EMPIRICAL MODELS FOR ESTIMATING FOURIER-AMPLITUDE SPECTRA OF GROUND ACCELERATION IN THE NORTHERN CAUCASUS (RACHA SEISMOGENIC ZONE)

Abstract

A collection of ground-motion recordings of small to moderate (Ms <= 6.2) earthquakes has been obtained in the epicentral area of the Racha earthquake, 29 April 1991 (Ms = 7.1), which occurred in the southern slope of the Big Caucasus Range (Northen Georgia). This zone has not shown a high level of seismic activity in historical times. The horizontal components of 68 strong-motion accelerograms were used to study source spectra scaling and ground-motion attenuation in this region. The acceleration spectra of shear waves recorded at rock sites from small events (M s < 4.0 to 4.5) can be modeled accurately by the Brune source model using a stress parameter of 150 bar and cutoff frequency fmax = 6 to 8 Hz. The empirical function developed for eastern North America by Atldnson (1993b) gives a better fit to empirical data observed during larger events. The use of a trilinear form proposed by Atkinson and Boore (1995) for the attenuation curve with Q = 29.411 + (f/0.3)2"9/(f/0.3) 2] is necessary to simulate spectra at long (R > 70 kin) distances. Stochastic simulation of ground motions (peak ground acceleration and response spectra) using obtained models of source spectra and attenuation shows good agree-ment with observed data. The comparison of modeled bedrock spectra and observed ones allowed us to estimate the local site response in terms of frequency-dependent amplification func-tions. The parameters of the amplification functions strongly correlate with the site geological characteristics.