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.