for natural crystals and one must exercise caution when extrapolating to the natural scenarios; however, the results provide insight into the behaviour of differently shaped particles.

Description: Monoclinic 4C pyrrhotite (Fe 7 S 8 ) is ferrimagnetic due to an ordered defect structure with alternating vacancy and vacancy-free sublattices. Its low-temperature magnetic transition near 35 K is characterized by the distinct increase in coercivity and remanent magnetization. The increase of these parameters has been attributed to changes in the domain wall structure. We present static and dynamic magnetization data of a powder sample to study the domain-wall dynamics across the low-temperature transition. The amplitude-dependent ac susceptibility and the ferromagnetic resonance spectroscopy indicate that the hardening of the domain-wall pinning at the transition occurs simultaneously with the decrease in initial saturation remanent magnetization. These two effects are explained by the enhanced inhomogeneity of the bulk material caused by the persistency of the ordered vacancies and by newly formed defects due to localized distortion of Fe(II) sites in the vacancy-free sublattice. The generated localized defects are the link between the domain wall dynamics and the low-temperature transition in 4C pyrrhotite.

Description: Thermal anomalies associated with ore-mineralization (Pb–Cu–Zn and Fe) were studied using thermal infrared data collected over Mamandur polymetal prospect, India, with the aid of satellite, field, and laboratory measurements. Day and night ASTER data were analysed in conjunction with field measurements to estimate thermal inertia of the ore body, altered zones and country rocks. Representative samples collected from field were also analysed for thermal conductivity, diffusivity, and inertia using a self-fabricated setup. Spatial changes in thermal inertia were mapped by look up table (LUT) and advanced thermal inertia mapping (ATIM) approaches. Mineralized zones show very high thermal contrast ( T ) both in field (15–25°C) and satellite data (14.9–16.9 o C). They also exhibit the lowest thermal inertia in field-(2118–5474 J m –2 K –1 s –1/2 ) and satellite-based (3783–4037 J m –2 K –1 s –1/2 ) measurements. In non-mineralized areas, acidic rocks (granite, migmatite and granite gneiss) have lower inertia than basic rocks (basic granulite, dolerite and charnockite). Results estimated by LUT and ATIM approaches correlate very well at satellite ( R 2  = 0.97) and field ( R 2  = 0.89) scales. Similarly, field- and satellite-based results also have good correlation ( R 2  = 0.69–0.72). This study illustrates the potential of thermal inertia mapping in delineating ore bodies and deciphering the lithological changes even under veneer of soil.

Description: We present a novel global 3-D electromagnetic (EM) inverse solution that allows to work in a unified and consistent manner with frequency-domain data that originate from ionospheric and magnetospheric sources irrespective of their spatial complexity. The main idea behind the approach is simultaneous determination of the source and conductivity distribution in the Earth. Such a determination is implemented in our solution as a looped sequential procedure that involves two steps: (1) determination of the source using a fixed 3-D conductivity model and (2) recovery of a 3-D conductivity model using a fixed source. We focus in this paper on analysis of Sq data and numerically verify each step separately and combined using data synthesized from 3-D models of the Earth induced by a realistic Sq source. To determine the source we implement an approach that makes use of a known conductivity structure of the Earth with non-uniform oceans. Based on model studies we show that this approach outperforms the conventional potential method. As for recovery of 3-D conductivity in the mantle, our inverse scheme relies on a regularized least-square formulation, exploits a limited-memory quasi-Newton optimization method and makes use of the adjoint source approach to calculate efficiently the misfit gradient. We perform resolution studies with checkerboard conductivity structures at depths between 10 and 1600 km for different inverse setups and conclude from these studies that: (1) inverting Z component gives much better results than inverting all ( X , Y and Z ) components; (2) data from the Sq source allows for resolving 3-D structures in depth range between 100 and 520 km; (3) the best resolution is achieved in the depth range of 100–250 km.

Description: The discontinuity surface between Earth crust and mantle, the so-called Moho, is commonly estimated by means of seismic or gravimetric methods. Usually these methods do not yield the same result since they are based on different geological and geophysical hypotheses, as well as different data types, also in terms of quality and spatial distribution. In particular, global crust models based only on seismic data (e.g. the CRUST2.0 model) can be locally very accurate since seismic profiles give an almost direct observation of the actual crust structure, but can be quite uninformative in large regions where no data are available or they are too inhomogeneous. On the contrary, when using satellite gravity observations like those provided by the ESA mission GOCE, information on the Moho can be inferred from a uniform and global data set. However, Moho models estimated by gravity data are in general characterized by simplified hypotheses to guarantee the uniqueness of the solution of the inverse gravitational problem. The aim of this work is to attenuate these drawbacks by combining the seismic global model CRUST2.0 with gravity observations from the GOCE satellite mission. More specifically, the used GOCE data are grid values at mean satellite altitude estimated by the so-called space-wise approach. After reducing the data to a two-layer model by removing the effect of topography, bathymetry and sediments, a combined inversion driven by a priori information on the CRUST2.0 accuracy and by the error covariance structure of the GOCE grids is performed. In addition, the observation errors as well as the error due to the data reduction are tentatively taken into account to estimate the accuracy of the final Moho model. The result is an update of the CRUST2.0 Moho model with a 0.5°  x 0.5° resolution, which at the same time contains seismic and geological information and it is consistent, at 20 mE level, to the actually observed gravity field. A first comparison with the CRUST2.0 Moho shows that in the continental crust the mean difference between the two models is of the order of 1.5 km with standard deviations depending on the considered region. As expected, the main variations (standard deviation of the order of 7 km) are located in South America, Africa and Antarctica where very few data in the CRUST2.0 were originally used. In the rest of the world, differences have a standard deviation of about 4 km. As for the oceanic crust, it can be noted that the corrections to the CRUST2.0 model are of the order of 3 km (mean value) with a standard deviation of 6 km. Finally, the solution computed in this paper has been compared with a set of Moho models at different scales from global to local ones showing that it is reasonably consistent (differences of about 5 km standard deviation) also with seismic observations.

Description: Gravity gradiometry, whether from ground or airborne surveys, for geodesy and geophysics typically requires that the gravitational effect of the visible terrain is removed from the measured values. A systematic, algorithmic approach is developed to determine the extent of terrain data needed to maintain a truncation error below a desired level. The algorithm is based on a geostatistical analysis of the topography and applies to gradient differences over a particular size of survey area. It was found that a suitable modification of the kernel of the integral for the terrain effect can reduce the needed extent significantly in some cases. The determination of the needed extent for a given truncation error (standard deviation) and survey area size requires knowing a reasonable amplitude of the power spectral density (PSD) of the local topography, but otherwise is based on the fractal nature of topography, which also assumes that it is stationary. The algorithm is illustrated for ground and airborne cases in both moderate and rough terrain. For example, for an airborne gradiometer survey at 5 km altitude over 50 km of moderate terrain, the algorithm predicts requiring a topographic data extent of about 48 km for 1 Eötvös error (standard deviation) in the terrain effect. This extent can be reduced to 35 km with the kernel modification. In addition, the developed PSD models may be extended to determine the data resolution required for the terrain effect using a simple analytic formula.

Description: The commonest technique for determination of the continental-oceanic crustal boundary or transition (COB) zone is based on locating and visually correlating bathymetric and potential field anomalies and constructing crustal models constrained by seismic data. In this paper, we present a simple method for spatial correlation of bathymetric and potential field geophysical anomalies. Angular differences between gradient directions are used to determine different types of correlation between gravity and bathymetric or magnetic data. It is found that the relationship between bathymetry and gravity anomalies can be correctly identified using this method. It is demonstrated, by comparison with previously published models for the southwest African margin, that this method enables the demarcation of the zone of transition from oceanic to continental crust assuming that this it is associated with geophysical anomalies, which can be correlated using gradient directions rather than magnitudes. We also applied this method, supported by 2-D gravity modelling, to the more complex Liberia and Cote d'Ivoire-Ghana sectors of the West African transform margin and obtained results that are in remarkable agreement with past predictions of the COB in that region. We suggest the use of this method for a first-pass interpretation as a prelude to rigorous modelling of the COB in frontier areas.

Description: Gravity inversion allows us to constrain the interior mass distribution of a planetary body using the observed shape, rotation and gravity. Traditionally, techniques developed for gravity inversion can be divided into Monte Carlo methods, matrix inversion methods and spectral methods. Here we employ both matrix inversion and Monte Carlo in order to explore the space of exact solutions, in a method which is particularly suited for arbitrary shape bodies. We expand the mass density function using orthogonal polynomials, and map the contribution of each term to the global gravitational field generated. This map is linear in the density terms, and can be pseudo-inverted in the underdetermined regime using QR decomposition, to obtain a basis of the affine space of exact interior structure solutions. As the interior structure solutions are degenerate, assumptions have to be made in order to control their properties, and these assumptions can be transformed into scalar functions and used to explore the solutions space using Monte Carlo techniques. Sample applications show that the range of solutions tend to converge towards the nominal one as long as the generic assumptions made are correct, even in the presence of moderate noise. We present the underlying mathematical formalism and an analysis of how to impose specific features on the global solution, including uniform solutions, gradients and layered models. Analytical formulas for the computation of the relevant quantities when the shape is represented using several common methods are included in the Appendix.

Description: Tortuosity is a key parameter for investigating the pore structure of sedimentary rock. The conventional model for calculating the tortuosity is derived from Archie's law which is only valid for one conductive phase contained in a porous rock. However, there have been increasingly cases to show the non-Archie phenomenon in instances where the rock matrix has extra conductive phases such as surface conductivity caused by the electrical double layer. Therefore, such model may be inapplicable in these cases and in calculations involving partial melting. We investigated the relation between formation factor and porosity ( F– ) over a wide range of porosities by the lattice-gas automata method (LGA) with the electrical double layer. The digital rock samples were constructed by packing up solid grains with different shapes and size distribution, as previously measured in the laboratory on real rock specimens. Our purpose was to identify the origin of non-Archie behaviour of the F– relation sometimes observed. The simulations show that Archie's law may only be an approximation of the F– relation in a high porosity range. Based on our LGA simulations and additional laboratory experimental data, we developed new equations for non-Archie F– relation and tortuosity potentially useful for improving the evaluation of pore structure.

Description: In this study, we investigate the seismic wave attenuation beneath Sikkim Himalaya using P , S and coda waves from 68 local earthquakes registered by eight broad-band stations of the SIKKIM network. The attenuation quality factor ( Q ) depends on frequency as well as lapse time and depth. The value of Q varies from (i) 141 to 639 for P waves, (ii) 143 to 1108 for S waves and (iii) 274 to 1678 for coda waves, at central frequencies of 1.5 Hz and 9 Hz, respectively. The relations that govern the attenuation versus frequency dependence are Q α  = (96 ± 0.9) f (0.94 ± 0.01) , Q β  = (100 ± 1.4) f (1.16 ± 0.01) and Q c  = (189 ± 1.5) f (1.2 ± 0.01) for P , S and coda waves, respectively. The ratio between Q β and Q α is larger than unity, implying larger attenuation of P compared to S waves. Also, the values of Q c are higher than Q β . Estimation of the relative contribution of intrinsic ( Q i ) and scattering ( Q s ) attenuation reveals that the former mechanism is dominant in Sikkim Himalaya. We note that the estimates of Q c lie in between Q i and Q s and are very close to Q i at lower frequencies. This is in agreement with the theoretical and laboratory experiments. The strong frequency and depth dependence of the attenuation quality factor suggests a highly heterogeneous crust in the Sikkim Himalaya. Also, the high Q values estimated for this region compared to the other segments of Himalaya can be reconciled in terms of moderate seismic activity, unlike rest of the Himalaya, which is seismically more active.

Description: The seismic wavefield mainly contains reflected, refracted and direct waves but energy related to elastic scattering can also be identified at frequencies of 1 Hz and higher. The scattered, high-frequency seismic wavefield contains information on the small-scale structure of the Earth's crust, mantle and core. Due to the high thermal conductivity of mantle materials causing rapid dissipation of thermal anomalies, the Earth's small-scale structure most likely reveals details of the composition of the interior, and, is therefore essential for our understanding of the dynamics and evolution of the Earth. Using specific ray configurations we can identify scattered energy originating in the lower mantle and under certain circumstances locate its point of origin in the Earth allowing further insight into the structure of the lowermost mantle. Here we present evidence, from scattered PKP waves, for a heterogeneous structure at the core–mantle boundary (CMB) beneath southern Africa. The structure rises approximately 80 km above the CMB and is located at the eastern edge of the African LLSVP. Mining-related and tectonic seismic events in South Africa, with m b from 3.2 to 6.0 recorded at epicentral distances of 119.3° to 138.8° from Yellowknife Array (YKA) (Canada), show large amplitude precursors to PKP df arriving 3–15 s prior to the main phase. We use array processing to measure slowness and backazimuth of the scattered energy and determine the scatterer location in the deep Earth. To improve the resolution of the slowness vector at the medium aperture YKA we present a new application of the F -statistic. The high-resolution slowness and backazimuth measurements indicate scattering from a structure up to 80 km tall at the CMB with lateral dimensions of at least 1200 km by 300 km, at the edge of the African Large Low Shear Velocity Province. The forward scattering nature of the PKP probe indicates that this is velocity-type scattering resulting primarily from changes in elastic parameters. The PKP scattering data are in agreement with dynamically supported dense material related to the Large Low Shear Velocity Province.

Description: In this paper, some methods for scoring the performances of an earthquake forecasting probability model are applied retrospectively for different goals. The time-dependent occurrence probabilities of a renewal process are tested against earthquakes of M w ≥ 5.3 recorded in Italy according to decades of the past century. An aim was to check the capability of the model to reproduce the data by which the model was calibrated. The scoring procedures used can be distinguished on the basis of the requirement (or absence) of a reference model and of probability thresholds. Overall, a rank-based score, information gain, gambling scores, indices used in binary predictions and their loss functions are considered. The definition of various probability thresholds as percentages of the hazard functions allows proposals of the values associated with the best forecasting performance as alarm level in procedures for seismic risk mitigation. Some improvements are then made to the input data concerning the completeness of the historical catalogue and the consistency of the composite seismogenic sources with the hypotheses of the probability model. Another purpose of this study was thus to obtain hints on what is the most influential factor and on the suitability of adopting the consequent changes of the data sets. This is achieved by repeating the estimation procedure of the occurrence probabilities and the retrospective validation of the forecasts obtained under the new assumptions. According to the rank-based score, the completeness appears to be the most influential factor, while there are no clear indications of the usefulness of the decomposition of some composite sources, although in some cases, it has led to improvements of the forecast.

Description: We propose a numerical algorithm for solving first arrival transmission traveltime tomography problems where the underlying slowness is piecewise continuous. The idea is based upon our previously efficient approach for smooth slowness inversion (Leung & Qian) using the fast sweeping method and the adjoint state method. In this work, we further incorporate the level set method to implicitly represent the discontinuity in the velocity. One main advantage of such implicit representation is that there is no assumption on the number of disjoint components in the inverted structure. The evolution of the level set function will naturally take care of the change in the topology. Like in the previous work, the gradient of the mismatch functional is derived using the adjoint state method. The forward problem and the adjoint equation are efficiently solved by the fast sweeping method. To further improve the computational efficiency, we also propose a local level set method so that most computer power of updating the level set evolution is spent near the discontinuity in the slowness. Numerical results will be given to demonstrate the robustness of the algorithm.

Description: We present a 1-D velocity model of the Earth's crust in Campania–Lucania region obtained by solving the coupled hypocentre–velocity inverse problem for 1312 local earthquakes recorded at a dense regional network. The model is constructed using the VELEST program, which calculates 1-D ‘minimum’ velocity model from body wave traveltimes, together with station corrections, which account for deviations from the simple 1-D structure. The spatial distribution of station corrections correlates with the P -wave velocity variations of a preliminary 3-D crustal velocity model that has been obtained from the tomographic inversion of the same data set of P traveltimes. We found that station corrections reflect not only inhomogeneous near-surface structures, but also larger-scale geological features associated to the transition between carbonate platform outcrops at Southwest and Miocene sedimentary basins at Northeast. We observe a significant trade-off between epicentral locations and station corrections, related to the existence of a thick low-velocity layer to the NE. This effect is taken into account and minimized by re-computing station corrections, fixing the position of a subset of well-determined hypocentres, located in the 3-D tomographic model.

Description: High-rate GPS and seismic sensors are mutually contributing to seismological applications for capturing earthquake-induced coseismic displacements. In this study, we propose an approach for tightly integrating GPS and strong motion data on raw observation level to increase the quality of the derived displacements. The performance of the proposed approach is demonstrated using 5 Hz high-rate GPS and 200 Hz strong motion data collected during the El Mayor–Cucapah earthquake ( M w 7.2, 2010 April 4) in Baja California, Mexico. The new approach not only takes advantages of both GPS and strong motion sensors, but also improves the reliability of the displacement by enhancing GPS integer-cycle phase ambiguity resolution, which is very critical for deriving displacements with highest quality.

Description: The Pegasus Bay aftershock sequence is the most recent aftershock sequence of the 2010 September 3 UTC moment magnitude ( M w ) 7.1 Darfield earthquake in the Canterbury region of New Zealand. The Pegasus Bay aftershock sequence began on 2011 December 23 UTC with three events of M w 5.4–5.9 located in the offshore region of Pegasus Bay, east of Christchurch city. We present a summary of key aspects of the sequence derived using various geophysical methods. Relocations carried out using double-difference tomography show a well-defined NNE–SSW to NE–SW series of aftershocks with most of the activity occurring at depths 〉5 km and an average depth of ~10 km. Regional moment tensor solutions calculated for the Pegasus Bay sequence indicate that the vast majority (45 of 53 events) are reverse-faulting events with an average P -axis azimuth of 125°. Strong-motion data inversion favours a SE-dipping fault plane for the largest event ( M w 5.9) with a slip patch of 18 km  x 15 km and a maximum slip of 0.8 m at 3.5 km depth. Peak ground accelerations ranging up to 0.98 g on the vertical component were recorded during the sequence, and the largest event produced horizontal accelerations of 0.2–0.4 g in the Christchurch central business district. Apparent stress estimates for the two largest events are 1.1 MPa ( M w 5.9) and 0.2 MPa ( M w 5.8), which are compatible with global averages, although lower than other large events in the Canterbury aftershock sequence. Coulomb stress analysis indicates that previous large earthquakes in the Canterbury sequence generate Coulomb stress increases for the two events only at relatively shallow depths (3–5 km). At greater depths, Coulomb stress decreases are predicted at the locations of the two events. The trend of the aftershocks is similar to mapped reverse faults north of Christchurch, and the high number of reverse-faulting mechanisms suggests that similar reverse-faulting structures are present in the offshore region east of Christchurch.

Description: Magma dynamics and time scales during the VEI 5, 2000 bp eruption of El Misti volcano, southern Peru (EM2000BP) are investigated to address cyclic explosive activity at this hazardous volcano. The 1·4 km 3 of pumice falls and flows have abundant mingled pumice of high-K, calc-alkaline rhyolite and andesite composition. Phenocryst zoning and compositions reveal mutual exchange of plagioclase between the two magmas; amphibole in the rhyolite was derived from the andesite. Amphiboles in the andesite are predominantly unrimmed crystals whereas those in the rhyolite mostly exhibit reaction rims. Phase equilibria indicate that the andesite formed at ~900–950°C and 2–3 kbar pressure and was water-saturated with 5·1–6·0 wt % H 2 O, broadly similar to El Misti magmas overall. Amphibole, plagioclase, Ti-magnetite, and two pyroxenes were the crystallizing phases. A separate rhyolite magma existed higher in the crust at a temperature of 816 ± 30°C and ~5% H 2 O in which only plagioclase and Fe–Ti oxides were stable. The lack of cognate amphibole in the rhyolite despite H 2 O saturation requires that it staged above the stability limit of amphibole (〈100 MPa). Exchange reactions in amphibole (dominantly pargasitic) and trace element partitioning in plagioclase indicate that both andesite and rhyolite magmas were broadly constant in temperature and H 2 O content. These constraints suggest that the initially separate rhyolite and deeper andesite magmas interacted by an initial andesite recharge event that resulted in mingling and crystal exchange. A period of 50–60 days is required for amphibole introduced into the rhyolite to develop reaction rims owing to decompression. These rims are dominated by plagioclase, a consequence of the Al-rich nature of the amphibole. The lack of reaction rims on amphibole in the andesite implicates a second, more-forceful and voluminous eruption-triggering recharge event during which andesite rose rapidly from source to surface in ≤5 days at ascent rates of at least 0·023 m s –1 . Further decompression-driven crystallization is recorded in plagioclase rims and microlite growth that may have contributed to a rapid increase in viscosity leading to explosive eruption. This VEI 5 plinian eruption shares characteristics with other explosive events at El Misti on a time scale of 2000–4000 years, suggesting periodic recharge-driven explosive activity.

Description: We report structural evidence of ductile strain localization in mantle pyroxenite from the spinel to plagioclase websterite transition in the Ronda Peridotite (southern Spain). Mapping shows that, in this domain, small-scale shear zones occurring at the base of the lithospheric section are systematically located within thin pyroxenite layers, suggesting that the pyroxenite was locally weaker than the host peridotite. Strain localization is associated with a sudden decrease of grain size and increasing volume fractions of plagioclase and amphibole as a result of a spinel to plagioclase phase transformation reaction during decompression. This reaction also fostered hydrogen extraction (‘dehydroxylation’) from clinopyroxene producing effective fluid saturation that catalyzed the synkinematic net-transfer reaction. This reaction produced fine-grained olivine and plagioclase, allowing the onset of grain-size sensitive creep and further strain localization in these pyroxenite bands. The strain localization in the pyroxenites is thus explained by their more fertile composition, which allowed earlier onset of the phase transition reactions. Geothermobarometry undertaken on compositionally zoned constituent minerals suggests that this positive feedback between reactions and deformation is associated with cooling from at least 1000°C to 700°C and decompression from 1·0 to 0·5 GPa.

Description: We present a method for estimating the errors on local and global wavelet power spectra using the jackknife approach to error estimation, and compare results with jackknifed multitaper (MT) spectrum estimates. We test the methods on both synthetic and real data, the latter being free air gravity over the Congo Basin. To satisfy the independence requirement of the jackknife we investigate the orthogonality properties of the 2-D Morlet wavelet. Although Morlet wavelets are non-orthogonal, we show that careful selection of parameters can yield approximate orthogonality in space and azimuth. We also find that, when computed via the Fourier transform, the continuous wavelet transform (CWT) contains errors at very long wavelengths due to the discretization of large-scale wavelets in the Fourier domain. We hence recommend the use of convolution in the space-domain at these scales, even though this is computationally more expensive. Finally, in providing an investigation into the bandwidth resolution of CWT and MT spectra and errors at long wavelengths, we show that the Morlet wavelet is superior in this regard to Slepian tapers. Wavelets with higher bandwidth resolution deliver smaller spectral error estimates, in contrast to the MT method, where tapers with higher bandwidth resolution deliver larger errors. This results in the fan-WT having better spectral estimation properties at long wavelengths than Slepian MTs.

Description: Core flows inverted from time-dependent geomagnetic field models image the geodynamo at the top of its generation region, the Earth’s outer core. Physical assumptions incorporated in these inversions affect the resulting flows. Based on rapid rotation dominance, two assumptions similar in form yet different in essence have been proposed: tangential geostrophy (TG, LeMouël 1984 ) and columnar flow (CF, Amit & Olson 2004 ). We recall that CF is theoretically consistent with the quasi-geostrophy (QG) theory for an incompressible fluid with spherical solid boundaries whereas TG is not. As such, we highlight the importance of applying the CF assumption when inverting geomagnetic data for interior core (columnar) flows that can be used in kinematic dynamo and thermal convection models in the Boussinesq approximation. Next we evaluate the non-uniqueness associated with CF flows. The areas of ambiguous patches at the core surface where invisible TG or CF flows reside are roughly comparable. The spatial distribution of ambiguous patches for both TG and CF is quite asymmetric about the equator, so assuming equatorial symmetry is expected to reduce the non-uniqueness significantly. In fact, for assumed equatorial symmetry, the only possible non-unique flows will be those along hypothetical -contours in the opposite hemispheres that their equatorial plane projections are parallel. TG flows exhibit a strong Atlantic/Pacific hemispheric dichotomy and a well-defined eccentric gyre whereas in CF flows the dichotomy between these two hemispheres is weaker and the gyre is less clear suggesting that the eccentric gyre might not conserve mass. Both TG and CF upwelling/downwelling patterns are strongly localized in the equatorial region. In addition, in both cases upwelling/downwelling is correlated with equatorward/poleward flow respectively, as expected for QG convection. CF upwelling is more intense than TG upwelling but the magnitude ratio is smaller than the factor 2 distinguishing the analytical expressions of the two assumptions. This smaller magnitude ratio is due to the fact that presently observed geomagnetic secular variation features are mostly explained by magnetic field advection by toroidal core flow in the frozen-flux approximation. Robust upwelling features below India/Indonesia may be viewed as geomagnetic evidence for whole core convection.

Description: Electric field variations coincident with the passage of seismic waves are commonly observed irrespective of whether the seismic events are natural or artificial. We present 10 examples of electric field variations obtained for artificial seismic waves whose typical frequency is a few times higher than that of natural seismic waves. In several cases, the electric fields showed left- and/or right-handed circular polarization, indicating the motion of ions with positive and/or negative electric charge, respectively, generated by ground motion in the Earth's magnetic field. In three cases, we have estimated transfer functions relating the electric field to the ground velocity. Furthermore, we have performed time-frequency analysis with the continuous wavelet transform and have constructed spectrograms of the electric field and ground velocity. In both results, we have found some peaks at the specific frequencies where the resonance of the motion of ions in groundwater with the Earth's magnetic field is expected, thereby supporting the proposed mechanism in terms of the seismic dynamo effect.

Description: The approximate forward modelling method using Fourier analysis has been used in 2-D applications for several decades. It involves decomposition of the terrain parameters, either the resistivity or the layer thickness, into a Fourier series expansion to simplify the problem to that of a 1-D situation. In this study, the Fourier analysis is applied to 3-D forward modelling for the purposes of shallow DC resistivity imaging with pole–pole array. Our work is to assess advantages and drawbacks of the simplified approach by comparing to exact 3-D solutions, method of moments (MoM) and surface integrals and to the Born approximation applied to MoM. While the Fourier analysis method offers very short calculation times, it shows a significant, albeit systematic, reduction of the anomaly amplitudes; and its ability to delineate anomaly sources is lower than the other methods. Nevertheless, its rapidity makes it an interesting first approach in the modelling of DC resistivity results.

Description: A 3-D density model of Greece was developed by gravity modelling constrained by 2-D seismic profiles. Densities were defined from seismic velocities using the Nafe & Drake and Birch empirical functions for the sediments, crust and upper mantle. Sediments in the North Aegean are 6 km thick, and are deposited in transtensional basins developing by dextral strike slip motion of the North Anatolian Fault. The Cyclades, central Aegean Sea, are free of sediments. South of Crete, in the Libyan Sea, sediments are approximately 11 km thick. At the western Hellenides sediments of up to 8 km thickness have been accumulated in basins formed by crustal bending and southwestwards thrusting of the Hellenic napes. At a deeper crustal level variations of crustal type and thickness cause density variations explaining large part of the observed gravity field. The North Aegean domain is characterized by a 24-km-thick continental crust, including sediments, whereas the western Cyclades, in central Aegean area, have a slightly thickened crust of 26 km. Crustal thicknesses vary between 16 km in the deep Ionian and Cretan Seas to 40 km in the western Hellenides. In western Crete crust is 30–32 km thick, thinning eastwards to only 26 km. The deep Ionian basin, the Mediterranean Ridge, as well as most of the Libyan Sea are underlain by oceanic crust. In western Turkey the crust thickens from 30 km along the coast to 34 km to the interior. A third deeper level of density variations occurs in the upper mantle. Subduction of the oceanic lithosphere below the Aegean continental domain destabilizes the thermal field, uplifting the isotherms by convection and conduction below the Aegean Sea. Consequently, volume expansion of the upper mantle and lithological changes reduce its density and depress the gravity intensity. This low density–velocity upper mantle extends from the Sporades islands in the North Aegean to the Cretan Sea, occupying the space between the cold subducted Ionian oceanic lithosphere and the Aegean continental Moho. Upper mantle densities vary from 3.24 g cm –3 in the Aegean area to 3.29 g cm –3 below western Greece and the Ionian and Libyan Seas.

Description: The magnetic susceptibility measured in alternating field can be resolved into a component that is in-phase with applied field and a component that is out-of-phase. While the former component is widely used for solving various geophysical, geological and environmental problems, the latter component is paid only minor attention. The theoretical relationship between the frequency-dependent in-phase susceptibility and the out-of-phase susceptibility is described by the /2 law valid for materials in which the latter is due to presence of magnetic particles on transition between superparamagnetic and stable single-domain states possessing sufficiently wide distribution of relaxation times. To advantageously use the out-of-phase susceptibility, which is measured simultaneously with the in-phase susceptibility during one measuring process, in magnetic granulometry, new parameters, X ON and X OD , are proposed approximately converting the out-of-phase susceptibility into the X FN and X FD parameters of the frequency-dependent susceptibility. The validity of the new parameters was tested through mathematical modelling and through investigating samples of various sediments. The correlations found seem to be acceptable from the practical point of view. In addition, simple test is proposed for checking that the out-of-phase susceptibility is solely due to the viscous phenomena and not due to electrical eddy currents or weak field hysteresis. As the out-of-phase susceptibility is measured automatically along the in-phase-susceptibility with some instruments and can be directly interpreted in magnetic granulometry terms, it is to recommend to be routinely investigated in solving various problems of environmental magnetism.

Description: The analysis of marine magnetotelluric data is often complicated by disturbing signals that are caused by small-scale periodic movements of the instrument. The motion-induced noise leads to a bias and/or severe scattering in the derived magnetotelluric transfer functions. Both the motion itself and its effects on the magnetic and telluric time-series are investigated in this study using an 80 d magnetotelluric data set that includes dynamic tilt records measured in the Pacific Ocean off Costa Rica. We apply a standard motion removal technique as well as a newly developed method to correct for motion-induced noise. The resulting magnetotelluric transfer functions are of significantly better quality than the uncorrected ones. Furthermore, the study of the properties of motion noise leads to conclusions about the optimal processing approach even in case of data sets where an explicit correction for that noise is not possible.

Description: Numerous monitoring applications make use of seismic coda waves to evaluate velocity changes in the Earth. This raises the question of the spatial sensitivity of coda wave-based measurements. Here, we investigate the depth sensitivity of coda waves to local velocity perturbations using 2-D numerical wavefield simulations. We calculate the impulse response at the surface before and after a slight perturbation of the velocity within a thin layer at depth is introduced. We perform a parametric analysis of the observed apparent relative velocity changes, obs , versus the depth of the thin perturbed layer. Through the analysis of the decay of obs , we can discriminate two different regimes: one for a shallow perturbation and the other for a deep perturbation. We interpret the first regime as the footprint of the 1-D depth sensitivity of the fundamental surface wave mode. To interpret the second regime, we need to model the sensitivity of the multiply scattered body waves in the bulk. We show that the depth sensitivity of coda waves can be modelled as a combination of bulk wave sensitivity and surface wave sensitivity. The transition between these two regimes is governed by mode conversions due to scattering. We indicate the importance of surface waves for the sensitivity of coda waves at shallow depths and at early times in the coda. At later times, bulk waves clearly dominate the depth sensitivity and offer the possibility of monitoring changes at depths below the sensitivity of the surface waves. Based on the transition between the two regimes, we can discriminate a change that occurs at the surface from a change that occurs at depth. This is illustrated for shallow depth perturbations through an example from lunar data.

Description: Local and regional seismicity jointly recorded by two dense small aperture arrays, one installed at surface and one at 1.3 km depth, constitutes an interesting data set useful for coda observations. Applying array techniques to earthquakes recorded at the two arrays we measure slowness, backazimuth and correlation coefficient of the coherent coda wave signals in five frequency bands in the range 1–10 Hz. Slowness distributions show marked differences between surface and underground, with slow signals at surface (slowness greater than 1.0 s km –1 ) that are not observed underground. We interpret these coherent signals as surface waves produced by the interaction of body waves with the free surface characterized by rough topography. The backazimuth values measured in the frequency bands centred at 1.5 and 3 Hz are almost uniformly distributed between 0 and 360°, while those measured at higher frequencies show different distributions between surface and underground. On the contrary, the earthquake envelopes show very similar coda shapes between surface and underground recordings, with an almost constant coda-amplitude ratio (between 4 and 8) in a wide frequency range.

Description: We quantify the seismicity on the island of Taiwan using the frequency–magnitude statistics of earthquakes since 1900. A break in Gutenberg–Richter scaling for large earthquakes in global seismicity has been observed, this break is also observed in our Taiwan study. The seismic data from the Central Weather Bureau Seismic Network are in good agreement with the Gutenberg–Richter relation taking b   1 when M  〈 7. For large earthquakes, M ≥ 7, the seismic data fit Gutenberg–Richter scaling with b   1.5. If the Gutenberg–Richter scaling for M  〈 7 earthquakes is extrapolated to larger earthquakes, we would expect a M  〉 8 earthquake in the study region about every 25 yr. However, our analysis shows a lower frequency of occurrence of large earthquakes so that the expected frequency of M  〉 8 earthquakes is about 200 yr. The level of seismicity for smaller earthquakes on Taiwan is about 12 times greater than in Southern California and the possibility of a M   9 earthquake north or south of Taiwan cannot be ruled out. In light of the Fukushima, Japan nuclear disaster, we also discuss the implications of our study for the three operating nuclear power plants on the coast of Taiwan.

Description: We study the problem of determining an unknown microseismic event location relative to previously located events using a single monitoring array in a monitoring well. We show that using the available information about the previously located events for locating new events is advantageous compared to locating each event independently. By analysing confidence regions, we compare the performance of two previously proposed location methods, double-difference and interferometry, for varying signal-to-noise ratio and uncertainty in the velocity model. We show that one method may have an advantage over another depending on the experiment geometry, assumptions about uncertainty in velocity and recorded signal, etc. We propose a unified approach to relative event location that includes double-difference and interferometry as special cases, and is applicable to velocity models and well geometries of arbitrary complexity, producing location estimators that are superior to those of double-difference and interferometry.

Description: The rapid expansion of broad-band seismic networks over the last decade has paved the way for a new generation of global tomographic models. Significantly improved resolution of global upper-mantle and crustal structure can now be achieved, provided that structural information is extracted effectively from both surface and body waves and that the effects of errors in the data are controlled and minimized. Here, we present a new global, vertically polarized shear speed model that yields considerable improvements in resolution, compared to previous ones, for a variety of features in the upper mantle and crust. The model, SL2013sv, is constrained by an unprecedentedly large set of waveform fits (~3/4 of a million broad-band seismograms), computed in seismogram-dependent frequency bands, up to a maximum period range of 11–450 s. Automated multimode inversion of surface and S -wave forms was used to extract a set of linear equations with uncorrelated uncertainties from each seismogram. The equations described perturbations in elastic structure within approximate sensitivity volumes between sources and receivers. Going beyond ray theory, we calculated the phase of every mode at every frequency and its derivative with respect to S - and P -velocity perturbations by integration over a sensitivity area in a 3-D reference model; the (normally small) perturbations of the 3-D model required to fit the waveforms were then linearized using these accurate derivatives. The equations yielded by the waveform inversion of all the seismograms were simultaneously inverted for a 3-D model of shear and compressional speeds and azimuthal anisotropy within the crust and upper mantle. Elaborate outlier analysis was used to control the propagation of errors in the data (source parameters, timing at the stations, etc.). The selection of only the most mutually consistent equations exploited the data redundancy provided by our data set and strongly reduced the effect of the errors, increasing the resolution of the imaging. Our new shear speed model is parametrized on a triangular grid with a ~280 km spacing. In well-sampled continental domains, lateral resolution approaches or exceeds that of regional-scale studies. The close match of known surface expressions of deep structure with the distribution of anomalies in the model provides a useful benchmark. In oceanic regions, spreading ridges are very well resolved, with narrow anomalies in the shallow mantle closely confined near the ridge axis, and those deeper, down to 100–120 km, showing variability in their width and location with respect to the ridge. Major subduction zones worldwide are well captured, extending from shallow depths down to the transition zone. The large size of our waveform fit data set also provides a strong statistical foundation to re-examine the validity field of the JWKB approximation and surface wave ray theory. Our analysis shows that the approximations are likely to be valid within certain time–frequency portions of most seismograms with high signal-to-noise ratios, and these portions can be identified using a set of consistent criteria that we apply in the course of waveform fitting.

Description: For the present 2-D lithospheric density modelling, we selected three geotransects of more than 1000 km in length each crossing the southern Indian shield, south of 16°N, in N–S and E–W directions. The model is based on the assumption of local isostatic equilibrium and is constrained by the topography, gravity and geoid anomalies, by geothermal data, and where available by seismic data. Our integrated modelling approach reveals a crustal configuration with the Moho depth varying from ~40 km beneath the Dharwar Craton, and ~39 km beneath the Southern Granulite Terrane to about 15–20 km beneath the adjoining oceans. The lithospheric thickness varies significantly along the three profiles from ~70–100 km under the adjoining oceans to ~130–135 km under the southern block of Southern Granulite Terrane including Sri Lanka and increasing gradually to ~165–180 km beneath the northern block of Southern Granulite Terrane and the Dharwar Craton. This step-like lithosphere–asthenosphere boundary (LAB) structure indicates a normal lithospheric thickness beneath the adjoining oceans, the northern block of Southern Granulite Terrane and the Dharwar Craton. The thin lithosphere below the southern block of Southern Granulite Terrane including Sri Lanka is, however, atypical considering its age. Our results suggest that the southern Indian shield as a whole cannot be supported isostatically only by thickened crust; a thin and hot lithosphere beneath the southern block of Southern Granulite Terrane including Sri Lanka is required to explain the high topography, gravity, geoid and crustal temperatures. The widespread thermal perturbation during Pan-African (550 Ma) metamorphism and the breakup of Gondwana during late Cretaceous are proposed as twin cause mechanism for the stretching and/or convective removal of the lower part of lithospheric mantle and its replacement by hotter and lighter asthenosphere in the southern block of Southern Granulite Terrane including Sri Lanka. Unusually thinned LAB beneath the region near Bangalore apparently indicates the preserved tectonocompositional effect of late Proterozoic rifted margin of Dharwar Craton.

Description: We derive a generalized theory for gravitationally self-consistent, static sea level variations on earth models of arbitrary complexity that takes into account the redistribution of sediments. The theory is an extension of previous work that incorporated, into the governing equations, shoreline migration due to local sea level variations and changes in the geometry of grounded, marine-based ice. In addition, we use viscoelastic Love number theory to present a version of the new theory valid for spherically symmetric earth models. The Love number theory accounts for the gravitational, deformational and rotational effects of the sediment redistribution. As a first, illustrative application of the new theory, we compute the perturbation in sea level driven by an idealized pulse of sediment transport into the Gulf of Mexico. We demonstrate that incorporating a gravitationally self-consistent water load in this case significantly improves the accuracy of sea level predictions relative to previous simplified treatments of the sediment redistribution.

Description: Models for glacial isostatic adjustment (GIA) can provide constraints on rheology of the mantle if past ice thickness variations are assumed to be known. The Pleistocene ice loading histories that are used to obtain such constraints are based on an a priori 1-D mantle viscosity profile that assumes a single deformation mechanism for mantle rocks. Such a simplified viscosity profile makes it hard to compare the inferred mantle rheology to inferences from seismology and laboratory experiments. It is unknown what constraints GIA observations can provide on more realistic mantle rheology with an ice history that is not based on an a priori mantle viscosity profile. This paper investigates a model for GIA with a new ice history for Fennoscandia that is constrained by palaeoclimate proxies and glacial sediments. Diffusion and dislocation creep flow law data are taken from a compilation of laboratory measurements on olivine. Upper-mantle temperature data sets down to 400 km depth are derived from surface heatflow measurements, a petrochemical model for Fennoscandia and seismic velocity anomalies. Creep parameters below 400 km are taken from an earlier study and are only varying with depth. The olivine grain size and water content (a wet state, or a dry state) are used as free parameters. The solid Earth response is computed with a global spherical 3-D finite-element model for an incompressible, self-gravitating Earth. We compare predictions to sea level data and GPS uplift rates in Fennoscandia. The objective is to see if the mantle rheology and the ice model is consistent with GIA observations. We also test if the inclusion of dislocation creep gives any improvements over predictions with diffusion creep only, and whether the laterally varying temperatures result in an improved fit compared to a widely used 1-D viscosity profile (VM2). We find that sea level data can be explained with our ice model and with information on mantle rheology from laboratory experiments, heatflow and seismology and a pure olivine rheology above 400 km. Moreover, laterally heterogeneous models provide a significantly better fit to relative sea level data than the VM2 viscosity, for our ice model as well as for the ICE-5G model that is based on the VM2 profile. The new ice model gives different constraints on mantle rheology than the ICE-5G model, indicating a possible bias towards mantle viscosity in the latter or shortcomings in our ice model. Present-day uplift rates for a dry rheology are close to GPS observed uplift rate for certain combinations of grain size and temperature fields. Sea level data show a preference for a wet olivine rheology, but in that case uplift rates are too low for all grain sizes and temperature fields. The difficulty to fit sea level data and uplift rate data simultaneously can not be resolved by varying creep parameters below 400 km. Uncertainties in the flow law and the neglect of other materials in the upper mantle, as well as the neglect of flow in the crust could affect our conclusions.

Description: Thermal and dynamical evolution of planets is controlled by thermal convection in planetary mantles. Mantle compressibility, which measures volume change due to pressure change and its associated energetic effects, can have important effects on planetary mantle convection. However, key issues including marginal stability analysis, thermal boundary properties and heat transfer in compressible mantle convection are not well understood. This paper studies the influence of mantle compressibility on thermal convection in an isoviscous and compressible fluid with infinite Prandtl number, using both marginal stability analysis and numerical modelling. For the marginal stability analysis, a new formulation of the propagator matrix method is implemented to compute the critical Rayleigh number Ra c and the corresponding eigenfunctions for compressible convection at different wavelengths (i.e. wavenumber k x ) and dissipation number Di which measures the compressibility. Ra c from the analysis is in a good agreement with that determined from the numerical experiment using the eigenfunctions as initial perturbations. Our study suggests that if Ra is defined by the surface density, the minimum Ra c may occur at non-zero Di . Finite element models are computed for compressible mantle convection at different Ra and Di . Heat flux and thermal boundary layer (TBL) properties including boundary layer thickness and temperature difference are quantified and analysed from the numerical results. Scaling laws of temperature differences across TBLs and of the heat flux are derived analytically for compressible mantle convection and are verified by the numerical results. This study shows that while TBL thicknesses and the heat flux are still scaled with Ra to the –1/3 and 1/3 power, respectively, as those for incompressible convection, they also strongly depend on Di . In particular, compressibility breaks the symmetry for the top and bottom TBLs, and the ratios of thickness and temperature difference for the top TBL to those for the bottom TBL are exp ( Di /2). These results have important implications for compressible mantle convection.

Description: We present a detailed study of tsunami-induced tilt at in-land sites, to test the interest and feasibility of such analysis for tsunami detection and modelling. We studied tiltmeter and broadband seismometer records of northern Chile, detecting a clear signature of the tsunamis generated by the 2007 Tocopilla ( M  = 7.6) and the 2010 Maule ( M  = 8.8) earthquakes. We find that these records are dominated by the tilt due to the elastic loading of the oceanic floor, with a small effect of the horizontal gravitational attraction. We modelled the Maule tsunami using the seismic source model proposed by Delouis et al. and a bathymetric map, correctly fitting three tide gauge records of the area (Antofagasta, Iquique and Arica). At all the closest stations (7 STS2, 2 long-base tiltmeters), we correctly modelled the first few hours of the tilt signal for the Maule tsunami. The only phase mismatch is for the site that is closer to the ocean. We find a tilt response of 0.005–0.01 μm at 7 km away from the coastline in response to a sea level amplitude change of 10 cm. For the Maule earthquake, we observe a clear tilt signal starting 20 min before the arrival time of the tsunami at the nearest point on the coastline. This capability of tilt or seismic sensors to detect distant tsunamis before they arrive has been successfully tested with a scenario megathrust in the southern Peru-northern Chile seismic gap. However, for large events near the stations, this analysis may no longer be feasible, due to the large amplitude of the long-period seismic signals expected to obscure the loading signal. Inland tilt measurements of tsunamis smooth out short, often unmodelled wavelengths of the sea level perturbation, thus providing robust, large-scale images of the tsunami. Furthermore, tilt measurements are not expected to saturate even for the largest run-ups, nor to suffer from near-coast tsunami damages. Tiltmeters and broadband seismometers are thus valuable instruments for monitoring tsunamis in complement with tide gauge arrays.

Description: Tidal gravity observations can be used to validate theoretical estimation for inelastic response of the Earth and its latitude dependence using gravimetric factors. We used superconducting gravimeter (SG) data at five sites (METSÄHOVI, STRASBOURG, SUTHERLAND, CANBERRA and SYOWA) to validate theoretical estimation based on an existing method. For the correction of ocean loading effect, we tested eight global ocean tide models, four OLD (CSR4.0, GOT99.2b, NAO.99b and FES2004) and four NEW (TPXO7-atlas, EOT11a, DTU10 and HAMTIDE11a) were tested. We selected an optimal ocean tide model based on the criterion of the smallest misfit for the three main waves (O1, K1 and M2) at each station, as determined by the residual between the observed loading effect and the modelled ocean loading effect. Large discrepancies among the results of ocean tide models remained at METSÄHOVI and SYOWA; however, the correction of the ocean loading effect using NEW ocean tide models exhibited consistency with body tide theoretical values of inelasticity. Regarding the validation of elasticity/inelasticity, the properties at all stations approached the inelastic theoretical value for K1. It was difficult to clarify the latitude dependence of tidal response, because of still inaccurate ocean loading correction at SYOWA and SUTHERLAND. In summary, revision of the calibration factor at METSÄHOVI reduced the real (in-phase) component of the final residual; however, a more accurate calibration factor is still needed. Data from STRASBOURG showed consistency with theoretical values for inelasticity including OLD and NEW ocean tide models. Results from SUTHERLAND gave a larger gravimetric factor, even using the NEW optimal ocean tide model, and the data at CANBERRA supported OLD ocean tide model. The NEW ocean tide models TPXO7-atlas and DTU10 reduced the residual in both the in-phase and out-phase components for O1, K1 and M2 as compared with FES2004, but relatively large residuals still remained in both components for K1 at SUTHERLAND, in the in-phase component for M2 at CANBERRA and in the out-phase component for M2 at SYOWA. These residuals mean that there is a room for improvement in the estimation of ocean loading effect and/or the determination of calibration factors at these sites.

Description: Based on studies of continuous waveform data recorded on broad-band seismograph stations in Africa, Europe and North America, we report evidences for two temporally persistent and spatially localized monochromatic vibrating sources (around 0.036 and 0.038 Hz, respectively) in the Gulf of Guinea, instead of just one source (0.038 Hz or 26 s) found 50 yr ago. The location of the 0.036 Hz source is close to the Sao Tome Volcano, therefore it may be related to volcano processes. However, the 0.038 Hz source cannot be explained with known mechanisms, such as tectonic or oceanic processes. The most likely mechanism is volcano processes, but there is no reported active volcano in source region. Such repetitive vibration sources may provide valuable tools for detecting temporal variation of crustal structure of the Earth.

Description: We propose a direct estimation of dampings in surface layers based on the normalized energy density (NED). The ratio of the NED, defined as the NED for the uppermost layer divided by the NED for the basement, correlates well with the total damping, $t_S^*$ . We apply the relation between the NED ratio and the total damping to estimate the total damping at an actual site, Katagihara (KTG) site in Japan. The total damping at the KTG site is directly estimated as $t_S^*=0.038$ . This value agrees well with the estimated values determined from a conventional method, incorporating the non-linear inversion scheme.

Description: The scattering of SH waves induced by a partially filled semi-elliptic alluvial valley is explored via the region-point-matching technique (RPMT). Appropriate radial and angular Mathieu functions are well utilized to express the antiplane displacement fields. With the aid of the coordinate-transformed relation, which is intended as a substitute for the addition theorem (involving multiple summations of Mathieu functions), the application of the RPMT facilitates the rapid construction of simultaneous equations. Enforcement of zero-stress conditions on the curved valley surface and continuity conditions on the soil-bedrock interface leads to the determination of unknown coefficients. Comparisons with published data for several limiting cases (e.g. those pertaining to the truncated semi-circular canyon, the partially filled semi-circular alluvial valley, the totally empty semi-elliptic canyon and the fully filled semi-elliptic alluvial valley) show good agreement. Effects of pertinent parameters on steady-state surface motions are demonstrated. Transient changes in surface displacement amplitude are also included.

Description: We have used the iterative spectral fitting method to measure both the elastic and anelastic splitting functions of 20 inner core sensitive normal modes. These modes show significant improvement in spectral fit when anelastic splitting function coefficients d st are introduced in addition to the elastic splitting function coefficients c st . We employ two separate anelastic treatments: (i) fully anelastic measurement, in which a complete set of anelastic splitting function coefficients is measured in addition to the elastic coefficients, and (ii) zonal anelastic measurement, in which anelasticity is only allowed in zonal splitting function coefficients. Together, these two approaches confirm that normal modes sensitive to the Earth’s inner core resolve zonally dominant elastic and anelastic structures. The zonal dominance of anelasticity suggests that the inner core exhibit cylindrical attenuation anisotropy in addition to cylindrical velocity anisotropy. In particular, the zonally dominant anelasticity correlates with zonal elastic structure, that is, directions of higher velocity in the inner core also appear more attenuating.

Description: In this study, microseism recordings from a near coast seismic station and concurrent significant sea wave heights ( $H_{\frac{1}{3}}$ ) are analysed to calibrate an empirical relation for predicting sea wave height in the Ligurian Sea. The study stems from the investigation of the damaging sea storms occurred in the Ligurian Sea between 2008 October and November. Analysing data collected in this time frame allows identification of two types of microseism signal, one associated to the local sea wave motion and one attributable to a remote source area. The former is dominated by frequencies greater than 0.2 Hz and the latter by frequencies between 0.07 and 0.14 Hz. Moreover, comparison of microseism spectrogram and significant sea wave heights reveals a strong correlation in that the spectral energy content of microseism results proportional to the sea wave height observed in the same time window. Hence, an extended data set including also observations from January to December 2011 is used to calibrate an empirical predictive relation for sea wave height whose functional form is a modified version of the classical definition of $H_{\frac{1}{3}}$ . By means of a Markov chain Monte Carlo algorithm we set up a procedure to investigate the inverse problem and to find a set of parameter values for predicting sea wave heights from microseism.

Description: We invert for the state of stress in the southern California crust using a catalogue of high quality earthquake focal mechanisms (1981–2010). The stress field is best resolved where seismicity rates are high and sufficient data are available to constrain the stress field across most of the region. From the stress field, we determine the maximum horizontal compressive stress ( S Hmax ) orientations and the style of faulting across southern California. The trend of S Hmax exhibits significant regional and local spatial heterogeneities. The regional trend of S Hmax varies from north along the San Andreas system to NNE to the east in the Eastern California Shear Zone as well as to the west, within the Continental Borderland and the Western Transverse Ranges. The transition zones from one state of stress to the other occur over a distance of only a few kilometres, following a trend from Yucca Valley to Imperial Valley to the east, and the western edge of the Peninsular Ranges to the west. The local scale heterogeneities in the S Hmax trend include NNW trends along the San Andreas Fault near Cajon Pass, Tejon Pass and the Cucapah Range, as well as NNE trends near the northern San Jacinto Fault and the Wheeler Ridge area. The style of faulting exhibits similar complexity, ranging from predominantly normal faulting in the high Sierra Nevada, to strike-slip faulting along the San Andreas system, to three consecutive bands of thrust faulting in the Wheeler Ridge area and the Western Transverse Ranges. The local variations in the style of faulting include normal faulting at the north end of the San Jacinto Fault and scattered areas of thrust faulting. The regional variations in the S Hmax trends are very similar to the pattern of the GPS-measured maximum shortening axes of the surface strain rate tensor field although the strain field tends to be smoother and appears to capture some of the upper-mantle deformation field. The mean trend of S Hmax departs about approximately 14° to the east from the trend of the maximum shortening directions derived from anisotropy in the upper mantle.

Description: We have discovered evidence of a previously unrecognized, large-scale rotation of the Ontong Java Plateau (OJP) recorded in its basement palaeolatitudes. When palaeolatitude differences computed among Ocean Drilling Program Sites 807 and 1183–1187 are plotted versus their present-day site latitude differences, a systematic 2:1 slope bias is evident. While it is possible to resolve this bias by introducing ad hoc tilt corrections at all six sites, drilling records indicate relatively undisturbed conditions at Sites 1183 and 1185–1187. Of the possible causes of the bias, only whole plateau rotation resolves it while honouring the majority of published palaeolatitudes. This implies that only Sites 807 and 1184 palaeolatitudes, both questioned in the literature, are erroneous. A 9° northward dip previously reported at Site 1184 appears to stem from inclined deposition rather than post-emplacement deformation. We also estimate an 8° southward tilt correction at Site 807 to make the data set self-consistent. Based on the six sites analysed, we find that OJP may have experienced ~40° of clockwise rotation since its formation at ~123 Ma. In contrast, available Pacific absolute plate motion (APM) models predict less than 10° of rotation. If our analysis is correct, it suggests that the plateau moved independently of the Pacific Plate early in its history or that Pacific APM models for the Lower Cretaceous are unreliable. While our corrections to Sites 807 and 1184 combined with ~40° rotation resolve the internal inconsistencies, the mean palaeolatitude value of Ontong Java remains largely unchanged and is still anomalous with respect to the Pacific apparent polar wander path at ~123 Ma.

Description: The timing of development of the magnetic fabric is a major issue in the application of anisotropy of magnetic susceptibility (AMS) as a strain marker. Analysis of AMS in unconcealed synsedimentary structures can be a sound approximation to this task. In this work, three types of early compactional structures (ECS) were studied by means of AMS, since they can help to understand the timing of development of the magnetic fabric. All three types of ECS are found in fine-grained detrital rocks (to avoid other influences such as palaeocurrents), claystones and marls of the Enciso Group within the Cameros Basin (NE Spain): dinosaur footprints, load structures due to differential compaction and dish-and-flame structures associated with fluid migration related to seismites. In addition, to determine possible influences of lithology on the magnetic fabric, different rock types (siltstones and limestones) were also sampled. In general, the influence of ECS results in scattering of the three magnetic axes, higher at the margins of the structure than at its centre. This fact suggests that ECS occurs during the development of the magnetic fabric, disturbing the incipient magnetic fabric stages, and strongly conditions its later evolution during diagenesis. The later homogeneous compaction process due to sedimentary load and physicochemical processes reorient the susceptibility carriers to some extent (i.e. the magnetic fabric is still under development), but not totally, since AMS still records the previous scattering due to ECS imprint. For the Enciso Group deposits, the magnetic fabric begins to develop at the earliest stages after deposition and it stops when diagenetic processes have finished.

Description: Fundamental to complex analysis is the Cauchy integral theorem, and the derivation of Cauchy-type integrals. For over 40 yr, Cauchy-type integrals have been used to describe analytical continuation, establish the location of singular points, and study non-single-valued solutions of inverse problems in 2-D potential field theory. In this paper, we revive this interesting and fundamental area of potential field theory to introduce Cauchy-type integrals for 3-D potential fields. In particular, we show how one can evaluate the gravity and gravity gradiometry responses of 3-D bodies as surface integrals over arbitrary volumes that may contain spatially variable densities. This method of 3-D spatial-domain potential field modelling has never been realized before, and we show how it is particularly suited to the terrain correction of airborne gravity and gravity gradiometry data. The surface integrals are evaluated numerically on a topographically conforming grid with a resolution equal to the digital elevation model. Thus, our method directly avoids issues related to prismatic discretization of the digital elevation model and their associated volume integration which may result in inappropriate discretization of the earth model, particularly for regions of rugged topography. We demonstrate our method with a model study for airborne gravity gradiometry data simulated for a next-generation 1 Eö/ $\sqrt{\text{Hz}}$ system over the Kauring test site in Western Australia.

Description: Significant progress has been made with regard to the quantitative integration of geophysical and hydrological data at the local scale for the purpose of improving predictions of groundwater flow and solute transport. However, extending corresponding approaches to the regional scale still represents one of the major challenges in the domain of hydrogeophysics. To address this problem, we have developed a regional-scale data integration methodology based on a two-step Bayesian sequential simulation approach. Our objective is to generate high-resolution stochastic realizations of the regional-scale hydraulic conductivity field in the common case where there exist spatially exhaustive but poorly resolved measurements of a related geophysical parameter, as well as highly resolved but spatially sparse collocated measurements of this geophysical parameter and the hydraulic conductivity. To integrate this multi-scale, multi-parameter database, we first link the low- and high-resolution geophysical data via a stochastic downscaling procedure. This is followed by relating the downscaled geophysical data to the high-resolution hydraulic conductivity distribution. After outlining the general methodology of the approach, we demonstrate its application to a realistic synthetic example where we consider as data high-resolution measurements of the hydraulic and electrical conductivities at a small number of borehole locations, as well as spatially exhaustive, low-resolution estimates of the electrical conductivity obtained from surface-based electrical resistivity tomography. The different stochastic realizations of the hydraulic conductivity field obtained using our procedure are validated by comparing their solute transport behaviour with that of the underlying "true" hydraulic conductivity field. We find that, even in the presence of strong subsurface heterogeneity, our proposed procedure allows for the generation of faithful representations of the regional-scale hydraulic conductivity structure and reliable predictions of solute transport over long, regional-scale distances.

Description: The Ethiopia/Afar hotspot has been frequently explained as an upper mantle continuation of the African superplume, with anomalous material in the lower mantle under southern Africa, rising through the transition zone beneath eastern Africa. However, the significantly larger amplitude low velocity anomaly in the upper mantle beneath Ethiopia/Afar, compared to the anomalies beneath neighboring regions, has led to questions about whether or not along-strike differences in the seismic structure beneath eastern Africa and western Arabia are consistent with the superplume interpretation. Here we present a new P -wave model of the hotspot's deep structure and use it to evaluate the superplume model. At shallow (〈 ~400 km) depths, the slowest velocities are centered beneath the Main Ethiopian Rift, and we attribute these low velocities to decompression melting beneath young, thin lithosphere. At deeper depths, the low velocity structure trends to the northeast, and the locus of the low velocity anomaly is found beneath Afar. The northeast-trending structure with depth is best modeled by northeastward flow of warm superplume material beneath eastern Africa. The combined effects of shallow decompression melting and northeastward flow of superplume material explain why upper mantle velocities beneath Ethiopia/Afar are significantly slower than those beneath neighboring East Africa and western Arabia. The superplume interpretation can thus explain the deep seismic structure of the hotspot if the effects of both decompression melting and mantle flow are considered.

Description: The low spatial resolution of GRACE causes leakage, where signals in one location spread out into nearby regions. Because of this leakage, using simple techniques such as basin averages may result in an incorrect estimate of the true mass change in a region. A fairly simple least squares inversion technique can be used to more specifically localize mass changes into a pre-determined set of basins of uniform internal mass distribution. However, the accuracy of these higher resolution basin mass amplitudes has not been determined, nor is it known how the distribution of the chosen basins affects the results. We use a simple ‘truth’ model over Greenland as an example case, to estimate the uncertainties of this inversion method and expose those design parameters which may result in an incorrect high-resolution mass distribution. We determine that an appropriate level of smoothing (300–400 km) and process noise (0.30 cm 2 of water) gets the best results. The trends of the Greenland internal basins and Iceland can be reasonably estimated with this method, with average systematic errors of 3.5 cm yr –1 per basin. The largest mass losses found from GRACE RL04 occur in the coastal northwest (–19.9 and –33.0 cm yr –1 ) and southeast (–24.2 and –27.9 cm yr –1 ), with small mass gains (+1.4 to +7.7 cm yr –1 ) found across the northern interior. Acceleration of mass change is measurable at the 95 per cent confidence level in four northwestern basins, but not elsewhere in Greenland. Due to an insufficiently detailed distribution of basins across internal Canada, the trend estimates of Baffin and Ellesmere Islands are expected to be incorrect due to systematic errors caused by the inversion technique.

Description: Pore space geometry of granitic rocks and its evolution with depth are key factors in large-scale seismics or in projects of enhanced geothermal systems or of deep hazardous waste repositories. In this study, we studied macroscopically anisotropic schlieren-bearing granite by experimental P -wave velocity ( V P ) measurements on spherical sample in 132 directions at seven different confining pressures in the range 0.1–400 MPa. In order to discriminate the phenomena affecting the rock elastic properties we analysed the orientation of microcracks and of grain boundaries and we measured the anisotropy of magnetic susceptibility of the rock. Three sets of microcracks were defined, with two of them linked to the massif exfoliation process and one to cooling contraction of the massif. During pressurization the measured mean V P and V P anisotropy degree at ambient pressure and at highest confinement (400 MPa) yielded 3.3 km s –1 and 24 per cent, and 6.2 km s –1 and 3 per cent, respectively. The associated V P anisotropy pattern was transversely isotropic and governed by the schlieren, with a minimum V P direction perpendicular to them and a girdle of high V P directions parallel to them. The highest change in V P was observed between 0.1 and 10 MPa, suggesting a significant closure of porosity below depths of 500 m. Change of the V P anisotropy pattern to orthorhombic together with increase of mean V P and V P anisotropy degree during depressurization was attributed to inelastic response of one of the sets of microcracks to the loading-unloading cycle.

Description: An elastic wavefield propagating in an inhomogeneous elastic medium is only sensitive in an effective way to inhomogeneities much smaller than its minimum wavelength. The corresponding effective medium, or homogenized medium, can be computed thanks to the non-periodic homogenization technique. In the seismic imaging context, limiting ourselves to layered media, we numerically show that a tomographic elastic model which results of the inversion of limited frequency band seismic data is an homogenized model. Moreover, we show that this homogenized model is the same as the model that can be computed with the non-periodic homogenization technique. We first introduce the notion of residual homogenization, which is computing the effective properties of the difference between a reference model and a ‘real’ model. This is necessary because most imaging technique parametrizations use a reference model that often contains small scales, such as elastic discontinuities. We then use a full-waveform inversion method to numerically show that the result of the inversion is indeed the homogenized residual model. The full-waveform inversion method used here has been specifically developed for that purpose. It is based on the iterative Gauss–Newton least-square non-linear optimization technique, using full normal mode coupling to compute the partial Hessian and gradient. The parametrization has been designed according to the residual homogenized parameters allowing to build a real multiscale inversion with progressive frequency band enrichment along the Gauss–Newton iterations.

Description: As a marine hazard, submarine slope failures have the potential to directly destroy offshore infrastructure, and, if a tsunami is generated, it also endangers the life of those who live and work at the coastline. The hazard and risk from tsunamis generated by submarine mass failure is difficult to quantify and evaluate due to the problems to constrain the characteristics of the triggered submarine landslide, which introduces unquantifiable uncertainty to hazard assessments based on numerical modelling. To lower the uncertainty, we present a method that determines material parameters for the slide body to constrain the generated tsunami waves. Our method employs the distribution of landslide run-out masses and their comparison with simulations. It assumes that the slide material can be approximated by bulk values during the slide motion. To demonstrate our method, we make use of Valdes slide run-out masses off the Chilean coast.

Description: The moment tensors are unique and describe the body force equivalents of rupture processes in any medium including faulting at a material interface, defined as the contact of two media with non-zero velocity or density contrasts. From a practical point of view, however, the moment tensor inversion of sources near or at a material interface is more involved than if the medium is smooth in the source area. First, the moment tensors of sources characterized by the same displacement discontinuity display jumps when the source crosses the interface. Consequently, the moment tensors become sensitive to the source location. If the source lies near the interface, the location into an incorrect half-space can introduce errors in the moment tensor. Second, if the source lies at the material interface, some of the spatial derivatives of the Green's function are, in general, discontinuous and the radiated wave field must be calculated using a generalized representation theorem. Third, the moment tensors are functions of averaged elastic parameters known from effective medium theory. The theory implies that shear faulting at a material interface in isotropic media is represented by the standard double-couple moment tensor. The scalar seismic moment is calculated as a product of the displacement discontinuity across the fault, the fault size and the effective rigidity at the fault. The effective rigidity is the harmonic mean of rigidities at the individual sides of the fault.

Description: We propose a three-dimensional (3-D) inversion scheme for deep ocean magnetotelluric (MT) data that enables us to incorporate both the local small-scale topography effect and regional large-scale topography effect, whose length scale is comparable with that of the mantle structure to be resolved by the inversion. We assume that the MT impedance tensor Z is approximately equal to the MT response of the total structure Z ts , expressed by the product of the local topographic distortion term D lt and the response to the regional structure Z rs that consists of the regional topography over the mantle electrical conductivity structure Z Z ts = D lt Z rs . We also assume that D lt may be treated as a site correction term, which is not dependent on the conductivity structure to be solved in the inversion. D lt is calculated before the inversion iteration is carried out through the forward modelling of Z ts and Z rs using a good initial guess for the mantle structure. However, the initial Z ts is separately modelled by another forward modelling program, which is more efficient in terms of computation cost. The model space of the inversion includes only the regional structure so that the modelled response is Z rs . Thus, in the inversion, Z ts is calculated from Z rs and D lt ; Z rs is modelled at every iteration but D lt is fixed throughout the inversion. The sensitivity of Z ts to the conductivity is calculated considering D lt as a constant. This scheme requires modelling only of the regional scale structure in the inversion process, saving computational resources and time, which is critical in the 3-D case. Tests using a synthetic model and data demonstrate that incorporating the local topography effect in this scheme was successful and produced an accurate reconstruction of the given mantle structure.

Description: To enhance the feasibility of seismic full waveform inversion (FWI) for various types of geological structures, the model parameters should be updated along directions such that both long- and short-wavelength structures can be properly resolved. These long- and short-wavelength structures are primarily influenced by the low- and high-frequency components of the gradients, respectively. In some cases, however, the gradients are not flexible to reconstruct both the long- and the short-wavelength structures. This problem can be related to the scaling method using the Hessian matrix and the effect of the source spectrum. In this study, we analyse the problems of conventional scaling methods in frequency-domain FWI and propose a weighting method to compensate for these problems. The weighting method is applied to the conventional elastic FWI, where the gradient is scaled by the diagonal of the pseudo-Hessian matrix inside the frequency loop so that the effect of the source spectrum can be removed through cancellation. The weighting factors are designed using the backpropagated wavefields incited by the deconvolved residuals, which play a role in making the descent directions appropriately reflect the spectral differences between the observed data and the initial (or the inverted) modelling responses. We analyse the characteristics of the Jacobians and residuals and compare the descent directions of the two conventional waveform inversion methods with descent directions of the weighting method for thick rectangular-shaped and thin-layers models. The results indicate that the descent directions computed using the conventional inversion methods do not reflect the characteristics of deconvolved residuals and that particular frequency components are always emphasized regardless of geological models, while the spatial resolution of the descent direction calculated using the weighting method is flexibly determined depending on the differences between the true and the assumed models. Inversion results for the Marmousi-2 model show that the weighting method is not sensitive to the initial guess even though we do not apply the frequency marching strategy. Numerical examples for the SEG/EAGE salt model show that the weighting method can properly recover the high velocities of the salt body and the low velocities below the salt body. Our numerical examples are based on the assumption that low frequencies are available. Further study is needed to apply the weighting method to real field data that are noisy and do not have low-frequency components.

Description: Western Turkey provides spectacular examples of the two end-member models of deformation of the continental lithosphere, with strain localization along the North Anatolian fault and distributed north–south extension along the Aegean coast. To provide constraints on the mechanisms of continental deformation, we present a new high-resolution image of lithospheric structure along a ~650 km transect crossing Western Anatolia at 28°E longitude from the Black Sea to the Mediterranean. More than 2600 receiver functions are computed from records of teleseismic earthquakes at 40 broadband seismic stations with an average spacing of 15 km. Lateral variations of crustal thickness and Vp / Vs ratio are inferred from both H - k and common conversion point stacks. We observe long-wavelength variations of Moho depth from ~31 km in the Thrace basin to ~25 km beneath the Marmara Sea, ~32 km beneath the Izmir–Ankara suture, ~25 km beneath the Menderes Massif and ~20 km on the coast of the Mediterranean. No mid- or lower-crustal interface is visible in the migrated depth section and seismic discontinuities are confined to the shallow crust. The small-amplitude and long-wavelength lateral variations of the Moho topography suggest that viscous flow in a hot lower crust has smoothed out the lateral variations of crustal thickness induced by Cenozoic continent–continent collision. The crust–mantle boundary is flat beneath the central and southern Menderes Massif. The rougher Moho topography and more heterogeneous crust of the Marmara region likely result from transtension in the North Anatolian Fault Zone superimposed onto Aegean extension. The Moho of the subducted African lithosphere is observed dipping northward between ~40 and ~60 km depth at the southern end of the profile. The abrupt termination of the subducted slab only 50 km to the north of the Mediterranean coast confirms the slab tear inferred from previous tomographic studies.

Description: We develop and apply a full waveform inversion method that incorporates seismic data on a wide range of spatio-temporal scales, thereby constraining the details of both crustal and upper-mantle structure. This is intended to further our understanding of crust–mantle interactions that shape the nature of plate tectonics, and to be a step towards improved tomographic models of strongly scale-dependent earth properties, such as attenuation and anisotropy. The inversion for detailed regional earth structure consistently embedded within a large-scale model requires locally refined numerical meshes that allow us to (1) model regional wave propagation at high frequencies, and (2) capture the inferred fine-scale heterogeneities. The smallest local grid spacing sets the upper bound of the largest possible time step used to iteratively advance the seismic wave field. This limitation leads to extreme computational costs in the presence of fine-scale structure, and it inhibits the construction of full waveform tomographic models that describe earth structure on multiple scales. To reduce computational requirements to a feasible level, we design a multigrid approach based on the decomposition of a multiscale earth model with widely varying grid spacings into a family of single-scale models where the grid spacing is approximately uniform. Each of the single-scale models contains a tractable number of grid points, which ensures computational efficiency. The multi-to-single-scale decomposition is the foundation of iterative, gradient-based optimization schemes that simultaneously and consistently invert data on all scales for one multi-scale model. We demonstrate the applicability of our method in a full waveform inversion for Eurasia, with a special focus on Anatolia where coverage is particularly dense. Continental-scale structure is constrained by complete seismic waveforms in the 30–200 s period range. In addition to the well-known structural elements of the Eurasian mantle, our model reveals a variety of subtle features, such as the Armorican Massif, the Rhine Graben and the Massif Central. Anatolia is covered by waveforms with 8–200 s period, meaning that the details of both crustal and mantle structure are resolved consistently. The final model contains numerous previously undiscovered structures, including the extension-related updoming of lower-crustal material beneath the Menderes Massif in western Anatolia. Furthermore, the final model for the Anatolian region confirms estimates of crustal depth from receiver function analysis, and it accurately explains cross-correlations of ambient seismic noise at 10 s period that have not been used in the tomographic inversion. This provides strong independent evidence that detailed 3-D structure is well resolved.

Description: We present a semi-automatic seismic waveform selection algorithm that can be used in full 3-D waveform inversions for earthquake source parameters and/or earth structure models. The algorithm is applied on pairs of observed and synthetic seismograms. A pair of observed and synthetic seismograms are first segmented in the wavelet domain into a number of wave packets using a topological watershed algorithm. A set of user-adjustable criteria based on waveform similarities is then applied to match each wave packet obtained from the observed seismogram with the corresponding wave packet obtained from the synthetic seismogram. The selected wave packet pairs are then used for extracting frequency-dependent phase and amplitude misfit measurements, which can be used in seismic source and/or structural inversions. The algorithm takes advantage of time–frequency representations of seismograms and is able to separate seismic phases in both time and frequency domains. We demonstrate the flexibility of this algorithm using examples of full 3-D waveform inversions for earthquake centroid moment tensors and earth structure models at different geographic scales.

Description: The use of seismic noise interferometry to retrieve Green's functions and the analysis of volcanic tremor are both useful in studying volcano dynamics. Whereas seismic noise interferometry allows long-range extraction of interpretable signals from a relatively weak noise wavefield, the characterization of volcanic tremor often requires a dense seismic array close to the source. We here show that standard processing of seismic noise interferometry yields volcanic tremor signals observable over large distances exceeding 50 km. Our study comprises 2.5 yr of data from the U.S. Geological Survey Hawaiian Volcano Observatory short period seismic network. Examining more than 700 station pairs, we find anomalous and temporally coherent signals that obscure the Green's functions. The time windows and frequency bands of these anomalous signals correspond well with the characteristics of previously studied volcanic tremor sources at Pu'u 'O'o and Halema'uma'u craters. We use the derived noise cross-correlation functions to perform a grid-search for source location, confirming that these signals are surface waves originating from the known tremor sources. A grid-search with only distant stations verifies that useful tremor signals can indeed be recovered far from the source. Our results suggest that the specific data processing in seismic noise interferometry—typically used for Green's function retrieval—can aid in the study of both the wavefield and source location of volcanic tremor over large distances. In view of using the derived Green's functions to image heterogeneity and study temporal velocity changes at volcanic regions, however, our results illustrate how care should be taken when contamination by tremor may be present.

Description: We present a new interpretation of anisotropy of magnetic susceptibility (AMS) fabrics in basaltic lava flows based on the detailed study of magnetic mineralogy and silicate crystallographic fabric of a Quaternary lava flow from the French Massif Central (La Palisse). We consider the model of AMS fabric imbrication between magnetic foliation and flow surface, as initially proposed for dykes. At the two sampling sites, the concordance between the flow direction deduced from the AMS foliation and that deduced from field observations indicates that the imbrication model could apply to the lava flows. However, the flow senses inferred from AMS are systematically opposed between the two sampling sites suggesting permutations between K 1 and K 3 AMS axes, a configuration referred to as inverse fabric. Electron backscatter diffraction (EBSD) measurements show strong lattice-preferred orientations (LPO) for plagioclase, especially the (010) plagioclase plane, which tends to be parallel to the flow. Clinopyroxene LPO remains less marked than plagioclase LPO, whereas titanomagnetite does not display a significant LPO. Comparison between magnetic and crystallographic fabrics suggests that the AMS fabric of the lava flow results from the distribution of titanomagnetite grains, which is in turn controlled by the fabric of the silicate framework. Magnetic hysteresis parameters and anisotropy of remanent magnetization (ARM) measurements exclude a significant contribution from single-domain grains, often called upon to explain inverse magnetic fabrics. The origin of the observed inverse magnetic fabric may relate to the dip of the palaeosurface, which is the only remarkable difference between the two sampling sites. AMS appears as a good tool to determine the direction of basaltic lava flows and coupling with local crystallographic fabric data provides a valuable control of relationships between magnetic fabrics and flow and thus contributes to better constrain the AMS signature of lava flows.

Description: Insights into plate-boundary deformation are gained from the shear-wave splitting of local S phases that originate within the lithosphere of the South Island, New Zealand. Analysis of the splitting parameters from land stations reveals changes in both delay times and fast azimuths with earthquake depth, earthquake-station back-azimuth and initial polarization azimuth, suggesting both laterally and depth varying anisotropy. When the average results are examined as a whole via tomographic inversion and spatial averaging, consistent patterns in delay times and fast azimuths exist. Spatially averaged fast azimuths reveal a localized high-strain zone in the southern central region of the South Island. Based on fast azimuths observed above 100 km depth, we suggest that the plate-boundary subparallel anisotropy that is produced by pervasive shear is mainly distributed within a zone extending ~130 km SE of the Alpine fault in southern South Island and is widely distributed in northern South Island. Average station delay times of ~0.1–0.4 s compared to 1.7 s SKS delay times from previous studies in South Island further suggests a deeper seated anisotropic zone in some areas, but in other regions frequency-dependent anisotropy could explain the observed splitting.

Description: Numerous non-ideal factors can influence paleointensity data, but the detection of these factors remains problematic and new approaches to understanding how paleointensity data behave are needed. In this study, a recently developed stochastic model of single domain (SD) paleointensity behaviour is expanded to investigate the effects that anisotropic and non-linear thermoremanent magnetizations (TRMs) have on the paleointensity results and the parameters used to select data. The model results indicate that before applying any form of correction these non-ideal factors can produce results that are self-consistent, but highly inaccurate. The methods that are currently used to correct for anisotropic and non-linear TRMs are effective and greatly increase the likelihood of obtaining accurate results. The corrections, however, do not restore the results to those of ideal SD samples measured with the same laboratory-to-ancient field ratio, but the data are restored to those of ideal SD samples with the equivalent laboratory-to-ancient magnetization ratios ( M Lab / M Anc ). The simulations indicate that non-linear and anisotropic TRM have no or only a weak influence on the parameters commonly used to select paleointensity data, which means that these non-ideal factors are effectively undetectable. These new models suggest that the paleointensity behaviour of thermally/chemically stable SD samples, whether they are ideally behaved, anisotropy or non-linear TRM corrected, is near universal and depends only on M Lab / M Anc and the choice of paleointensity protocol (i.e. Coe-type versus Thellier). Given the high self-consistency and highly inaccurate results that anisotropic and non-linear TRM can yield, it is essential to test for such effects and all Thellier-type paleointensity studies must include tests for anisotropic and non-linear TRM to assert the reliability of the data obtained.

Description: We present a nodal finite-element method that can be used to compute in parallel highly accurate solutions for 3-D controlled-source electromagnetic forward-modelling problems in anisotropic media. Secondary coupled-potential formulation of Maxwell's equations allows to avoid the singularities introduced by the sources, while completely unstructured tetrahedral meshes and mesh refinement support an accurate representation of geological and bathymetric complexity and improve the solution accuracy. Different complex iterative solvers and an efficient pre-conditioner based on the sparse approximate inverse are used for solving the resulting large sparse linear system of equations. Results are compared with the ones of other researchers to check the accuracy of the method. We demonstrate the performance of the code in large problems with tens and even hundreds of millions of degrees of freedom. Scalability tests on massively parallel computers show that our code is highly scalable.

Description: We present a two-and-half-dimensional (2.5-D) forward and inversion algorithm for the interpretation of surface seismic elastic full-waveform data. The 2-D modelling approach for elastic waves might not be sufficiently accurate because of its line-source assumption. On the other hand, a full 3-D modelling of elastic waves is still computationally very expensive for seismic exploration applications. By employing the 2.5-D modelling approach, we assume that the elastic medium is 2-D while the source is a 3-D point source. We solve the 2.5-D problem by first transforming the elastic wave equation in the spatial domain into the wavenumber domain. Then, for each wavenumber we solve a 2-D problem using the finite difference method with staggered grids. After that an inverse wavenumber transform is performed to compute the 3-D field distribution. To handle the inverse transform, we develop an efficient numerical integration scheme by subdividing the integration domain and applying different integration rules to each subinterval. We show that this approach works well for surface seismic applications. The 2.5-D approach offers a more realistic modelling of the elastic wave data, hence it produces more accurate inversion results than the 2-D inversion approach. Finally, we validate this approach by using a numerical test. In this test we used our 2.5-D full-waveform inversion algorithm to invert the synthetic data generated by a 3-D finite-difference time-domain simulation.

Description: We introduce a novel finite-difference (FD) approach for seismic wave extrapolation in time. We derive the coefficients of the finite-difference operator from a lowrank approximation of the space-wavenumber, wave-propagator matrix. Applying the technique of lowrank finite-differences, we also improve the finite difference scheme of the two-way Fourier finite differences (FFD). We call the new operator lowrank Fourier finite differences (LFFD). Both the lowrank FD and lowrank FFD methods can be applied to enhance accuracy in seismic imaging by reverse-time migration. Numerical examples confirm the validity of the proposed technique.

Description: First-arrival traveltime is commonly used in problems that involve static correction, pre-stack migration, earthquake location, seismic tomography, etc. The classical eikonal equation discretized with regular rectilinear grids is effective for calculating the first-arrival traveltimes in a rectangular domain, but is less efficient for an Earth model that has an irregular surface. Here, we present a topography-dependent eikonal equation in 2-D that makes use of the wave equation in a curvilinear coordinate system, and is equivalent to a direct derivation of the classical eikonal equation together with a transformation from Cartesian to curvilinear coordinates. The topography-dependent eikonal equation is reduced to the classical version when the surface is flat. The topography-dependent equation (in the curvilinear coordinate system) displays the mathematical form of an anisotropic eikonal equation (even though the medium is isotropic in the Cartesian coordinate system). Then, we use a Lax–Friedrichs sweeping scheme, which has been developed as an iterative method for Hamilton–Jacobi equations, to approximate the viscosity solutions (first-arrival traveltimes) of the topography-dependent eikonal equation formulated in the curvilinear coordinate system. Several numerical experiments performed with different models illustrate that the method is stable and accurate in calculating seismic traveltimes with an irregular (non-flat) surface.

Description: A seismic experiment provides new insights on the crustal structure of the head area of the Three Gorges Reservoir in central China. The region is characterized by a relatively high rate of reservoir-induced seismicity that is often triggered within those areas associated with the ascending water level. Our 3-D velocity model of the Three Gorges region shows that the Huangling anticline (HLA) is characterized by a high-velocity crust, and the Zigui Basin (ZGB) has lower crustal velocities. The 3-D tomographic inversions are conducted using 11 901 P -wave and 12 032 S -wave arrival times from 1342 events recorded by the local network of seismic stations from early 2001 to late 2006. Initial models with varying velocity gradients are extracted to constrain a data-driven optimum 1-D model for 3-D iterative inversion scheme. Checkerboard tests are applied to assess model reliability, indicating a reasonable level of lateral and vertical resolutions. The P - and S -tomographic models reveal a local high-velocity anomaly from 5 to 10 km beneath SW portion of the HLA and a strong, large low-velocity anomaly between about 5–10 km depths at the south margin of the ZGB. Moreover, the southwest border of HLA underthrust the ZGB with slightly bigger angle. Also, a prominent high-velocity anomaly is located below 5 km beneath Shazhenxi, and to the west, the velocity anomaly turns out to be negative. There is no record help explaining the dramatic feature since incorporating local tectonic and topography, it suggests sharp gap in velocity near surface is primarily due to several secondary fracture zones. The surface responses of velocity discontinuity are generally aligned parallel to the trending of fault. Relatively good agreement between regional features and the velocity perturbations promotes further interpretation. Earthquake swarm activities from source relocation occur on the outer portions of high-velocity anomaly with nearly perpendicular dip angle. Thus, the seismicity pattern could be interpreted based on the velocity models, regional geostress and dynamic load of reservoir.

Description: In this study, we modify and extend a data analysis technique to determine the stress orientations between data clusters by adding an additional constraint governing the probability algorithm. We apply this technique to produce a map of the maximum horizontal compressive stress ( S Hmax ) orientations in the greater European region (including Europe, Turkey and Mediterranean Africa). Using the World Stress Map data set release 2008, we obtain analytical probability distributions of the directional differences as a function of the angular distance, . We then multiply the probability distributions that are based on pre-averaged data within 〈 3° of the interpolation point and determine the maximum likelihood estimate of the S Hmax orientation. At a given distance, the probability of obtaining a particular discrepancy decreases exponentially with discrepancy. By exploiting this feature observed in the World Stress Map release 2008 data set, we increase the robustness of our S Hmax determinations. For a reliable determination of the most likely S Hmax orientation, we require that 90 per cent confidence limits be less than ±60° and a minimum of three clusters, which is achieved for 57 per cent of the study area, with uncertainties of less than ±30° for 19 per cent of the area. When the data density exceeds 0.8  x 10 –3 data km –2 , our method provides a means of reproducing significant local patterns in the stress field. Several mountain ranges in the Mediterranean display 90° changes in the S Hmax orientation from their crests (which often experience normal faulting) and their foothills (which often experience thrust faulting). This pattern constrains the tectonic stresses to a magnitude similar to that of the topographic stresses.

Description: The existence of an active compression on the frontal fold-and-thrust belt (FTB) of the NW Borneo Wedge is a long debate. Because of the absence of seismicity, the frontal FTB is traditionally considered as inactive and generally attributed to the thin-skin gravity-driven Baram Basin. However, there are some signs of convergence and compression (GPS velocities and horizontal stress field measured from borehole analysis) do exist between the NW Borneo area and Sunda Plate (Dangerous-Grounds). Revisited GPS data, combined with a rigorous structural study of the NW Borneo Wedge suggest that the recent compression recorded on the frontal FTB is the result of a crustal-scale gravity-driven mechanism, the orogenic collapse of the NW Borneo in the Sabah–northern Sarawak area since 1.9 Myr. These results provide a new understanding of the recent behaviour of the NW Borneo Wedge which can be included in a continuum of the wedge history.