ALBERT

Description: Siwi caldera, in the Vanuatu arc (Tanna island), is a rare volcanic complex where both persistent eruptive activity (Yasur volcano)and rapid block resurgence (Yenkahe horst) can be investigated simultaneously during a post-caldera stage. Here we provide new constraints on the feeding system of this volcanic complex, based on a detailed study of the petrology, geochemistry and volatile content of Yasur^Siwi bulk-rocks and melt inclusions, combined with measurements of the chemical composition and mass fluxes of Yasur volcanic gases. Major and trace element analyses of Yasur^ Siwi volcanic rocks, together with literature data for other volcanic centers, point to a single magmatic series and possibly long-lived feeding of Tanna volcanism by a homogeneous arc basalt. Olivine-hosted melt inclusions show that the parental basaltic magma, which produces basaltic-trachyandesites to trachyandesites by 50^70% crystal fractionation, is moderately enriched in volatiles ( 1wt % H2O, 0·1wt % S and 0·055 wt % Cl). The basaltic-trachyandesite magma, emplaced at between 4^5 km depth and the surface, preserves a high temperature (1107 158C) and constant H2O content ( 1wt %) until very shallow depths, where it degasses extensively and crystallizes. These conditions, maintained over the past 1400 years of Yasur activity, require early water loss during basalt differentiation, prevalent open-system degassing, and a relatively high heat flow ( 109W). Yasur volcano releases on average 13·4 103 tons d 1 of H2O and 680 tons d 1 of SO2, but moderate amounts of CO2 (840 tons d 1), HCl (165 tons d 1), and HF (23 tons d 1). Combined with melt inclusion data, these gas outputs constrain a bulk magma degassing rate of 5 107 m3 a 1, about a half of which is due to degassing of the basaltic-trachyandesite. We compute that 25 km3 of this magma have degassed without erupting and have accumulated beneath Siwi caldera over the past 1000 years, which is one order of magnitude larger than the accumulated volume uplift of the Yenkahe resurgent block. Hence, basalt supply and gradual storage of unerupted degassed basaltictrachyandesite could easily account for (or contribute to) the Yenkahe block resurgence.

Description: Published

Description: 1077-1105

Description: 2.3. TTC - Laboratori di chimica e fisica delle rocce

Description: 2.4. TTC - Laboratori di geochimica dei fluidi

Description: JCR Journal

Description: restricted

Description: Stromboli is known for its persistent degassing and rhythmic strombolian activity occasionally punctuated by paroxysmal eruptions. The basaltic pumice and scoria emitted during paroxysms and strombolian activity, respectively, differ in their textures, crystal contents and glass matrix compositions, which testify to distinct conditions of crystallization, degassing and magma ascent. We present here an extensive dataset on major elements and volatiles (CO2, H2O, S and Cl) in olivine-hosted melt inclusions and embayments from pyroclasts emplaced during explosive eruptions of variable magnitude. Magma saturation pressures were assessed from the dissolved amounts of H2O and CO2 taking into account the melt composition evolution. Both pressures and melt inclusion compositions indicate that (1) Ca-basaltic melts entrapped in high-Mg olivines (Fo89–90) generate Stromboli basalts through crystal fractionation, and (2) the Stromboli plumbing system can be imaged as a succession of magma ponding zones connected by dikes. The 7–10 km interval, where magmas are stored and differentiate, is periodically recharged by new magma batches, possibly ranging from Ca-basalts to basalts, with a CO2-rich gas phase. These deep recharges promote the formation of bubbly basalt blobs, which are able to intrude the shallow plumbing system (2–4 km), where CO2 gas fluxing enhances H2O loss, crystallization and generation of crystal-rich, dense, degassed magma. Chlorine partitioning into the H2O–CO2-bearing gas phase accounts for its efficient degassing (≥69%) under the open-system conditions of strombolian activity. Paroxysms, however, are generated through predominantly closed-system ascent of basaltic magma batches from the deep storage zone. In this situation crystallization is negligible and sulfur exsolution starts at ≤170 MPa. Chlorine remains dissolved in the melt until lower pressures, only 16% being lost upon eruption. Finally, we propose a continuum in explosive eruption energy, from strombolian activity to large paroxysmal events, ultimately controlled by variable pressurization of the deep feeding system associated with magma and gas recharges.

Description: Published

Description: 603-626

Description: 2.3. TTC - Laboratori di chimica e fisica delle rocce

Description: 3.5. Geologia e storia dei vulcani ed evoluzione dei magmi

Description: JCR Journal

Description: reserved

Description: SUMMARY In his comment, De Basabe criticises our paper ignoring the advantages of unstructured element mesh used in the finite element method, and argues that the Gaussian quadrature grid (GQG) approach is limited to a homogenous or constant layered geological model and does not have the spectral accuracy. In this reply, we give our response to his criticism and comment, and further clarify the accuracy and capability of the GQG approach.

Description: SUMMARY High-porosity channels are important pathways for melt migration in the mantle. To better understand the lithology and geometry of high-porosity melt channels, we conduct high-order accurate numerical simulations of reactive dissolution along a solubility gradient in an upwelling and viscously deformable porous column. In contrast to earlier studies, we assume the dissolution reaction to be at equilibrium, consider a finite soluble mineral abundance, and employ a high-order accurate numerical scheme. Using sustained perturbations in porosity and soluble mineral abundance at the inflow boundary, we explore the structure of steady-state high-porosity melt channels and their associated lithologies over a range of parameters, including soluble mineral abundance, solubility gradient, amplitude and lateral variation in inflow melt flux, melt fraction and upwelling rate. In general, high-porosity dunite channels are transient and shallow parts of pathways for melt migration in the mantle. The lower parts of a high-porosity channel are orthopyroxene-bearing dunite, harzburgite and possibly lherzolite. A wide orthopyroxene-free dunite channel may contain two or three high-porosity melt channels. The depth of dunite channel initiation depends on the solubility gradient, soluble mineral abundance, inflow melt flux, melt suction rate and upwelling rate. The amplitude and length scale of lateral variation in porosity and orthopyroxene abundance at the base of the upwelling column are important in determining the size and dimension of dunite channels, the strength of melt focusing and the melt suction rate, the presence of compacting boundary layer, as well as the number of high-porosity melt branches within a dunite channel. The spatial relations among the high-porosity melt channels, dunite and harzburgite channels documented in our numerical simulations may shed new light on a number of field, petrological and geochemical observations related to melt migration in the mantle.

Description: SUMMARY Geothermal energy is becoming an important clean energy source, however, the stimulation of a reservoir for an Enhanced Geothermal System (EGS) is associated with seismic risk due to induced seismicity. Seismicity occurring due to the water injection at depth have to be well recorded and monitored. To mitigate the seismic risk of a damaging event, an appropriate alarm system needs to be in place for each individual experiment. In recent experiments, the so-called traffic-light alarm system, based on public response, local magnitude and peak ground velocity, was used. We aim to improve the pre-defined alarm system by introducing a probability-based approach; we retrospectively model the ongoing seismicity in real time with multiple statistical forecast models and then translate the forecast to seismic hazard in terms of probabilities of exceeding a ground motion intensity level. One class of models accounts for the water injection rate, the main parameter that can be controlled by the operators during an experiment. By translating the models into time-varying probabilities of exceeding various intensity levels, we provide tools which are well understood by the decision makers and can be used to determine thresholds non-exceedance during a reservoir stimulation; this, however, remains an entrepreneurial or political decision of the responsible project coordinators. We introduce forecast models based on the data set of an EGS experiment in the city of Basel. Between 2006 December 2 and 8, approximately 11 500 m 3 of water was injected into a 5-km-deep well at high pressures. A six-sensor borehole array, was installed by the company Geothermal Explorers Limited (GEL) at depths between 300 and 2700 m around the well to monitor the induced seismicity. The network recorded approximately 11 200 events during the injection phase, more than 3500 of which were located. With the traffic-light system, actions where implemented after an M L 2.7 event, the water injection was reduced and then stopped after another M L 2.5 event. A few hours later, an earthquake with M L 3.4, felt within the city, occurred, which led to bleed-off of the well. A risk study was later issued with the outcome that the experiment could not be resumed. We analyse the statistical features of the sequence and show that the sequence is well modelled with the Omori–Utsu law following the termination of water injection. Based on this model, the sequence will last 31+29/−14 years to reach the background level. We introduce statistical models based on Reasenberg and Jones and Epidemic Type Aftershock Sequence (ETAS) models, commonly used to model aftershock sequences. We compare and test different model setups to simulate the sequences, varying the number of fixed and free parameters. For one class of the ETAS models, we account for the flow rate at the injection borehole. We test the models against the observed data with standard likelihood tests and find the ETAS model accounting for the on flow rate to perform best. Such a model may in future serve as a valuable tool for designing probabilistic alarm systems for EGS experiments.

Description: SUMMARY In principle, we can never know the true Green's function, which is a major error source in seismic waveform inversion. So far, many studies have devoted their efforts to obtain a Green's function as precise as possible. In this study, we propose a new strategy to cope with this problem. That is to say, we introduce uncertainty of Green's function into waveform inversion analyses. Due to the propagation law of errors, the uncertainty of Green's function results in a data covariance matrix with significant off-diagonal components, which naturally reduce the weight of observed data in later phases. Because the data covariance matrix depends on the model parameters that express slip distribution, the inverse problem to be solved becomes non-linear. Applying the developed inverse method to the teleseismic P -wave data of the 2006 Java, Indonesia, tsunami earthquake, we obtained a reasonable slip-rate distribution and moment-rate function without the non-negative slip constraint. The solution was independent of the initial values of the model parameters. If we neglect the modelling errors due to Green's function as in the conventional formulation, the total slip distribution is much rougher with significant opposite slip components, whereas the moment-rate function is much smoother. If we use a stronger smoothing constraint, more plausible slip distribution can be obtained, but then the moment-rate function becomes even smoother. By comparing the observed waveforms with the synthetic waveforms, we found that high-frequency components were well reproduced only by the new formulation. The modelling errors are essentially important in waveform inversion analyses, although they have been commonly neglected.

Description: SUMMARY Geological and geodetic records show that motion is distributed over a broad and complex system of faults in southern California, with many large earthquakes occurring off the main San Andreas fault. This unusually complex plate boundary geometry is imposed by the big bend of the San Andreas Fault and the dynamics of the underlying lithosphere, but lithosphere rheology and distribution of applied forces are still poorly constrained. Geodynamic modelling is thus very uncertain, and distribution of lithospheric deformations highly controversial. Here, exploiting the property that deformation orients olivine crystals, we show that deep lithospheric deformations can be mapped directly with a new imaging approach of seismic anisotropy, with a resolution fine enough for detailed geodynamic interpretation. This method relies entirely on finite-frequency effects in the splitting of SKS waves. We build an extensive data set of more than 3400 SKS splitting measurements from the analysis of SKS waves recorded by all the broad-band stations in southern California. Resolution tests demonstrate that, using this data set, we are able to resolve anisotropic structures smaller than the size of the first Fresnel zone of SKS waves. The good vertical resolution in our tomographic images results from the short inter-station spacing in southern California. In our 3-D anisotropy model, we find no evidence for localized lithospheric shear deformation beneath the San Andreas Fault. Instead, the lithospheric plate boundary is localized along a broad shear zone beneath the East California Shear Zone. Therefore, surface and deep deformation patterns are poorly correlated and most likely decoupled. Active N–S convergence in the Transverse Ranges results in strongly coherent E–W alignment of olivine fast axes in the shallow lithosphere. At the top of the asthenosphere, the observed low level of anisotropy suggests weak lithospheric basal tractions and thus a strong decoupling between the lithosphere and the asthenosphere.

Description: SUMMARY Earthquakes often occur on faults separating materials with different elastic properties. On theoretical grounds, it is expected that earthquakes on such bimaterial interfaces might have a preferred rupture propagation direction, that being the direction of motion of the more compliant material. The goal of this paper is to determine whether a large sample of natural earthquakes on a bimaterial interface exhibits this tendency. Since the creeping section of San Andreas Fault has a large across-fault velocity contrast and has produced thousands of microearthquakes over the last few decades, the rupture directions of ∼3100 magnitude 0.5–3.0 earthquakes were studied using spectral ratio analysis. The spectral ratios of all earthquake pairs in spatially defined clusters were fitted with synthetic spectral ratios at qualified stations. The synthetics were computed from a simple moving point source model in which each modelled earthquake has four parameters: two rupture lengths (one to the SE and one to the NW) and their propagation velocities. The resolution of rupture directivity increases with event size, such that nearly 900 events, mostly those larger than ∼70 m, appear reasonably well resolved. The inversion results suggest that ∼40 per cent of the well-resolved events are roughly bilateral, although more than ∼80 per cent of the 144 events classified as "strongly unilateral" rupture to the SE, consistent with the theoretical prediction. For those rupture halves that were large enough for the propagation speed to be somewhat resolved, that speed was greater by roughly 10 per cent for those halves propagating to the SE, qualitatively consistent with numerical and laboratory experiments. We also found that events with nearby foreshocks within several hours tend to rupture away from those foreshocks, whether to the NW or to the SE, indicating that asymmetry of prior stressing history can exert a stronger influence on rupture directivity than the material contrast.

Description: SUMMARY We present results on the composition, structure and particle size distribution (PSD) of pulverized and damaged granitic rocks in a 42-m-deep core adjacent to the San Andreas Fault near Littlerock, CA. The cored section is composed of pulverized granites and granodiorites, and is cut by numerous mesoscopic secondary shears. The analysis employs XRD, XRF, thin sections and semi-automated particle size analyser methods, including a novel calibration method. The mean particle size for the majority of samples falls between 50 and 470 μm. The PSDs can be fitted by a power law, with D -values ranging between 2.5 and 3.1, as well as by a superposition of two Gaussians. Fracture surface energy calculations based on the observed particle distributions provide very low values, indicating that the part of the total earthquake energy budget expended for breaking or shattering rocks is small. Shear deformation is likely to dominate near secondary faults. The most pronounced fault-related alteration occurs along the secondary shears, and is a function of both composition and depth. The alteration to clay appears to be the result of fluid–rock interaction and brittle deformation under low temperature conditions, rather than of surface-related weathering. The particle size reduction noted in the core reflects multiple mechanisms of comminution. The zones of pulverization that lack significant weathering likely result from repeating episodes of dynamic dilation and contraction.

Description: SUMMARY Broad-band seismic data from the southern African seismic experiment and the AfricaArray network are used to investigate the seismic velocity structure of the upper mantle beneath southern Africa, and in particular beneath the Kaapvaal Craton. A two-plane approximation method that includes a finite frequency sensitivity kernel is employed to measure Rayleigh wave phase velocities, which are inverted to obtain a quasi-3-D shear wave velocity model of the upper mantle. We find phase velocities for the Kaapvaal Craton and surrounding mobile belts that are comparable to those reported by previous studies, and we find little evidence for variation from east to west across the Namaqua–Natal Belt, a region not well imaged in previous studies. A high-velocity upper-mantle lid is found beneath the Kaapvaal Craton and most of southern Africa. For the Kaapvaal Craton, the thickness of the lid (∼150–200 km) is consistent with the lid thicknesses reported in many previous studies. The cratonic lid is underlain by a ∼100-km thick low-velocity zone with a 3.9 per cent maximum velocity reduction. By comparing the velocity model to those published for other Archean cratons, we find few differences, and therefore conclude that there is little evidence in the shear wave velocity structure of the mantle to indicate that the southern African plateau is supported by an upper-mantle thermal anomaly.

Description: SUMMARY To better constrain Baltica's position within Pangea, we conducted a palaeomagnetic study of Permo-Triassic dykes from the Oslo Graben, as a follow-up to an initial, but rather limited, study by Torsvik and colleagues in 1998. The age of these so-called Lunner dykes had previously been determined as ∼240 Ma in that study, but details in their analyses and new 40 Ar/ 39 Ar ages reveal that there may have been some argon loss in the initially dated dyke minerals and that a combined (weighted mean) age of 271 ± 2.7 (2σ) Myr for the dykes is preferable. We find two major components of magnetization in our samples: one carried by an Fe-sulphide (likely pyrrhotite) and the other carried by low-Ti magnetite; these magnetization components may be found together (superposed) in a given sample or they may occur apart. Micronmetre-sized crystals of Ti-Fe oxides, observed with a scanning electron microscope (SEM) show exsolution lamellae, formed upon cooling from intrusion temperatures. Assuming that the submicronmetre-sized (Ti)-magnetite grains that carry a stable remanence are of the same generation as the observed larger grains, we interpret the magnetite remanence in the dykes as of primary, thermoremanent origin. The sulphide remanence appears to be slightly younger, as seen by the SEM observations of pyrite framboids and a Fe-sulphide grain invading a Ti-magnetite grain. Moreover, the sulphide mineralization is likely of region-wide hydrothermal origin. The magnetizations carried by the pyrrhotite and magnetite have nearly identical directions and so, must be nearly of the same age. For this study, we sampled 56 sites including 39 dykes, 10 baked-contact rocks and 7 host rocks removed from the immediate dyke contacts. The dykes and the contact rocks have the same SW and up directions of magnetization, and contain the Fe-sulphide or the magnetite magnetization or both, as diagnosed by their relative unblocking temperatures. However, all the sampled carbonate and igneous host rocks far away from the dykes also have the same directions. Thus, all of the 10 originally planned contact tests are inconclusive. The new palaeopoles of this study are a few degrees apart; the magnetite pole (from dykes only, N = 25) is located at 51°N, 164°E, K = 69, A 95 = 3.5°, whereas the pole calculated from iron sulphide magnetic directions (all rock types, N = 20) is at 54°N, 166°E, K = 112, A 95 = 3.1°. All directions are of reversed polarity, suggesting that the magnetization was acquired during the Kiaman Reversed Superchron. The palaeomagnetic mean result from the magnetite-bearing sites implies a palaeolatitude of Oslo of 23°N, whereas the palaeolatitude calculated from the pyrrhotite magnetizations is 25–27°N, depending on choice of host lithologies. As noted in many previous publications, the palaeomagnetic poles for the late Palaeozoic and Early-Middle Triassic are in conflict with classical Pangea reconstructions. The poles with ages of 250 ± 10 Ma, in particular, previously showed a discrepancy of some 25° or more, when the Gondwana and Laurussia continents are restored to their juxtapositions in the Pangea-A fit, before the opening of the Atlantic Ocean. Proposed solutions to this conundrum have been controversial, involving doubts about (1) the geocentric coaxial dipole field model, (2) the reliability of the palaeomagnetic results or their ages, or (3) the validity of the Pangea-A reconstruction, leading to proposals of a Pangea B reconstruction in which Gondwana is displaced some 3500 km to the east with respect to Laurussia. The significance of our new result for this Pangea controversy resides in its improved age within an early Guadelupian (mid to late Permian) time interval where few results exist from well-dated igneous rocks in either Baltica or Laurentia. There are quite a few results from sedimentary rocks, but these may be suspected to suffer inclination shallowing, and are therefore less suitable to settle a palaeolatitudinal argument. Our new result of the magnetite magnetization, granted it is primary and acquired at about 270 Ma, combined with a new ∼265 Ma result from Argentina and selected other poles from igneous rocks, leaves enough room for the north–south configuration of Pangea A at 270 Ma and avoids the overlap between Baltica and Gondwana that necessitated Pangea B, at least for the Late Permian.

Description: SUMMARY We investigate 3-D basin structures and site resonance frequencies in the İzmit Bay area of Turkey by new geophysical surveys based on 239 single station microtremor and 405-point gravity measurements. A fundamental resonance frequency map of the İzmit Bay was produced from the main peak in the horizontal-to-vertical component spectral ratio (HVSR) of microtremors. The HVSR analysis of the microtremor data reveals single, double, broad peaked or no peak type HVSR curves varying in accordance with the surface geology and spatial extent of the three basins present in the İzmit Bay area. In the deepest sections of the İzmit, Gölcük and Derince basins of the İzmit Bay, the fundamental resonance frequencies are dominantly 0.24–0.30 Hz. These resonance frequencies should be taken into consideration along with higher mode frequencies to construct earthquake resistant structures in the İzmit Bay area. The 3-D gravimetric bedrock depth map of the İzmit basin shows that the basin has an asymmetric shape with its deepest section coinciding with the surface trace of the North Anatolian Fault. The northern shoulder of the basin has a gentle dip on the Kocaeli Peneplain side and the southern shoulder is much steeper, and it is bounded by the Samanlı Mountains. We derive a power-law relationship that correlates the fundamental site resonance frequencies with the sedimentary cover thickness obtained from the gravity and shear wave velocity data in the İzmit Bay. We use this relationship to estimate bedrock depths beneath Gölcük and Derince basins. Our estimation of maximum basin depths is 1400 m for the İzmit and is 800 m for the Gölcük and Derince basins. Finally, we have analysed a converted Sp phase from a local earthquake recording made at site CMP to calculate and verify the sediment thickness estimations obtained from our gravimetric and microtremor analyses. This calculation shows close agreement with that of the gravimetric and microtremor results. Our results show that the basins in the İzmit Bay area have a very thick sedimentary cover with very low shear velocities underlined by hard bedrock, forming a sharp impedance contrast. We anticipate that these results will be a key contribution to the quantitative assessment of seismic hazard for the İzmit Bay area before the occurrence of strong earthquakes in the Marmara region.

Description: SUMMARY To constrain the forecast horizon of geomagnetic data assimilation, it is of interest to quantify the range of predictability of the geodynamo. Following earlier work in the field of dynamic meteorology, we investigate the sensitivity of numerical dynamos to various perturbations applied to the magnetic, velocity and temperature fields. These perturbations result in some errors, which affect all fields in the same relative way, and grow at the same exponential rate  λ=τ −1 e , independent of the type and the amplitude of perturbation. Errors produced by the limited resolution of numerical dynamos are also shown to produce a similar amplification, with the same exponential rate. Exploring various possible scaling laws, we demonstrate that the growth rate is mainly proportional to an advection timescale. To better understand the mechanism responsible for the error amplification, we next compare these growth rates with two other dynamo outputs which display a similar dependence on advection: the inverse  τ −1 SV  of the secular-variation timescale, characterizing the secular variation of the observable field produced by these dynamos; and the inverse  (τ mag diss ) −1  of the magnetic dissipation time, characterizing the rate at which magnetic energy is produced to compensate for Ohmic dissipation in these dynamos. The possible role of viscous dissipation is also discussed via the inverse  (τ kin diss ) −1  of the analogous viscous dissipation time, characterizing the rate at which kinetic energy is produced to compensate for viscous dissipation. We conclude that  τ e  tends to equate  τ mag diss  for dynamos operating in a turbulent regime with low enough Ekman number, and such that  τ mag diss 〈 τ kin diss . As these conditions are met in the Earth's outer core, we suggest that  τ e  is controlled by magnetic dissipation, leading to a value  τ e =τ mag diss ≈ 30  yr . We finally discuss the consequences of our results for the practical limit of predictability of the geodynamo.

Description: SUMMARY A closed-form solution is given for a 2-D, transverse electric mode, magnetotelluric (MT) problem. The model system consists of a finite vertical thin conductor with variable integrated conductivity over a perfectly conducting base. A notable property of the solution is that the frequency response possesses a single pole in the complex plane. Systems with finitely many resonances play a central role in the 1-D MT inverse problem based on finite data sets, but until now, no 2-D system of this kind was known. The particular model is shown to be just one of a large class of thin conductors with same the property, and further examples are given. The solutions of the induction problem for members of this family can often be written in compact closed form, making them the simplest known solutions to the 2-D MT problem.

Description: SUMMARY The eikonal equation in an attenuating medium has the form of a complex-valued Hamilton–Jacobi equation and must be solved in terms of the complex-valued traveltime (complex-valued action function). A very suitable approximate method for calculating the complex-valued traveltime right in real space is represented by the perturbation from the reference traveltime calculated along real-valued reference rays to the complex-valued traveltime defined by the complex-valued Hamilton–Jacobi equation. The real-valued reference rays are calculated using the reference Hamiltonian function. The perturbation Hamiltonian function is parametrized by one or more perturbation parameters, and smoothly connects the reference Hamiltonian function with the Hamiltonian function corresponding to a given complex-valued Hamilton–Jacobi equation. Both the reference Hamiltonian function and the perturbation Hamiltonian function may be constructed in different ways, yielding differently accurate perturbation expansions of traveltime. All present perturbation methods use reference rays calculated in a reference anisotropic non-attenuating medium. In this paper, the reference Hamiltonian function is constructed directly using the Hamiltonian function corresponding to a given complex-valued Hamilton–Jacobi equation, and the perturbation Hamiltonian function is linear with respect to the perturbation parameter. The direct construction of the reference Hamiltonian function from the given complex-valued Hamilton–Jacobi equation is very general and accurate, especially for homogeneous Hamiltonian functions of degree N =−1 with respect to the slowness vector.

Description: SUMMARY The lithospheric structure in Oman has been determined by analyzing teleseismic P -receiver functions recorded at broad-band and short-period seismic stations of the Oman Seismological Network. Lithospheric structure is obtained by jointly inverting receiver functions and Rayleigh wave group velocities derived from continental-scale surface wave tomography of Pasyanos. We observe relatively thick crust (40–48 km) within the ophiolite formed mountains in northern Oman. The crustal thickness is about 35 km within the passive continental margin of the southern Oman region. Uppermost (〈5 km) crustal shear wave velocities are faster in the northern ophiolite regions compared to the southern Oman region, while shear velocities in the middle crust are faster in the Southern Oman region compared to the ophiolite region. This observation coincides well with cretaceous to Eocene marine platform sequences overlying Precambrian to Cambrian basement of the southern part. Joint inversion analysis shows that the Moho depth of Oman varies from 34 km in the southern region to 48 km in the northern part.

Description: We present experimental data on the partitioning of Li, Be, B, K, Mg, Sr, Ga, Rb, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Th, U, Hf, Zr, Nb and Ta between lawsonite and fluid, and zoisite and fluid at 3·0–3·5 GPa and 650–850°C. The aim is to provide data bearing on the trace element contents of fluids released during dehydration of subducting oceanic crust. Experimental trace element partition coefficients for lawsonite indicate a preference for the light rare earth elements (LREE) over the heavy REE (HREE) and for Be. These characteristics are consistent with the chemical composition of lawsonite in natural rocks. Experimental trace element partition coefficients for zoisite indicate a preference for HREE relative to LREE. This observation, consistent with earlier experimental data, is the reverse of the observed trace element compositions of natural zoisites, indicating the influence of other factors on the trace element contents of this phase. Lattice strain theory explains well the experimentally derived partitioning of divalent cations in the Ca-site between lawsonite and fluid. However, the weak relative fractionation of REE between lawsonite and fluid cannot be explained by lattice strain theory, as previously observed for zoisite–fluid REE partitioning. We combine our experimental data with thermodynamic models of mineral stability to model the compositions of fluids released during subduction of altered normal mid-ocean ridge basalt. The low La/Sm ratio associated with very high Ba/Th in arc magmas can be explained only if allanite is stable in the subducting oceanic crust. This suggests that the crustal fluid component involved in arc magma petrogenesis results from processes occurring in the warm, top part of the subducting slab. Decreasing lawsonite modal proportion with depth is associated with a large release of fluid characterized by low B/Be ratios that could explain the decreasing B/Be ratios in arc magmas with increasing distance from the trench. This implies that an important Be input in arc magma originates from the fluid generated during oceanic crust dehydration.

Description: To contribute to our understanding of the mechanisms and pathways of fluid movement through deeply subducted crust, we investigate high-pressure veins cutting eclogite-facies (~2·0 GPa and ~600°C) metagabbros of the Monviso Ophiolite, Italian Western Alps. The veins consist mainly of omphacite with minor garnet, rutile, talc and accessory zircon. Most of the vein minerals have major and trace element compositions that are comparable with the host-rock minerals, and vein and host-rock zircons have similar Hf isotopic compositions. These observations support the conclusions of previous studies that these veins largely formed from a locally sourced hydrous fluid during prograde or peak metamorphism. However, the bulk-rock Cr and Ni contents of the veins are significantly higher than those of the surrounding host eclogites. We also document distinct Cr-rich (up to weight per cent levels) zones in omphacite, garnet and rutile in some vein samples. Vein garnet and talc also have relatively high MgO and Ni contents. X-ray maps of vein garnet and rutile grains reveal complex internal zoning features, which are largely defined by micrometre-scale variations in Cr content. Some grains have concentric and oscillatory zoning in Cr, whereas others feature a chaotic fracture-like pattern. These Cr-rich zones are associated with high concentrations of Ni, B, As, Sb, Nb, Zr and high ratios of light rare earth elements (LREE) to middle REE (MREE) compared with low-Cr vein and host-rock minerals. Petrological and mass-balance constraints verify that the Cr-rich zones in the veins were not derived from internally sourced fluids, but represent precipitates from an external fluid. The external source that is consistent with the distinctive trace element characteristics of the vein components is antigorite serpentinite, which forms the structural basement of the high-pressure metagabbros. We propose at least two separate growth mechanisms for the Monviso veins. Most vein infillings were formed during progressive prograde metamorphism from locally derived fluid. Influx of the serpentinite-derived or other external fluid was transient and episodic and was probably achieved via brittle fractures, which preferentially formed along the pre-existing vein structures. The dehydration of serpentinite at high pressures in subduction zones may provide crucial volatiles and trace elements for arc magmas. Our results indicate that the movement of these fluids through subducted oceanic crust is likely to be highly channeled and transient so the progressive development of vein systems in mafic rocks may also be crucial for forming channelways for long-distance fluid flow at depth in subduction zones.

Description: Siwi caldera, in the Vanuatu arc (Tanna island), is a rare volcanic complex where both persistent eruptive activity (Yasur volcano) and rapid block resurgence (Yenkahe horst) can be investigated simultaneously during a post-caldera stage. Here we provide new constraints on the feeding system of this volcanic complex, based on a detailed study of the petrology, geochemistry and volatile content of Yasur–Siwi bulk-rocks and melt inclusions, combined with measurements of the chemical composition and mass fluxes of Yasur volcanic gases. Major and trace element analyses of Yasur–Siwi volcanic rocks, together with literature data for other volcanic centers, point to a single magmatic series and possibly long-lived feeding of Tanna volcanism by a homogeneous arc basalt. Olivine-hosted melt inclusions show that the parental basaltic magma, which produces basaltic-trachyandesites to trachyandesites by ~50–70% crystal fractionation, is moderately enriched in volatiles (~1 wt % H 2 O, 0·1 wt % S and 0·055 wt % Cl). The basaltic-trachyandesite magma, emplaced at between 4–5 km depth and the surface, preserves a high temperature (1107 ± 15°C) and constant H 2 O content (~1 wt %) until very shallow depths, where it degasses extensively and crystallizes. These conditions, maintained over the past 1400 years of Yasur activity, require early water loss during basalt differentiation, prevalent open-system degassing, and a relatively high heat flow (~10 9 W). Yasur volcano releases on average ≥ 13·4 x 10 3 tons d –1 of H 2 O and 680 tons d –1 of SO 2 , but moderate amounts of CO 2 (840 tons d –1 ), HCl (165 tons d –1 ), and HF (23 tons d –1 ). Combined with melt inclusion data, these gas outputs constrain a bulk magma degassing rate of ~5 x 10 7 m 3 a –1 , about a half of which is due to degassing of the basaltic-trachyandesite. We compute that 25 km 3 of this magma have degassed without erupting and have accumulated beneath Siwi caldera over the past 1000 years, which is one order of magnitude larger than the accumulated volume uplift of the Yenkahe resurgent block. Hence, basalt supply and gradual storage of unerupted degassed basaltic-trachyandesite could easily account for (or contribute to) the Yenkahe block resurgence.

Description: Primitive basalts are rarely found in arcs. The active NW Rota-1 volcano in the Mariana arc has erupted near-primitive lavas, which we have sampled with ROV Hyper-Dolphin (HPD). Samples from the summit (HPD480) and eastern flank (HPD488) include 17 magnesian basalts (51–52 wt % SiO 2 ) with 7·5–9·5 wt % MgO and Mg-number of 61–67, indicating little fractionation. Olivine phenocrysts are as magnesian as Fo 93 and contain 0·4 wt % NiO; the Cr/(Cr + Al) values of spinels are mostly 0·5–0·8, indicating equilibrium with depleted mantle. There are three petrographic groups, based on phenocryst populations: (1) cpx–olivine basalt (COB); (2) plagioclase–olivine basalt (POB); (3) porphyritic basalt. Zr/Y and Nb/Yb are higher in POB (3·1–3·2 and 1·2–1·5, respectively) than in COB (Zr/Y = 2·8–3·0 and Nb/Yb = 0·7–0·9), suggesting that POB formed from lower degrees of mantle melting, or that the COB mantle source was more depleted. On the other hand, COB have Ba/Nb (70–80) and Th/Nb (0·4–0·5) that are higher than for POB (Ba/Nb = 30–35 and Th/Nb = 0·1–0·2), and also have steeper light rare earth element (LREE)-enriched patterns. Moreover, COB have enriched 87 Sr/ 86 Sr and 143 Nd/ 144 Nd, and higher Pb isotope values, suggesting that COB has a greater subduction component than POB. 176 Hf/ 177 Hf between COB and POB are similar and Hf behavior in COB and POB is similar to that of Zr, Y and HREE, suggesting that Hf is not included in the subduction component, which produced the differences between COB and POB. The calculated primary basaltic magmas of NW Rota-1 volcano (primary COB and POB magmas) indicate segregation pressures of 2–1·5 GPa (equivalent to 65–50 km depth). These magmas formed by 24–18% melting of mantle peridotite having Mg-number ~89·5. Diapiric ascent of hydrous peridotite mixed heterogeneously with sediment melts may be responsible for the NW Rota-1 basalts. These two basalt magma types are similar to those found at Sumisu and Torishima volcanoes in the Izu–Bonin arc, with COB representing wetter and POB representing drier magmas, where subduction zone-derived melt components are coupled with the water contents.

Description: The origin of andesite is an important issue in petrology because andesite is the main eruptive product at convergent margins, corresponds to the average crustal composition and is often associated with major Cu–Au mineralization. In this study we present petrographic, mineralogical, geochemical and isotopic data for basaltic andesites of the latest Pleistocene Pilavo volcano, one of the most frontal volcanoes of the Ecuadorian Quaternary arc, situated upon thick (30–50 km) mafic crust composed of accreted Cretaceous oceanic plateau rocks and overlying mafic to intermediate Late Cretaceous–Late Tertiary magmatic arcs. The Pilavo rocks are basaltic andesites (54–57·5 wt % SiO 2 ) with a tholeiitic affinity as opposed to the typical calc-alkaline high-silica andesites and dacites (SiO 2 59–66 wt %) of other frontal arc volcanoes of Ecuador (e.g. Pichincha, Pululahua). They have much higher incompatible element contents (e.g. Sr 650–1350 ppm, Ba 650–1800 ppm, Zr 100–225 ppm, Th 5–25 ppm, La 15–65 ppm) and Th/La ratios (0·28–0·36) than Pichincha and Pululahua, and more primitive Sr ( 87 Sr/ 86 Sr ~0·7038–0·7039) and Nd ( Nd ~ +5·5 to +6·1) isotopic signatures. Pilavo andesites have geochemical affinities with modern and recent high-MgO andesites (e.g. low-silica adakites, Setouchi sanukites) and, especially, with Archean sanukitoids, for both of which incompatible element enrichments are believed to result from interactions of slab melts with peridotitic mantle. Petrographic, mineral chemistry, bulk-rock geochemical and isotopic data indicate that the Pilavo magmatic rocks have evolved through three main stages: (1) generation of a basaltic magma in the mantle wedge region by flux melting induced by slab-derived fluids (aqueous, supercritical or melts); (2) high-pressure differentiation of the basaltic melt (at the mantle–crust boundary or at lower crustal levels) through sustained fractionation of olivine and clinopyroxene, leading to hydrous, high-alumina basaltic andesite melts with a tholeiitic affinity, enriched in incompatible elements and strongly impoverished in Ni and Cr; (3) establishment of one or more mid-crustal magma storage reservoirs in which the magmas evolved through dominant amphibole and clinopyroxene (but no plagioclase) fractionation accompanied by assimilation of the modified plutonic roots of the arc and recharge by incoming batches of more primitive magma from depth. The latter process has resulted in strongly increasing incompatible element concentrations in the Pilavo basaltic andesites, coupled with slightly increasing crustal isotopic signatures and a shift towards a more calc-alkaline affinity. Our data show that, although ultimately originating from the slab, incompatible element abundances in arc andesites with primitive isotopic signatures can be significantly enhanced by intra-crustal processes within a thick juvenile mafic crust, thus providing an additional process for the generation of enriched andesites.

Description: SUMMARY We analyse the spatial distribution of the intensity data points surveyed after the M w 6.3, 2009 L’Aquila (central Italy) earthquake, with the aim to recognize and quantify finite-fault and directivity effects. The study is based on the analysis of the residuals, evaluated with respect to attenuation-with-distance models, calibrated for L’Aquila earthquake. We apply a non-parametric approach considering both the epicentral and the rupture distance, which accounts for the finite extension of the source. Then, starting from a simplified kinematic rupture model of the L’Aquila fault, we compute four directivity predictors proposed in literature, and assess their correlation with intensity residuals. We derive a so-called Intensity Directivity Factor by the correlation between theoretical predictors and observed residuals that allows us to identify and quantify the intensity data points affected by forward and backward directivity during L’Aquila earthquake. We find that the effects are more pronounced in the forward directivity direction and increments up to 1 MCS intensity unit are expected. Moreover, the directivity predictor that accounts for radiation pattern poorly correlates with residuals. These results show that the spatial distribution of the L’Aquila macroseismic field is affected by source effects and in particular that directivity-induced amplification effects can be recognized. We show that the quasi-unilateral rupture propagation along the fault can explain the high-intensity patterns observed along specific direction at relatively large distance from the instrumental epicentre, in accordance with the seismological source models derived from the analysis of instrumental observations.

Description: SUMMARY Temporal variations in the elastic behaviour of the Earth’s crust can be monitored through the analysis of the Earth’s seismic response and its evolution with time. This kind of analysis is particularly interesting when combined with the reconstruction of seismic Green’s functions from the cross-correlation of ambient seismic noise, which circumvents the limitations imposed by a dependence on the occurrence of seismic events. In fact, because seismic noise is recorded continuously and does not depend on earthquake sources, these cross-correlation functions can be considered analogously to records from continuously repeating doublet sources placed at each station, and can be used to extract observations of variations in seismic velocities. These variations, however, are typically very small: of the order of 0.1 per cent. Such accuracy can be only achieved through the analysis of the full reconstructed waveforms, including later scattered arrivals. We focus on the method known as Moving-Window Cross-Spectral Analysis that has the advantage of operating in the frequency domain, where the bandwidth of coherent signal in the correlation function can be clearly defined. We investigate the sensitivity of this method by applying it to microseismic noise cross-correlations which have been perturbed by small synthetic velocity variations and which have been randomly contaminated. We propose threshold signal-to-noise ratios above which these perturbations can be reliably observed. Such values are a proxy for cross-correlation convergence, and so can be used as a guideline when determining the length of microseismic noise records that are required before they can be used for monitoring with the moving-window cross-spectral technique.

Description: SUMMARY The young (〈 5 Ma) Corinth Rift is an ideal natural laboratory to investigate rift deformation mechanisms by comparing extension rates determined by various methods spanning different depth and time ranges. Corinth Rift geodetic extension rates averaged over 5–100 yr have been interpreted to increase from ∼5 mm yr –1 or less in the east to 〉10–15 mm yr –1 in the west. We quantify total upper-crust and whole-crust extension on three profiles across the Corinth Rift. Whole-crust extension is greater across the central rift (∼11–21 km) than across the western part of the rift (∼5–13 km). This correlates with the overall rift morphology, which shows maximum basement subsidence, sediment accumulation, rift width and greatest summed Late Quaternary fault displacements in the central basin, but contrasts with the pattern of geodetic extension rates which are greater to the west of the central basin. The E–W increase in strain rates interpreted from geodetic data cannot have persisted over rift history to produce the observed rift morphology. We suggest the discrepancy between short-term and long-term extension patterns is related to shifts in the loci of maximum extension due to fault growth and linkage during Corinth Rift history, and is likely a characteristic of rift development in general. Total upper-crust and whole-crust extension estimates in the western rift, where extension estimates are best constrained, are within error. We propose that uniform pure-shear extension is a viable extension mechanism in the western rift and crustal extension estimates do not require the existence of a major active N–S dipping detachment fault.

Description: SUMMARY This paper describes an innovative method of volcano deformation measurements, applied to camera images taken from the 2004–2008 eruption period at Mount St. Helens. Dome growth was thought to be characterized by sustained, near-linear rates of a solid dacite plug. Through spatial digital image correlation (DIC) analysis of the camera images, new evidences arise that the deformation and strain rate of the spine was more complex. DIC yielded cumulative and incremental displacements, strain and shear planes at decimetre resolution. It was found that dome extrusion rates are highly non-linear, decelerating prior to partial collapse, followed by a pronounced dome extrusion increase and direction change. Associated processes have been identified through DIC, such as shallow landslides and reworking of talus apron material. The work highlights the strengths of camera strain monitoring, and illustrates that dome growth and collapse is a very dynamic process complexly interplaying with the surrounding.

Description: SUMMARY Direct detection of Rayleigh-wave azimuthal anisotropy is reported by applying an array analysis to broad-band seismic data in Southern California, USA. Our approach has excellent resolution for frequencies between 30 and 60 mHz and good resolution between 10 and 30 mHz. Limitation from array size limits accuracy below 10 mHz and complicated wave propagation effects lead to difficulty above 60 mHz. Between 30 and 60 mHz, azimuthal anisotropy of Rayleigh-wave phase velocity is detected cleanly. Phase velocity results show that the 2θ components dominate the 4θ components. Strength of anisotropy hovers around 1 per cent for the 2θ components with standard error ( 1 σ ) of about 0.4 per cent. Fast axes orientations show stable orientation in the direction 110° clockwise from north and its opposite direction 290° . Depth sensitivity kernels for P -wave and S -wave anisotropy indicate that anisotropy is confined to depths in the upper 100 km and may even be to shallower depths. We speculate that the fast axes alignment may be associated with a strong shearing process over the thickness of lithosphere due to obliquely collisional motions between the Pacific Plate and the North America Plate in the region.

Description: SUMMARY Zhou & Greenhalgh have recently presented an application of the Gaussian quadrature grid to seismic modelling in which the authors propose a meshing scheme that partitions the domain independently of the discontinuities in the media parameters. This comment aims to clarify the implications that this strategy has on the accuracy.

Description: SUMMARY We obtained the crustal thickness ( H ) and average Vp / Vs ratio (κ) for the central (CNCC) and western North China Craton (WNCC) by H –κ stacking of receiver functions. Our results show that H and κ varies significantly but similarly in the northern and southern CNCC, reflecting possible effects of the Phanerozoic cratonotic reactivation on the region. Of the WNCC, the south is featured by κ values typical for Precambrian shields (∼1.77) and variations of H mirroring surface topography; the north presents large fluctuations of κ (1.68–1.93) and a crustal thickening (up to 50 km) uncorrelated with topography. Such N–S differences were thought to be associated with a ∼1.95-Ga suturing event occurred in the north with little effects on the south. Our observations suggest that distinct structural heterogeneities of the NCC not only reflect diverse regional tectonics in Phanerozoic, but also provide evidence for long-term survival of crustal fabrics in stable continental interior.

Description: SUMMARY The aim of this paper is to present an alternative method for the analysis of resistivity data. The methodology was developed during a study to evaluate if electrical resistivity can be used as a tool for analysing subsurface gas dynamics and gas emissions from landfills. The main assumption of this study was that variations in time of resistivity data correspond to variations in the relative amount of gas and water in the soil pores. Field measurements of electrical resistivity, static chamber gas flux and weather data were collected at a landfill in Helsingborg, Sweden. The resistivity survey arrangement consisted of nine lines each with 21 electrodes in an investigation area of 16 ×20 m. The ABEM Lund Imaging System provided vertical and horizontal resistivity profiles every second hour. The data were inverted in Res3Dinv using L 1 -norm–based optimization method with a standard least-squares formulation. Each horizontal soil layer was then represented as a linear interpolated raster model. Different areas underneath the gas flux measurement points were defined in the resistivity model of the uppermost soil layer, and the vertical extension of the zones could be followed at greater depths in deeper layer models. The average resistivity values of the defined areas were calculated and plotted on a time axis, to provide graphs of the variation in resistivity with time in a specific section of the ground. Residual variation of resistivity was calculated by subtracting the resistivity variations caused by the diurnal temperature variations from the measured resistivity data. The resulting residual resistivity graphs were compared with field data of soil moisture, precipitation, soil temperature and methane flux. The results of the study were qualitative, but promising indications of relationships between electrical resistivity and variations in the relative amount of gas and water in the soil pores were found. Even though more research and better data quality is necessary for verification of the results presented here, we conclude that this alternative methodology of working with resistivity data seems to be a valuable and flexible tool for this application.

Description: SUMMARY We analysed high-frequency body waves of local earthquakes to image the damage zone of the Calico fault in the eastern California shear zone. We used generalized ray theory and finite difference methods to compute synthetic seismograms for a low-velocity fault zone (FZ) to model the direct and FZ-reflected P and S traveltimes of local earthquakes recorded by a temporary array across the fault. The low velocity zone boundaries were determined by apparent traveltime delays across the fault. The velocity contrast between the fault zone and host rock was constrained by the traveltime delays of P and S waves and differential traveltimes between the direct and FZ-reflected waves. The dip and depth extent of the low velocity zone were constrained by a systematic analysis of direct P traveltimes of events on both sides of the fault. We found that the Calico fault has a ∼1.3-km-wide low velocity zone in which the P - and S -wave velocity decreased 40 and 50 per cent, respectively, with respect to the host rock. The low velocity zone dips 70° northeast and extends 3 km in depth.

Description: SUMMARY High-grade metamorphic rocks of the upper amphibolite to granulite facies in the Higher Himalayan crystalline (HHC) were studied for their appropriateness for palaeomagnetic investigations. Sampling and fieldwork was accomplished on a south–north transect at ∼87°E in the Khumbu region south of Everest. Paragenesis of iron-sulphides and oxides was found to be variable due to different source rocks and metamorphic conditions in this area. Unblocking temperature ranges, isothermal remanence acquisition, low-temperature magnetic properties and microscopic observations confirm the presence of pyrrhotite and magnetite. Two remanence components were separated with unblocking temperature intervals of 250–330 °C and 370–550 °C, which can be attributed to characteristic remanent magnetizations (ChRM) of monoclinic pyrrhotite (ChRM pyr ) and magnetite (ChRM mag ), respectively. Mainly the ChRM pyr showed a consistent demagnetization behaviour, probably residing in pyrrhotite particles within the single domain to pseudo-single domain range. Normal and reverse polarities were observed, partly co-existing within the unblocking spectrum of single specimens; this (i) supports a thermoremanent (TRM) origin of the ChRM pyr and (ii) indicates that the time span of remanence acquisition was long enough to average out palaeosecular variation. The age of the ChRM pyr can be related to the last metamorphic cooling event at the Early to Late Miocene boundary (16 Ma). The ChRM mag is probably a secondary chemical or thermochemical remanence acquired by hydrothermal processes during retrogression. ChRM pyr and ChRM mag directions are similar suggesting that pyrrhotite and magnetite were magnetized approximately contemporaneously. Separation of the sampling area into three sectors reveals a south–north trend with (i) variable block rotations in the southernmost part close to the Main Central Thrust (five sites), (ii) no rotation in the area between Lukla and Namche (10 sites) and (iii) a significant clockwise rotation ( N = 5 sites, D / I = 42.3°/29.4°, k = 20.7, α 95 = 17.2°) along the Imja Khola north of Namche. We interpret the clockwise rotation as an apparent rotation due to tilting by crustal doming in the Everest region. This doming obviously occurred north of a fault zone located at around Namche and was not affecting the region south of it. The clockwise sense of the apparent vertical-axis rotation indicates that the area north of Namche is located at the western side of the dome structure where westward tilting around a subhorizontal axis occurred (best fit: tilt direction 277°, tilt angle 51°).

Description: SUMMARY The deep structure of the Upper Rhine Graben (URG), a continental rift in SW Germany and E France, is still poorly known. This deficit impedes a full understanding of the geodynamic evolution of this prominent rift. We study the lithosphere–asthenosphere structure using teleseismic waveforms obtained from the passive broad-band TIMO project across the central URG. The recovered, crust-corrected traveltime residuals relative to the iasp91 earth model are tiny (mostly less than 0.2–0.3 s). The average measured slowness (〈1 s deg –1 ) and backazimuth (〈5°) deviations are also very small and do not show any systematic wave front anomalies. These observed perturbation values are smaller than expected ones from synthetic 3-D ray tracing modelling with anomalies exceeding 2–3 per cent seismic velocity in the mantle. Thus there is no significant hint for any deep-seated anomaly such as a mantle cushion, etc. This result means that the rifting process did not leave behind a lower lithospheric signature, which could be clearly verified with high-resolution teleseismic experiments. The only significant traveltime perturbation at the central URG is located at its western side in the upper crust around a known geothermal anomaly. The upper crustal seismic anomaly with traveltime delays of 0.2–0.3 s cannot be explained with increased temperature alone. It is possibly related to a zone of highly altered granite. In the west of our network a traveltime anomaly (0.6–0.7 s delay) related with the Eifel plume is confirmed by the TIMO data set.

Description: Mid-ocean ridge basalts (MORB) from the Arctic Ocean have been significantly less studied than those from other oceans. The Arctic ridges (Gakkel Ridge and Lena Trough) are ultraslow-spreading ridges with low melt productivity and are thus the best locations to investigate mantle heterogeneity. We report the major and trace element and Sr–Nd–Pb–Hf isotope compositions of basalts generated along the Lena Trough and the westernmost part of the Gakkel Ridge in the Arctic Ocean. Basalts from the northern Lena Trough and westernmost Gakkel Ridge (NLT–WGR) have compositions close to normal MORB. The geochemical composition of the NLT–WGR lavas confirms a binary mixing model involving melts from a depleted MORB mantle source and a Spitsbergen amphibole-bearing subcontinental lithospheric mantle (SCLM) source. In contrast, in the central part of the Lena Trough (CLT), the basalts are alkalic with relatively high Mg-number (60–65), high SiO 2 (51·0–51·6 wt %), Al 2 O 3 (18·1–18·4 wt %), Na 2 O (4·0–4·2 wt %), K 2 O (1·0–1·6 wt %), K 2 O/TiO 2 (0·6–0·9) and (La/Sm) PM (1·4–1·8), and low FeO (6·5–6·8 wt %) contents. These basalts display isotope variations with 87 Sr/ 86 Sr ranging from 0·70361 to 0·70390, 143 Nd/ 144 Nd from 0·51283 to 0·51290 ( Nd + 3·7 to +5·2), 176 Hf/ 177 Hf from 0·28313 to 0·28322 ( Hf + 11·6 to +14·9) and 206 Pb/ 204 Pb from 17·752 to 17·884, 207 Pb/ 204 Pb from 15·410 to 15·423 and 208 Pb/ 204 Pb from 37·544 to 37·670. These isotope compositions clearly distinguish the CLT lavas from those generated along the Gakkel Ridge. For the CLT lavas, involvement of a phlogopite- or amphibole- and (possibly garnet)-bearing SCLM source component is proposed. Owing to SCLM contamination along the entire length of the Lena Trough, we classify the Lena Trough as an ocean–continent transition boundary. Magmatism similar to that observed in the Lena Trough would be expected to occur wherever ocean spreading initiates.

Description: SUMMARY Recent revisions to the satellite-derived vertical gravity gradient (VGG) data reveal more detail of the ocean bottom and have allowed us to develop a non-linear inversion method to detect seamounts in VGG data. We approximate VGG anomalies over seamounts as sums of individual, partially overlapping, elliptical polynomial functions, which allows us to form a non-linear inverse problem by fitting the polynomial model to the observations. Model parameters for a potential seamount include geographical location, peak VGG amplitude, major and minor axes of the elliptical base, and the azimuth of the major axis. The non-linear inversion is very sensitive to the initial values for the location and amplitude; hence, they are constrained by the centre and amplitude of the uppermost contours obtained with a 1-Eötvös contour interval. With these initial conditions from contouring, we execute a step-wise and fully automated inversion and obtain optimal model estimates for potential seamounts; these are statistically evaluated for significance using the Akaike Information Criterion and  F  tests. A logarithmic barrier technique is applied to ensure positivity of all seamount amplitudes. After automatic and manual inspections of the model parameters we estimate actual heights and basal ellipses of the inspected potential seamounts directly from the predicted bathymetry grid. In this study, we find globally 24 643 potential seamounts ( h  ≥ 0.1 km) that are located away from continental margins; 8458 potential seamounts are taller than 1 km. Although our global estimate is significantly lower than predictions from previous studies, a first-order reconciliation of the size-frequency statistics obtained from those studies reveals that the previous counts are systematically overestimated. Because of the ambiguity of gravity signals due to small seamounts of  h  〈 1 km and the overlap with abyssal hills, we estimate the global seamount census to lie in the 40 000–55 000 range. The seamount data from this study are accessible from http://www.soest.hawaii.edu/PT/SMTS .

Description: SUMMARY The growth of the Earthscope/USArray Transportable Array (TA) has prompted the development of new methods in surface wave tomography that track phase fronts across the array and map the traveltime field for each earthquake or for each station from ambient noise. Directionally dependent phase velocities are determined locally by measuring the gradient of the observed traveltime field without the performance of a formal inversion. This method is based on the eikonal equation and is, therefore, referred to as ‘eikonal tomography’. Eikonal tomography is a bent-ray theoretic method, but does not account for finite frequency effects such as wave interference, wave front healing, or backward scattering. This shortcoming potentially may lead to both systematic bias and random error in the phase velocity measurements, which would be particularly important at the longer periods studied with earthquakes. It is shown here that eikonal tomography can be improved by using amplitude measurements to construct a geographically localized correction via the Helmholtz equation. This procedure should be thought of as a finite-frequency correction that does not require the construction of finite-frequency kernels and is referred to as ‘Helmholtz tomography’. We demonstrate the method with Rayleigh wave measurements following earthquakes between periods of 30 and 100 s in the western US using data from the TA. With Helmholtz tomography at long periods (〉50 s): (1) resolution of small-scale isotropic structures, which correspond to known geological features, is improved, (2) uncertainties in the isotropic phase velocity maps are reduced, (3) the directionally dependent phase velocity measurements are less scattered, (4) spurious 1-psi azimuthal anisotropy near significant isotropic structural contrasts is reduced, and (5) estimates of 2-psi anisotropy are better correlated across periods.

Description: SUMMARY Geomagnetic polarity transitions may be significantly more complex than are currently depicted in many sedimentary and lava-flow records. By splicing together paleomagnetic results from earlier studies at Steens Mountain with those from three newly studied sections of Oregon Plateau flood basalts at Catlow Peak and Poker Jim Ridge 70–90 km to the southeast and west, respectively, we provide support for this interpretation with the most detailed account of a magnetic field reversal yet observed in volcanic rocks. Forty-five new distinguishable transitional (T) directions together with 30 earlier ones reveal a much more complex and detailed record of the 16.7 Ma reversed (R)-to-normal (N) polarity transition that marks the end of Chron C5Cr. Compared to the earlier R-T-N-T-N reversal record, the new record can be described as R-T-N-T-N-T-R-T-N. The composite record confirms earlier features, adds new west and up directions and an entire large N-T-R-T segment to the path, and fills in directions on the path between earlier directional jumps. Persistent virtual geomagnetic pole (VGP) clusters and separate VGPs have a preference for previously described longitudinal bands from transition study compilations, which suggests the presence of features at the core–mantle boundary that influence the flow of core fluid and distribution of magnetic flux. Overall the record is consistent with the generalization that VGP paths vary greatly from reversal to reversal and depend on the location of the observer. Rates of secular variation confirm that the flows comprising these sections were erupted rapidly, with maximum rates estimated to be 85–120 m ka −1 at Catlow and 130–195 m ka −1 at Poker Jim South. Paleomagnetic poles from other studies are combined with 32 non-transitional poles found here to give a clockwise rotation of the Oregon Plateau of 11.4°± 5.6° with respect to the younger Columbia River Basalt Group flows to the north and 14.5°± 4.6° with respect to cratonic North America (95 per cent confidence interval).

Description: SUMMARY The principal aim of performing a survey or experiment is to maximize the desired information within a data set by minimizing the post-survey uncertainty on the ranges of the model parameter values. Using Bayesian, non-linear, statistical experimental design (SED) methods we show how industrial scale amplitude variations with offset (AVO) surveys can be constructed to maximize the information content contained in AVO crossplots, the principal source of petrophysical information from seismic surveys. The design method allows offset dependent errors, previously not allowed in non-linear geoscientific SED methods. The method is applied to a single common-midpoint gather. The results show that the optimal design is highly dependent on the ranges of the model parameter values when a low number of receivers is being used, but that a single optimal design exists for the complete range of parameters once the number of receivers is increased above a threshold value. However, when acquisition and processing costs are considered we find that a design with constant spatial receiver separation survey becomes close to optimal. This explains why regularly-spaced, 2-D seismic surveys have performed so well historically, not only from the point of view of noise attenuation and imaging in which homogeneous data coverage confers distinct advantages, but also to provide data to constrain subsurface petrophysical information.

Description: SUMMARY Tsunami generated by the Late Bronze Age (LBA) eruption of Thera were simulated using synthetic tide records produced for selected nearshore (∼20 m depths) sites of northern Crete, the Cyclades Islands, SW Turkey and Sicily. Inundation distances inland were also calculated along northern Crete. Modelling was performed by incorporating fully non-linear Boussinesq wave theory with two tsunamigenic mechanisms. The first involved the entry of pyroclastic flows into the sea, assuming a thick (55 m; 30 km 3 ) flow entering the sea along the south coast of Thera in three different directions all directed towards northern Crete, then a thin pyroclastic flow (1 m; 1.2 km 3 ) entering the sea along the north coast of Thera directed towards the Cyclades Islands. Flows were modelled as a solid block that slowly decelerates along a horizontal surface. The second mechanism assumed caldera collapse, of 19 km 3 and 34 km 3 modelled as a dynamic landslide producing rapid vertical displacements. Calculated nearshore wave amplitudes varied from a few metres to 28 m along northern Crete from pyroclastic flows, and up to 19 m from caldera collapse (34 km 3 volume). Inundation distances on Crete were 250–450 m. Waves produced by pyroclastic flows were highly focused, however, as a function of sea entry direction. Smaller volume pyroclastic flows produced nearshore wave amplitudes up to 4 m in the Cyclades islands north of Thera. Wave amplitudes in the Cyclades from smaller volume caldera collapse (19 km 3 ) were up to 24 m, whereas in SW Turkey were as low as 2.1 and 0.8 m (Didim and Fethye where LBA tsunami deposits have been found). Wave amplitudes for the larger volume caldera collapse (34 km 3 ) were generally 2.5–3 times larger than those generated by the smaller volume collapse (19 km 3 ). These results provide estimates for understanding possible consequences of tsunami impact in LBA coastal zones, thus providing criteria at archaeological sites for detecting inundation damage, as well as for contemporary hazard assessment; they also provide additional criteria for deciphering homogenite layers in the abyssal stratigraphy of the Ionian and eastern Mediterranean Seas.

Description: SUMMARY The first archaeomagnetic secular variation (SV) curves for the whole Southern Balkan Peninsula are presented. These are based on all data within a 700 km circle centred at Thessaloniki (40.60 o N, 23.00 o E). This data set consists of 325 directional and 625 intensity data mainly from Greece, Bulgaria, Serbia and southern Hungary. Some data from southern Italy are also included. The sliding moving window technique, was used to calculate a continuous SV curve for intensity while the directional SV curves were calculated using the bivariate extension of the Fisher statistics. These curves are well constrained and clearly show the main features of the geomagnetic field variation in this region during the last eight millennia. Comparisons with the predictions of the SCHA.DIF.3K and SCHA.DIF.8K regional and the CALS7K.2 and ARCH3K.1 global geomagnetic field models show a good agreement for the last 3000 years but differences for older times. The Balkan SV curves identify several rapid changes of the geomagnetic field in eastern Europe and can be used as reference curves for archaeomagnetic dating in the Balkan Peninsula.

Description: SUMMARY Receiver function analysis reveals a 7 km step-like change in crustal thickness across the Taranaki–Ruapehu Line (TR-line) of North Island, New Zealand. The TR-line runs east–west between the active andesite volcanoes of Mt Taranaki and Ruapehu and marks the southern-most extent of subduction zone volcanism in New Zealand. North of the TR-line receiver functions show a strong and sharp P -to- S ( Ps ) conversion at 25 ± 1.5 km depth, which is interpreted as a shallow Moho. At the TR-line the Moho Ps conversion deepens across a step to ∼32 km depth and weakens. Further south the Moho deepens to 〉35 km. Most of the 7 km step-change in crustal thickness occurs over a lateral distance of ∼8 km, yet there is little surface or topographic manifestation of the line. Given Fresnel zone considerations, the dip of the Moho offset could vary between 90° and 45°. Gravity, seismic attenuation and electrical data all show that the TR-line is not only a step in crustal thickness but also a profound lithospheric boundary as mantle properties, such as attenuation ( Q p −1 ), implied density, and electrical resistivity change abruptly across the line. An east–west oriented cluster of earthquakes with hypocentral depths of 20–40 km is centred on the Moho step. We propose that the Moho step, the earthquakes and the rapid change in mantle properties across the TR boundary are causally related. Processes that could be responsible for the phenomena described here include the rapid removal of mantle lithosphere and lower crust to the north of the TR-line.

Description: SUMMARY We investigate the slip distribution of the 2008 May 12 Wenchuan  M s 7.9 earthquake using GPS data and InSAR measurements under a linear inversion scheme, with emphasis on the effect of three factors, including constraint on rake, different discretizations, and layered elastic model. Within our inversion parameterization context, we find the most influential factor would be constraint on rake. Without constraint on rake, the slip model seems physically wrong under the depth of 15 km, due to the limited depth resolution of the geodetic data used, especially the one orbit of InSAR measurements. Thus it is necessary to add  a priori  to the slip rake to obtain a reasonable fault source model. Different discretizations of the subfault patches have a notable impact on the slip distribution. Also, the layered elastic model predicts more slip at depth than does the half-space model, by about 15–20 per cent. The characteristics of slip distribution established through our inversions include some points as follows: (1) the most reliable results would be expected at depth range of 0–15 km; (2) three peak-slip asperities are inverted, at Yingxiu county (near epicentre), Yuejiashan county, and Beichuan county, respectively; (3) the inverted rake along the Yingxiu-Beichuan fault changes from dominantly thrusting motion at the southwest segments to dominant right-lateral or even pure right-lateral strike slip at the northeast segments, and (4) only thrust slip is occurred on the Guanxian-Jiangyou fault.

Description: SUMMARY Recordings from a temporary seismic network in the Rwenzori region of the East African Rift show seven earthquakes at a depth range between 53 and 60 km. Evidence from receiver functions indicates a crustal thickness of about 32 km in this region suggesting that the earthquakes are located well within the mantle beneath the rift. The occurrence of faulting at this depth is of relevance for models of continental-rift initiation and related deep magmatic processes.

Description: SUMMARY We present an analysis on thermal properties, density and porosity of clastic, chemical/biochemical and intrasedimentary volcanic rocks collected from petroleum exploration wells of the Po Basin (Northern Italy). Moreover, we investigate the applicability of the Hashin–Shtrikman's model for a mineral aggregate in combination with the Zimmerman's model that takes pore shape into account, to calculate the bulk thermal conductivity. In case of macroscopically isotropic rocks, deviations between predicted and measured values range from −2.2 per cent to 6.9 per cent, and significantly decrease if a proper pore aspect ratio is chosen. Regarding the volumetric heat capacity, approximate estimates were obtained by means of a weighted average of the volumetric heat capacity of the mineral grains and that of the pore-filling water. The differences between the computed and measured values range from –6.2 per cent to 4.9 per cent and, on average, the computed volumetric heat capacity is lower by 1.6 per cent. The water loss during compaction and the temperature increase with depth are main factors controlling thermal properties. An anisotropy effect occurs in the case of rocks rich in sheet silicates. Due to rotation of these minerals, the vertical thermal conductivity of sheet silicates decreases exponentially with burial depth from 2.13 W m −1 K −1 at the surface to 0.52 W m −1 K −1 at 4.5 km. The laboratory data allow the formulation of compaction curves for the different sedimentary rock types. Examples of estimations of in situ vertical thermal conductivity and heat flow are finally given for two petroleum wells at which lithostratigraphic information is known in good detail.

Description: SUMMARY We present forward and adjoint spectral-element simulations of coupled acoustic and (an)elastic seismic wave propagation on fully unstructured hexahedral meshes. Simulations benefit from recent advances in hexahedral meshing, load balancing and software optimization. Meshing may be accomplished using a mesh generation tool kit such as CUBIT, and load balancing is facilitated by graph partitioning based on the SCOTCH library. Coupling between fluid and solid regions is incorporated in a straightforward fashion using domain decomposition. Topography, bathymetry and Moho undulations may be readily included in the mesh, and physical dispersion and attenuation associated with anelasticity are accounted for using a series of standard linear solids. Finite-frequency Fréchet derivatives are calculated using adjoint methods in both fluid and solid domains. The software is benchmarked for a layercake model. We present various examples of fully unstructured meshes, snapshots of wavefields and finite-frequency kernels generated by Version 2.0 ‘Sesame’ of our widely used open source spectral-element package SPECFEM3D.

Description: SUMMARY Allègre et al. recently presented new experimental data regarding the dependence of the streaming potential coupling coefficient with the saturation of the water phase. Such experiments are important to model the self-potential response associated with the flow of water in the vadose zone and the electroseismic/seismoelectric conversions in unsaturated porous media. However, the approach used to interpret the data is questionable and the conclusions reached by Allègre et al. likely incorrect

Description: SUMMARY Revil and Linde recently commented our paper concerning streaming potential (SP) measurements in unsaturated sand during drainage experiments. This comment suggests that the approach used to infer SP coefficients was inappropriate for unsaturated conditions, and therefore yielded wrong conclusions and ‘unphysical’ results regarding the behaviour of the relative SP coefficient. This reply argues that even if in Allègre et al . we neglected some secondary electrokinetic sources, the resulting conclusions are still representative of the behaviour of the true SP coefficient, and that the remarks of Revil & Linde arose from a misunderstanding of the drainage experiment conditions. We also find support for our results from a comparison between our observations and previous experimental studies.

Description: SUMMARY A small-aperture (40 m) short-period seismic array was installed during four months on the Vercors massif (Western French Alps) at the top of a limestone column which collapsed one month and a half later. During this monitoring period, 193 seismic events were recorded by the seven seismometers of the array. Signal analysis yielded three main types of nearby seismic events to be identified from temporal and spectral characteristics: microearthquakes (single or multiple events), individual block falls and rock falls. It turned out that 60 per cent of the 193 events were classified as microearthquakes, exhibiting distinct  P  and  S  waves, while 17 per cent of these events remained unclassified. Out of the microearthquakes, 40 events with a good signal-to-noise ratio were selected and processed.  P-  and  S -wave traveltimes were picked on the records and the inferred hypocentral distances agree with the two zones of the scarp exhibiting fresh ruptures after the fall. Polarization analysis of the 3-C records, along with numerical simulations, allowed discriminating between the two possible rupture mechanisms (toppling and sliding). Shear rupture (sliding) was the predominant mode in the lower part of the column whereas traction rupture (toppling) affected the upper part. Finally, the comparison between the ground motions recorded on the column and on the rock mass showed a systematic amplification on the column. Signal processing and numerical modelling both suggest that this amplification resulted from the excitation of the natural frequencies of the column and is particularly high (〉3) for microearthquakes occurring at the column-to-mass interface.