ALBERT

Description: SUMMARY To constrain seismic anisotropy under and around the Alps in Europe, we study SKS shear wave splitting from the region densely covered by the AlpArray seismic network. We apply a technique based on measuring the splitting intensity, constraining well both the fast orientation and the splitting delay. Four years of teleseismic earthquake data were processed, from 723 temporary and permanent broad-band stations of the AlpArray deployment including ocean-bottom seismometers, providing a spatial coverage that is unprecedented. The technique is applied automatically (without human intervention), and it thus provides a reproducible image of anisotropic structure in and around the Alpine region. As in earlier studies, we observe a coherent rotation of fast axes in the western part of the Alpine chain, and a region of homogeneous fast orientation in the Central Alps. The spatial variation of splitting delay times is particularly interesting though. On one hand, there is a clear positive correlation with Alpine topography, suggesting that part of the seismic anisotropy (deformation) is caused by the Alpine orogeny. On the other hand, anisotropic strength around the mountain chain shows a distinct contrast between the Western and Eastern Alps. This difference is best explained by the more active mantle flow around the Western Alps. The new observational constraints, especially the splitting delay, provide new information on Alpine geodynamics.

Description: We compiled a dataset of continuous recordings from the temporary and permanent seismic networks to compute the high-resolution 3D S-wave velocity model of the Southeastern Alps, the western part of the external Dinarides, and the Friuli and Venetian plains through ambient noise tomography. Part of the dataset is recorded by the SWATH-D temporary network and permanent networks in Italy, Austria, Slovenia and Croatia between October 2017 and July 2018. We computed 4050 vertical component cross-correlations to obtain the empirical Rayleigh wave Green’s functions. The dataset is complemented by adopting 1804 high-quality correlograms from other studies. The fast-marching method for 2D surface wave tomography is applied to the phase velocity dispersion curves in the 2–30 s period band. The resulting local dispersion curves are inverted for 1D S-wave velocity profiles using the non-perturbational and perturbational inversion methods. We assembled the 1D S-wave velocity profiles into a pseudo-3D S-wave velocity model from the surface down to 60 km depth. A range of iso-velocities, representing the crystalline basement depth and the crustal thickness, are determined. We found the average depth over the 2.8–3.0 and 4.1–4.3 km/s iso-velocity ranges to be reasonable representations of the crystalline basement and Moho depths, respectively. The basement depth map shows that the shallower crystalline basement beneath the Schio-Vicenza fault highlights the boundary between the deeper Venetian and Friuli plains to the east and the Po-plain to the west. The estimated Moho depth map displays a thickened crust along the boundary between the Friuli plain and the external Dinarides. It also reveals a N-S narrow corridor of crustal thinning to the east of the junction of Giudicarie and Periadriatic lines, which was not reported by other seismic imaging studies. This corridor of shallower Moho is located beneath the surface outcrop of the Permian magmatic rocks and seems to be connected to the continuation of the Permian magmatism to the deep-seated crust. We compared the shallow crustal velocities and the hypocentral location of the earthquakes in the Southern foothills of the Alps. It revealed that the seismicity mainly occurs in the S-wave velocity range between ∼3.1 and ∼3.6 km/s.

Description: Probing seismic anisotropy of the lithosphere provides valuable clues on the fabric of rocks. We present a 3-D probabilistic model of shear wave velocity and radial anisotropy of the crust and uppermost mantle of Europe, focusing on the mountain belts of the Alps and Apennines. The model is built from Love and Rayleigh dispersion curves in the period range 5–149 s. Data are extracted from seismic ambient noise recorded at 1521 broad-band stations, including the AlpArray network. The dispersion curves are first combined in a linearized least squares inversion to obtain 2-D maps of group velocity at each period. Love and Rayleigh maps are then jointly inverted at depth for shear wave velocity and radial anisotropy using a Bayesian Monte Carlo scheme that accounts for the trade-off between radial anisotropy and horizontal layering. The isotropic part of our model is consistent with previous studies. However, our anisotropy maps differ from previous large scale studies that suggested the presence of significant radial anisotropy everywhere in the European crust and shallow upper mantle. We observe instead that radial anisotropy is mostly localized beneath the Apennines while most of the remaining European crust and shallow upper mantle is isotropic. We attribute this difference to trade-offs between radial anisotropy and thin (hectometric) layering in previous studies based on least-squares inversions and long period data (〉30 s). In contrast, our approach involves a massive data set of short period measurements and a Bayesian inversion that accounts for thin layering. The positive radial anisotropy (VSH 〉 VSV) observed in the lower crust of the Apennines cannot result from thin layering. We rather attribute it to ductile horizontal flow in response to the recent and present-day extension in the region.

Description: To constrain seismic anisotropy under and around the Alps in Europe, we study SKS shear wave splitting from the region densely covered by the AlpArray seismic network. We apply a technique based on measuring the splitting intensity, constraining well both the fast orientation and the splitting delay. Four years of teleseismic earthquake data were processed, from 723 temporary and permanent broad-band stations of the AlpArray deployment including ocean-bottom seismometers, providing a spatial coverage that is unprecedented. The technique is applied automatically (without human intervention), and it thus provides a reproducible image of anisotropic structure in and around the Alpine region. As in earlier studies, we observe a coherent rotation of fast axes in the western part of the Alpine chain, and a region of homogeneous fast orientation in the Central Alps. The spatial variation of splitting delay times is particularly interesting though. On one hand, there is a clear positive correlation with Alpine topography, suggesting that part of the seismic anisotropy (deformation) is caused by the Alpine orogeny. On the other hand, anisotropic strength around the mountain chain shows a distinct contrast between the Western and Eastern Alps. This difference is best explained by the more active mantle flow around the Western Alps. The new observational constraints, especially the splitting delay, provide new information on Alpine geodynamics.

Description: Ocean-bottom seismometers (OBSs) allow us to extend seismological research to the oceans to constrain offshore seismicity but also image the marine subsurface. A challenge is the high noise level on OBS records, which is created not only by bottom currents but also by the specific seismometer models used. We present a uantitative noise model for the LOBSTER OBS, which is the main instrument of DEPAS, currently the largest European OBS pool, stationed at Alfred-Wegener-Institut (AWI) Bremerhaven. Studying sensor noise in vault conditions and current sensitivity at an oceanographic measurement mast, we can show that the previously reported high noise level of the instrument is caused by the original sensor (Güralp CMG-40T-OBS) or its housing. We also show that a strong signal that has been reported between 1 and 5 Hz can be attributed to head- buoy cable strumming. This noise signal can actually be used to estimate bottom current velocities with relatively high precision to a few cm/s. We provide a current-dependent quantitative noise model that can be used for experiment design in future deployments. We further show that replacing the original sensor with a Trillium compact considerably improves the performance of the pool OBS at moderate cost.

Description: Current measurements in the oceans are of importance for understanding the global exchange of water masses and also to verify the output of Earth System Models (ESM). The benthic layer in abyssal depths, which makes up the largest part of the global sea floor, is poorly sampled in this respect, even though the current velocities there are of importance for understanding the global circulation, but also its impact on the benthic fauna. We show that the noise recorded by a widely used brand of ocean-bottom-seismometers (LOBSTER) be tween 1 and 10 Hz is highly sensitive to the current velocity. This is due to resonance frequency of the head buoy cable being very close to the Kármán vortex shedding frequency for currents of a few centimeter per second. This creates a clear, harmonic signal, which has been found at deployments of the instrument in various regions in the Atlantic and the Indian Ocean. Since OBS are measuring permanently for a year or more and are deployed over wide areas, this may become a completely novel dataset for physical oceanography. We tested the proposed relationship by installing three ocean bottom seismometers near an oceanographic measuring pile located at Darss Silt in the south-western Baltic sea. where hourly current records from an acoustic doppler current profiler (ADCP) are available.

Description: Ocean-bottom seismometers (OBSs) allow us to extend seismo-logical research to the oceans to constrain offshore seismicity but also image the marine subsurface. A challenge is the high noise level on OBS records, which is created not only by bottom currents but also by the specific seismometer models used. We present a quantitative noise model for the LOBSTER OBS, which is the main instrument of DEPAS, currently the largest European OBS pool, stationed at Alfred-Wegener-Institut (AWI) Bremerhaven. Studying sensor noise in vault conditions and current sensitivity at an oceanographic measurement mast, we can show that the previously reported high noise level of the instrument is caused by the original sensor (Güralp CMG-40T-OBS). We also show that a strong signal that has been reported between 1 and 5 Hz can be attributed to head-buoy cable strumming. We provide a current-dependent quantitative noise model that can be used for experiment design in future deployments and show that the performance of the pool OBS can be improved at moderate cost by replacing the CMG-40T-OBS with a sensor of a proven noise floor below 10−8 nm=s2 , for example, a Trillium compact.

Description: The DEPAS (DEutscher Pool für Amphibische Seismologie) is the largest pool of wideband ocean-bottom seismometers in Europe. It consists of LOBSTER OBS, manufactured by KUM, which are equipped with Güralp CMG-40T-OBS and Trillium compact sensors. Multiple operators reported a relatively high long-period noise level of the original CMG-40T-equipped design. It has been discussed whether the source of this noise is the integration of the seismometer into the frame or the instrument itself. Using vault installations of the seismometer and test deployments of different LOBSTER next to a current sensor, we can show that the long-period noise is caused by the CMG-40T-OBS sensor itself. Equipped with a Trillium compact seismometer, the LOBSTER has a median vertical noise level of -155 dB at 120s period for current velocities below 10cm/s, which is essentially the self-noise of the seismometer. Even at relatively high bottom currents above 20 cm/s, the noise level does not exceed the Peterson New High Noise Model at these periods. We present a quantitative noise model for the LOBSTER and conclude that its rugged design is suited for seismology on periods below 300 seconds, if it is equipped with a good seismometer.

Description: To constrain seismic anisotropy under and around the Alps in Europe, we study SKS shear wave splitting from the region densely covered by the AlpArray seismic network. We apply a technique based on measuring the splitting intensity, constraining well both the fast orientation and the splitting delay. Four years of teleseismic earthquake data were processed, from 723 temporary and permanent broad-band stations of the AlpArray deployment including ocean-bottom seismometers, providing a spatial coverage that is unprecedented. The technique is applied automatically (without human intervention), and it thus provides a reproducible image of anisotropic structure in and around the Alpine region. As in earlier studies, we observe a coherent rotation of fast axes in the western part of the Alpine chain, and a region of homogeneous fast orientation in the Central Alps. The spatial variation of splitting delay times is particularly interesting though. On one hand, there is a clear positive correlation with Alpine topography, suggesting that part of the seismic anisotropy (deformation) is caused by the Alpine orogeny. On the other hand, anisotropic strength around the mountain chain shows a distinct contrast between the Western and Eastern Alps. This difference is best explained by the more active mantle flow around the Western Alps. The new observational constraints, especially the splitting delay, provide new information on Alpine geodynamics. © 2021 The Author(s) 2021. Published by Oxford University Press on behalf of The Royal Astronomical Society.

Description: Published

Description: 1996–2015

Description: 1T. Struttura della Terra

Description: JCR Journal