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Bulletin of the Seismological Society of America; October 2005; v. 95; no. 5; p. 1749-1765; DOI: 10.1785/0120050005
© 2005 Seismological Society of America
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Hard Rock Spectral Models for Intermediate-Depth Vrancea, Romania, Earthquakes

Vladimir Sokolov1, Klaus-Peter Bonjer1, Mihnea Oncescu*,1 and Mihaela Rizescu*,2

1 Geophysical Institute
Karlsruhe University
Hertzstr. 16
Karlsruhe, Germany
Vladimir.Sokolov{at}gpi.uni-karlsruhe.de
Klaus.Bonjer{at}gpi.uni-karlsruhe.de
 (V.S., K.-P.B., M.O.)

2 National Institute for Earth Physics
P.O. Box MG-2
Bucharest-Magurele, Romania
 (M.R.)

* Present address: Geotech Instruments, LLC, 10755 Sanden Drive, Dallas, Texas.

The frequency-dependent amplification for rock sites, as well as apparent source spectra, were studied using an earthquake ground-motion database collected in Romania. Almost all events occurred in the Vrancea focal zone (Southeast Carpathians), which is characterized by a high rate of occurrence of large earthquakes in a narrow focal volume. The seismic activity ranges within an almost vertical stripe in depths between 60 and 170 km. The database used includes several hundred records from more than 100 small magnitude (MW 3.5–5.3) earthquakes that occurred in 1996–2004 and six acceleration records obtained during four large (MW 7.2, 6.9, 6.3, and 6.0) earthquakes. The characteristics of amplification were evaluated using the well-known technique of horizontal-to-vertical (H/V) Fourier spectral ratio of the S-wave phase (Lermo and Chavez-Garcia, 1993). The apparent source spectra were obtained from horizontal components using site (H/V), propagation path (geometric spreading and anelastic attenuation), and near-surface attenuation (kappa-factor) corrections.

Seismic radiation (within the frequency range from 0.2–0.3 to 15–20 Hz) during the earthquakes of various magnitudes in the Vrancea zone may be described by an omega-square single-corner-frequency spectral model ({omega}2 Brune spectrum). The stress parameter {Delta}{sigma}, which controls high-frequency spectral amplitudes, increases with magnitude from 20–30 bars for MW ≤3.5 up to 200–250 bars for MW 4.8–5.3, and up to 1000 bars for the case of large (MW >6.0–6.5) events. Empirical amplification (H/V) for rock sites, showing stability for particular locations, reveal high variability from station to station, which together with variations of values of kappa, reflects influence of local geologic and geomorphologic factors.




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A. Oth, D. Bindi, S. Parolai, and F. Wenzel
S-Wave Attenuation Characteristics beneath the Vrancea Region in Romania: New Insights from the Inversion of Ground-Motion Spectra
Bulletin of the Seismological Society of America, October 1, 2008; 98(5): 2482 - 2497.
[Abstract] [Full Text] [PDF]


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