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 (MS ≦ 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 Atkinson (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 = 29.4[1 + (f/0.3)2.9/(f/0.3)2] is necessary to simulate spectra at long (R > 70 km) distances. Stochastic simulation of ground motions (peak ground acceleration and response spectra) using obtained models of source spectra and attenuation shows good agreement 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 functions. The parameters of the amplification functions strongly correlate with the site geological characteristics.