ALBERT

Description: Inverted models of the deep mantle show a decorrelation between maps of shear VS and compressional VP wave velocities, an anti-correlation between the bulk sound velocity V$\phi$ and VS and a much larger variability of VS with respect to VP, expressed by large values of the ratio of their relative lateral variations. We carried out synthetic tests to verify if these features could be artifacts, explained by limits in tomographic resolution: synthetic data are calculated for an “input” model, and linearly inverted, as in tomography, to find an “output” model. Comparing the values of the aforementioned parameters for two different chemically homogeneous input models with the associated reconstructed output ones, we found that artifacts caused by realistic data noise and the nonuniform distribution of seismic sources and stations over the globe are not sufficient to introduce the features previously described. We confirm that compositional effects are required to explain them.

Description: [1]  The underestimation of the size of recent megathrust earthquakes illustrates our limited understanding of their spatiotemporal occurrence and governing physics. To unravel their relation to associated subduction dynamics and long-term deformation, we developed a 2D continuum viscoelastoplastic model that uses an Eulerian-Lagrangian finite difference framework with similar on- and off-fault physics. We extend the validation of this numerical tool to a realistic subduction zone setting that resembles Southern Chile. The resulting quasi-periodic pattern of quasi-characteristic M8-M9 megathrust events compares quantitatively with observed recurrence and earthquake source parameters, albeit at very slow coseismic speeds. Without any data fitting, surface displacements agree with GPS data recorded before and during the 2010 M8.8 Maule earthquake, including the presence of a second-order flexural bulge. These surface displacements show cycle-to-cycle variations of slip deficits, which overall accommodate ~5% of permanent internal shortening. We find that thermally (and stress) driven creep governs a spontaneous conditionally stable downdip transition zone between temperatures of ~350 °C and ~450 °C. Ruptures initiate above it (and below the fore-arc Moho), propagate within it, interspersed by small intermittent events, and arrest below it as ductile shearing relaxes stresses. Ruptures typically propagate upward along lithological boundaries and widen as pressures drop. The main thrust is constrained to be weak due to fluid-induced weakening required to sustain regular subduction and to generate events with natural characteristics (fluid pressures of ~75-99% of solid pressures). The agreement with a range of seismological, geodetic and geological observations demonstrates the validity and strength of this physically consistent seismo-thermo-mechanical approach.

Description: [1]  Ambient-noise seismology is of great relevance to high-resolution crustal imaging, thanks to the unprecedented dense data coverage it affords in regions of little seismicity. Under the assumption of uniformly distributed noise sources, it has been used to extract the Green’s function between two receivers. We determine the imprint of this assumption by means of wave propagation and adjoint methods in realistic 3D Earth models. In this context, we quantify the sensitivity of ambient-noise cross correlations from central Europe with respect to noise-source locations and shear wavespeed structure. We use ambient noise recorded over one year at 196 stations, resulting in a database of 864 cross-correlations. Our mesh is built upon a combined crustal and 3D tomographic model. We simulate synthetic ambient-noise cross-correlations in different frequency bands using a 3D spectral-element method. Traveltime cross-correlation measurements in these different frequency bands define the misfit between synthetics and observations as a basis to compute sensitivity kernels using the adjoint method. We perform a comprehensive analysis varying geographic station and noise-source distributions around the European seas. The deterministic sensitivity analysis allows for estimating where the starting crustal model shows better accordance with our dataset and gain insight into the distribution of noise sources in the European region. This highlights the potential importance to consider localized noise distributions for tomographic imaging and forms the basis of a tomographic inversion in which the distribution of noise sources may be treated as a free parameter similar to earthquake tomography.

Description: [1]  We present a new tomographic model of radially anisotropic shear-velocity variations in the Earth's mantle based on a new compilation of previously published datasets and a variable block parameterization, adapted to local ray-path density. We employ ray-theoretical sensitivity functions to relate surface-wave and body wave data with radially anisotropic velocity perturbations. Our database includes surface-wave phase delays from fundamental modes up to the 6 th overtone, measured at periodsbetween 25 and 350 s, as well as cross-correlation traveltimes of major body-wave phases. Prior to inversion we apply a crustal correction using the crustal model CRUST2.0 [ Bassin et al., 2000] and we account for azimuthal anisotropy in the upper mantle using ray-theoretical corrections based on a global model of azimuthal anisotropy. While being well correlated with earlier models at long spatial wavelength, our preferred solution, savani , additionally delineates a number of previously unidentified structures, due to its improved resolution in areas of dense coverage. This is because the density of the inverse grid ranges between 1.25 ° in well sampled to 5 ° in poorly sampled regions, allowing us to resolve regional structure better than it is typically the case in global S -wave tomography. Important features of our model include: (i) A distinct ocean-continent anisotropic signature in the uppermost mantle; (ii) an oceanic peak in above average ξ  〉 1 which is shallower than in previous models and thus in better agreement with estimates of lithosphere thickness; (iii) a long wavelength pattern of ξ  〈 1 associated with the large low-shear-velocity provinces in the lowermost mantle. Furthermore we conduct a comprehensive comparison between various published isotropic and anisotropic upper- and whole-mantle tomographic models to identify regions in which anisotropic images have reached a stage of maturity, comparable to that of their isotropic counterparts.

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: The effect of network density and geometric distribution on kinematic non-linear source inversion is investigated by inverting synthetic ground motions from a buried strike-slip fault ( M w 6.5), that have been generated by dynamic spontaneous rupture modelling. For the inversion, we use a physics-based regularized Yoffe function as slip velocity function. We test three different cases of station network geometry: (i) single station, varying azimuth and epicentral distance; (ii) multistation circular configurations, that is stations at similar distances from the fault, and regularly spaced around the fault; (iii) irregular multistation configurations using different numbers of stations. Our results show: (1) single station tests suggest that it may be possible to obtain a relatively good source model even using a single station. The best source model using a single station is obtained with stations at which amplitude ratios between three components are not large. We infer that both azimuthal angle and source-to-station distance play an important role in the design of optimal seismic network for source inversion. (2) Multistation tests show that the quality of the inverted source systematically correlates neither with the number of stations, nor with waveform misfit. (3) Waveform misfit has a direct correlation with the number of stations, resulting in overfitting the observed data without any systematic improvement of the source. It suggests that the best source model is not necessarily derived from the model with minimum waveform misfit. (4) A seismic network with a small number of well-spaced stations around the fault may be sufficient to obtain acceptable source inversion.

Description: We present a time-independent gridded earthquake rate forecast for the European region including Turkey. The spatial component of our model is based on kernel density estimation techniques, which we applied to both past earthquake locations and fault moment release on mapped crustal faults and subduction zone interfaces with assigned slip rates. Our forecast relies on the assumption that the locations of past seismicity is a good guide to future seismicity, and that future large-magnitude events occur more likely in the vicinity of known faults. We show that the optimal weighted sum of the corresponding two spatial densities depends on the magnitude range considered. The kernel bandwidths and density weighting function are optimized using retrospective likelihood-based forecast experiments. We computed earthquake activity rates ( a - and b -value) of the truncated Gutenberg–Richter distribution separately for crustal and subduction seismicity based on a maximum likelihood approach that considers the spatial and temporal completeness history of the catalogue. The final annual rate of our forecast is purely driven by the maximum likelihood fit of activity rates to the catalogue data, whereas its spatial component incorporates contributions from both earthquake and fault moment-rate densities. Our model constitutes one branch of the earthquake source model logic tree of the 2013 European seismic hazard model released by the EU-FP7 project ‘Seismic HAzard haRmonization in Europe’ (SHARE) and contributes to the assessment of epistemic uncertainties in earthquake activity rates. We performed retrospective and pseudo-prospective likelihood consistency tests to underline the reliability of our model and SHARE's area source model (ASM) using the testing algorithms applied in the collaboratory for the study of earthquake predictability (CSEP). We comparatively tested our model's forecasting skill against the ASM and find a statistically significant better performance for testing periods of 10–20 yr. The testing results suggest that our model is a viable candidate model to serve for long-term forecasting on timescales of years to decades for the European region.

Description: The topography of the core–mantle boundary (CMB) is directly linked to the dynamics of both the mantle and the outer core, although it is poorly constrained and understood. Recent studies have produced topography models with mutual agreement up to degree 2. A broad-band waveform inversion strategy is introduced and applied here, with relatively low computational cost and based on a first-order Born approximation. Its performance is validated using synthetic waveforms calculated in theoretical earth models that include different topography patterns with varying lateral wavelengths, from 600 to 2500 km, and magnitudes (~10 km peak-to-peak). The source–receiver geometry focuses mainly on the P diff , PKP , PcP and ScS phases. The results show that PKP branches, PcP and ScS generally perform well and in a similar fashion, while P diff yields unsatisfactory results. We investigate also how 3-D mantle correction influences the output models, and find that despite the disturbance introduced, the models recovered do not appear to be biased, provided that the 3-D model is correct. Using cross-correlated traveltimes, we derive new topography models from both P and S waves. The static corrections used to remove the mantle effect are likely to affect the inversion, compromising the agreement between models derived from P and S data. By modelling traveltime residuals starting from sensitivity kernels, we show how the simultaneous use of volumetric and boundary kernels can reduce the bias coming from mantle structures. The joint inversion approach should be the only reliable method to invert for CMB topography using absolute cross-correlation traveltimes.

Description: We present a time-independent gridded earthquake rate forecast for the European region including Turkey. The spatial component of our model is based on kernel density estimation techniques, which we applied to both past earthquake locations and fault moment release on mapped crustal faults and subduction zone interfaces with assigned slip rates. Our forecast relies on the assumption that the locations of past seismicity is a good guide to future seismicity, and that future large-magnitude events occur more likely in the vicinity of known faults. We show that the optimal weighted sum of the corresponding two spatial densities depends on the magnitude range considered. The kernel bandwidths and density weighting function are optimized using retrospective likelihood-based forecast experiments. We computed earthquake activity rates ( a - and b -value) of the truncated Gutenberg–Richter distribution separately for crustal and subduction seismicity based on a maximum likelihood approach that considers the spatial and temporal completeness history of the catalogue. The final annual rate of our forecast is purely driven by the maximum likelihood fit of activity rates to the catalogue data, whereas its spatial component incorporates contributions from both earthquake and fault moment-rate densities. Our model constitutes one branch of the earthquake source model logic tree of the 2013 European seismic hazard model released by the EU-FP7 project ‘Seismic HAzard haRmonization in Europe’ (SHARE) and contributes to the assessment of epistemic uncertainties in earthquake activity rates. We performed retrospective and pseudo-prospective likelihood consistency tests to underline the reliability of our model and SHARE's area source model (ASM) using the testing algorithms applied in the collaboratory for the study of earthquake predictability (CSEP). We comparatively tested our model's forecasting skill against the ASM and find a statistically significant better performance for testing periods of 10–20 yr. The testing results suggest that our model is a viable candidate model to serve for long-term forecasting on timescales of years to decades for the European region.