ALBERT

Beschreibung: Abstract We present a new methodology for inverting P‐to‐S receiver function (RF) waveforms directly for mantle temperature and composition. This is achieved by interfacing the geophysical inversion with self‐consistent mineral phase equilibria calculations from which rock mineralogy and its elastic properties are predicted as a function of pressure, temperature, and bulk composition. This approach anchors temperatures, composition, seismic properties, and discontinuities that are in mineral physics data, while permitting the simultaneous use of geophysical inverse methods to optimize models of seismic properties to match RF waveforms. Resultant estimates of transition zone (TZ) topography and volumetric seismic velocities are independent of tomographic models usually required for correcting for upper mantle structure. We considered two end‐member compositional models: the equilibrated equilibrium assemblage (EA) and the disequilibrated mechanical mixture (MM) models. Thermal variations were found to influence arrival times of computed RF waveforms, whereas compositional variations affected amplitudes of waves converted at the TZ discontinuities. The robustness of the inversion strategy was tested by performing a set of synthetic inversions in which crustal structure was assumed both fixed and variable. These tests indicate that unaccounted‐for crustal structure strongly affects the retrieval of mantle properties, calling for a two‐step strategy presented herein to simultaneously recover both crustal and mantle parameters. As a proof of concept, the methodology is applied to data from two stations located in the Siberian and East European continental platforms.

Beschreibung: [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.

Beschreibung: [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.

Beschreibung: Abstract Hydraulic injection by the Pohang enhanced geothermal systems (EGS) has been suspected to trigger the 2017 moment magnitude (MW) 5.5 Pohang earthquake in South Korea. The last stimulation experiment in the EGS was conducted only two months before the disaster, which has led to this suspicion. In this study, we conducted a seismic analysis on the earthquakes that have occurred around the EGS site in the past 10 years. The study included the construction of a velocity model, earthquake detection, the determination of hypocenters, magnitudes, focal mechanisms, and stress inversion, and a clustering analysis. No seismic activity was detected near the study area until November 2015 when there was a loss of a large quantity of heavy drilling mud. For three stimulations of a geothermal well, earthquakes sequentially migrated to the southwest along a fault plane, leading to the location of the mainshock. The delineated fault plane crossed the injection well at approximately 3,800 m, which corresponds to the borehole interval of not only the mud loss but also the breakage of the well's casing due to the mainshock rupture. These findings can be treated as empirical evidence for the hypothesis that the 2017 MW 5.5 Pohang earthquake was initiated on a critically stressed fault zone by the anthropogenic activity of fluid injection, consequentially releasing accumulated strain energy via tectonic loading.

Beschreibung: We present a new methodology for inverting P-to-S receiver function (RF) waveforms directly for mantle temperature and composition. This is achieved by interfacing the geophysical inversion with self-consistent mineral phase equilibria calculations from which rock mineralogy and its elastic properties are predicted as a function of pressure, temperature, and bulk composition. This approach anchors temperatures, composition, seismic properties, and discontinuities that are in mineral physics data, while permitting the simultaneous use of geophysical inverse methods to optimize models of seismic properties to match RF waveforms. Resultant estimates of transition zone (TZ) topography and volumetric seismic velocities are independent of tomographic models usually required for correcting for upper mantle structure. We considered two end-member compositional models: the equilibrated equilibrium assemblage (EA) and the disequilibrated mechanical mixture (MM) models. Thermal variations were found to influence arrival times of computed RF waveforms, whereas compositional variations affected amplitudes of waves converted at the TZ discontinuities. The robustness of the inversion strategy was tested by performing a set of synthetic inversions in which crustal structure was assumed both fixed and variable. These tests indicate that unaccounted-for crustal structure strongly affects the retrieval of mantle properties, calling for a two-step strategy presented herein to simultaneously recover both crustal and mantle parameters. As a proof of concept, the methodology is applied to data from two stations located in the Siberian and East European continental platforms. ©2018. American Geophysical Union. All Rights Reserved.

Beschreibung: In the frame of the European Commission project "Seismic Hazard Harmonization in Europe" (SHARE), aiming at harmonizing seismic hazard at a European scale, the compilation of a homogeneous, European parametric earthquake catalogue was planned. The goal was to be achieved by considering the most updated historical dataset and assessing homogenous magnitudes, with support from several institutions. This paper describes the SHARE European Earthquake Catalogue (SHEEC), which covers the time window 1000-1899. It strongly relies on the experience of the European Commission project "Network of Research Infrastructures for European Seismology" (NERIES), a module of which was dedicated to create the European "Archive of Historical Earthquake Data" (AHEAD) and to establish methodologies to homogenously derive earthquake parameters from macroseismic data. AHEAD has supplied the final earthquake list, obtained after sorting duplications out and eliminating many fake events; in addition, it supplied the most updated historical dataset. Macroseismic data points (MDPs) provided by AHEAD have been processed with updated, repeatable procedures, regionally calibrated against a set of recent, instrumental earthquakes, to obtain earthquake parameters. From the same data, a set of epicentral intensity-to-magnitude relations has been derived, with the aim of providing another set of homogeneous Mw estimates. Then, a strategy focussed on maximizing the homogeneity of the final epicentral location and Mw, has been adopted. Special care has been devoted also to supply location and Mw uncertainty. The paper focuses on the procedure adopted for the compilation of SHEEC and briefly comments on the achieved results. © 2012 The Author(s).

Beschreibung: H/V spectral ratios from microtremors are used to retrieve the S-velocity structure from a single ambient vibration record, by using its relation to the ellipticity of the fundamental mode Rayleigh wave and the amplitude of observed H/V ratio. Constraints are needed in order to restrict the possible range of solutions, and the inversion is applied to sites where the thickness of the unconsolidated sediments is approximately known from borehole information. Within the uncertainty, the inverted structures agree well with the results from other S-wave measuring techniques such as downhole and cross-hole measurements, and the analysis of ambient vibrations measured on an array. The influence of the inversion uncertainty on site- amplification estimates for earthquakes is then investigated. For all inverted models, site response is computed for a large number of events, which allows to define the uncertainty by the apriori unknown source position and mechanism of a future earthquake. In most cases the variability between the results obtained for the different models is much smaller than the variability introduced by the unknown source position. The accuracy with which S-wave velocity structures can be retrieved from observed H/V ratios is therefore sufficient for an application of the method in seismic hazard analysis for a specific site.