SHAKEMAP METHODOLOGY BASED ON FOURIER AMPLITUDE SPECTRA AND ITS APPLICATION FOR THE CASE OF VRANCEA EARTHQUAKES IN ROMANIA

摘要

Shake maps are generated within a few minutes on a routine basis by extrapolation ground motion parameters such as PGA, PGV, response spectra, and computed intensity from observational sites equipped with accelerometers with distances in the range of kilometers to tens of kilometers between them. Extrapolation is based on empirical hard-rock attenuation relations, geological classification of the sites, and on empirical relations of ground motion amplification for a given geological class. We suggest a pure seismological approach based on Fourier amplitude spectrum (FAS) that can be used in most cases and that allows obtaining site-dependent assessment in terms of various ground motion parameters. First, source scaling and attenuation models for FAS are evaluated using recordings obtained on rock stations. Second, the site effect at non-rock stations is analyzed as ratios between the spectra of observed records and the obtained spectral models. Third, the generalized site amplification functions are constructed for typical soil conditions. Fourth, using stochastic technique, the site-dependent attenuation models are constructed to be used directly in shake map generation. The method is applied for the case of intermediate-depth (70-140 km) earthquakes of the Vrancea (Romania) source zone, which produce the most significant seismic hazard to Romania, including the city of Bucharest, and its neighboring countries. We modeled ground motion parameters distribution for major Vrancea earthquakes occurred during the last century, e.g November 10, 1940 (M = 7.7), and March 4, 1977 (M = 7.4). The theoretical data were compared with available observations.