Estimating Geophysical Bedrock Depth Using Single Station Analysis and Geophysical Data in the Extra-Carpathian Area of
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Pure and Applied Geophysics
Estimating Geophysical Bedrock Depth Using Single Station Analysis and Geophysical Data in the Extra-Carpathian Area of Romania ELENA FLORINELA MANEA,1,2
CARMEN ORTANZA CIOFLAN,1 ALINA COMAN,1,3 CLOTAIRE MICHEL,4 VALERIO POGGI,5 and DONAT FA¨H4
Abstract—Local site evaluation is an essential step in understanding the amplification of seismic motion induced by the complex geological structure and their estimation for future strong earthquakes in urban regions. One of the critical parameters on evaluating amplification effects is the depth of the geophysical bedrock, whose interface to soft sediments is responsible for the development of destructive resonance phenomena. The present study is focused on the estimation of the geophysical bedrock depth along the extra-Carpathian area of Romania (Moesian Platform and surroundings) by correlating and interpolating the results obtained from single station measurements with the available geological/geophysical data. Each site was investigated through the computation of horizontal-to-vertical (H/V) spectral ratios from three-component single station measurements of ambient vibrations. The geophysical bedrock depth was computed using a twostep inversion scheme, based on the retrieval of the Rayleigh-wave ellipticity peak at each seismic station using a regional generic velocity profile. The fundamental frequency of resonance reaches the lowest value in the deepest side (0.07 Hz) and is rising to 13 Hz in the South of the Moesian Platform, where a shallow bedrock is present. The computed bedrock depths (from 30 to * 3100 m) show a dipping tendency towards the Southern Carpathians and complex features such as local outcrops and lateral depth variations superpose this gradually dipping trend. In the Carpathian foreland, the bedrock is interpreted as the transition between different sediment layers of Neogene, while outside this area as the Neogene— Cretaceous transition. Keywords: Ambient vibrations, fundamental frequency of resonance, H/V spectral ratio, geophysical bedrock, two-step inversion, Moesian platform.
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00024-020-02548-3) contains supplementary material, which is available to authorized users. 1
National Institute of Earth Physics, Calugareni, 12, Magurele, Ilfov, Romania. E-mail: [email protected] 2 Department of Geological Sciences, San Diego State University, Campanile Dr., San Diego, CA 5500, USA. 3 Faculty of Physics, University of Bucharest, Atomistilor, 405, Magurele, Ilfov, Romania. 4 Swiss Seismological Service, ETH Zu¨rich, Sonneggstrasse, 5, Zurich, Switzerland. 5 Seismological Research Centre (CRS), National Institute of Oceanography and Applied Geophysics (OGS), Udine, Italy.
1. Introduction Seismic site effects evaluation (microzonation) has a substantial impact on the evaluation of seismic hazard and risk and it is a crucial step for the mitigation of potentially high seismic risk in densely populated urban regions. The extra-Ca
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