Ambient-noise tomography of the wider Vienna Basin region

We present a new 3-D shear-velocity model for the top 30 km of the crust in the wider Vienna Basin region based on surface waves extracted from ambient-noise cross-correlations. We use continuous seismic records of 63 broad-band stations of the AlpArray project to retrieve interstation Green’s functions from ambient-noise cross-correlations in the period range from 5 to 25 s. From these Green’s functions, we measure Rayleigh group traveltimes, utilizing all four components of the cross-correlation tensor, which are associated with Rayleigh waves (ZZ, RR, RZ and ZR), to exploit multiple measurements per station pair. A set of selection criteria is applied to ensure that we use high-quality recordings of fundamental Rayleigh modes. We regionalize the interstation group velocities in a 5 km × 5 km grid with an average path density of ∼20 paths per cell. From the resulting group-velocity maps, we extract local 1-D dispersion curves for each cell and invert all cells independently to retrieve the crustal shear-velocity structure of the study area. The resulting model provides a previously unachieved lateral resolution of seismic velocities in the region of ∼15 km. As major features, we image the Vienna Basin and Little Hungarian Plain as low-velocity anomalies, and the Bohemian Massif with high velocities. The edges of these features are marked with prominent velocity contrasts correlated with faults, such as the Alpine Front and Vienna Basin transfer fault system. The observed structures correlate well with surface geology, gravitational anomalies and the few known crystalline basement depths from boreholes. For depths larger than those reached by boreholes, the new model allows new insight into the complex structure of the Vienna Basin and surrounding areas, including deep low-velocity zones, which we image with previously unachieved detail. This model may be used in the future to interpret the deeper structures and tectonic evolution of the wider Vienna Basin region, evaluate natural resources, model wave propagation and improve earthquake locations, among others

Arrival angles of teleseismic fundamental mode Rayleigh waves across the AlpArray

The dense AlpArray network allows studying seismic wave propagation with high spatial resolution. Here we introduce an array approach to measure arrival angles of teleseismic Rayleigh waves. The approach combines the advantages of phase correlation as in the two-station method with array beamforming to obtain the phase-velocity vector. 20 earthquakes from the first two years of the AlpArray project are selected, and spatial patterns of arrival-angle deviations across the AlpArray are shown in maps, depending on period and earthquake location. The cause of these intriguing spatial patterns is discussed. A simple wave-propagation modelling example using an isolated anomaly and a Gaussian beam solution suggests that much of the complexity can be explained as a result of wave interference after passing a structural anomaly along the wave paths. This indicates that arrival-angle information constitutes useful additional information on the Earth structure, beyond what is currently used in inversions

The French Accelerometric Network (RAP) and National Data Centre (RAP-NDC)

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High-resolution surface wave tomography beneath the Aegean-Anatolia region: constraints on upper-mantle structure

International audienceThis study provides new constraints on the upper-mantle structure from western Greece to central Anatolia using seismic data of permanent broad-band networks recently installed in Greece and Turkey and from a two-year temporary array (SIMBAAD experiment). We used ∼200 seismic events recorded at 146 broad-band stations with a typical interstation distance of 60–100 km across the study area. The high-resolution 3-D shear wave velocity model of the mantle is obtained by inversion of fundamental-mode Rayleigh wave phase velocity maps for periods between 20 and 195 s. The tomography is based on ray tracing in heterogeneous media taking into account external propagation effects. The horizontal resolution is approximately 100 km, however small heterogeneities may suffer from some horizontal smearing and damp- ing. The vertical resolution is approximately 100 km. The vertical smoothing is necessary to avoid unresolved spurious shear wave velocity oscillations in the upper mantle. The errors on shear wave velocities in our 3-D model (0.02–0.1km s−1) are significantly smaller than the amplitude of Vs variations (0.3–0.5 km s−1). In spite of the vertical and horizontal smoothing, our model shows details in the upper-mantle structure never reached at regional scale in the area. The overall structure is characterized by a low-velocity zone (80–200km depth) re- flecting a slow and warm asthenosphere underlying a thin lithosphere. The southwesternmost termination of the low-velocity anomaly corresponds to the northward dipping Hellenic slab. The detailed shear velocity structure of the upper mantle beneath Anatolia appears to be far more geometrically complex than revealed in previous tomographic studies of the area. At depths larger than or equal to 160 km, velocities are overall high beneath Anatolia, partly due to the presence of dipping high-velocity anomalies which we tentatively interpret as remnant slabs. The southernmost high-velocity anomaly beneath Anatolia is separated from the eastern edge of the Hellenic slab by a major low-velocity anomaly which we interpret as the trace of asthenospheric mantle material rising inside a vertical slab tear beneath southwestern Anatolia

First Results from the UnderVolc High Resolution Seismic and GPS Network Deployed on Piton de la Fournaise Volcano

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