UNIVERSITY OF BUCHAREST
FACULTY OF PHYSICS

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2024-11-22 1:49
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Conference: Bucharest University Faculty of Physics 2003 Meeting


Section: Atmosphere and Earth Science; Environment Protection


Title:
Seismic Ground Motion Characteristics in the Bucharest Area: Source and Site Effects Contribution


Authors:
B. Grecu1, M. Popa1, M. Radulian1


Affiliation:
National Institute for Earth Physics, P.O.BOX MG-2, Bucharest, Romania


E-mail


Keywords:


Abstract:
The contribution of source vs. site effects on the seismic ground motion in Bucharest is controversial as the previous studies showed. The fundamental period of resonance for the sedimentary cover is emphasized by ambient noise and earthquake measurements, if the spectral ratio method (Nakamura, 1989) is applied (Bonjer et al., 1989). On the other hand, the numerical simulations (Moldoveanu et al., 2000.) and acceleration spectra analysis (Sandi et al., 2001) brought into the light the determinant role of the source effects. We considered all the available instrumental data related to Vrancea earthquakes recorded in Bucharest area to find how the source and site properties control the peak ground motion peculiarities. Our main results are summarized as follows: 1. The resonant period of oscillation, related to the shallow sediment layer, is practically present in all the H/V spectral ratios, no matter we consider ambient noise or earthquakes of any size. This argues in favor of the crucial role played by the sedimentary cover and proves that the ratio method is reasonably removing the source effects. However, the absolute spectra are completely different for earthquakes below and above magnitude 7: amplitudes in the range of 1-2 s periods are negligible in the first case, and predominant in the second. It looks like the resonant amplification by the sedimentary cover becomes effective only for the largest earthquakes (M > 7), when the source radiation coincides with the fundamental resonance range. We conclude that the damage in Bucharest is dramatically amplified when the earthquake size is above a critical value (M ~ 7). 2. Our analysis shows a rather weak variability of the peak motion values and spectral amplitudes over the study area, in agreement with the relatively small variability of the shallow structure topography.