AN EMPIRICAL-STUDY OF EARTHQUAKE SOURCE SPECTRA FOR CALIFORNIA EARTHQUAKES

We generalize source, path, and site effects for California earthquake s as functions of magnitude and distance, based on regression analysis of 1000 Fourier acceleration spectra from 43 California earthquakes i n the moment magnitude range from 4.4 to 7.4, recorded at rupture dist ances from 1 to 200 km. Empirically derived source spectra for Califor nia earthquakes are generally inconsistent with the spectral shape imp lied by a Brune (''omega-squared'') point-source model. This is manife sted by magnitude- and frequency-dependence of the Brune model paramet ers. For example, the Brune stress parameter that best matches the dat a at high frequencies decreases from a value of about 120 bars at M5.5 to a value near 50 bars at M7.5. At frequencies below 1 Hz, though, s ource spectra have much lower spectral amplitudes than predicted by th e Brune point-source model for these values of stress; this discrepanc y grows with increasing magnitude. Finite-fault simulations indicate t hat this is a consequence of the breakdown of the validity of the poin t-source approximation for large ruptures. A stochastic finite-fault m odel, in which the fault is discretized as a number of subfaults, each of which is represented by a Brune omega-squared point source, correc tly matches the observed spectral shapes and amplitudes. The spectral decay parameter kappa, representing average near-surface attenuation o f high-frequency motion at rock sites, increases with increasing magni tude, from values near 0.035 sec at M5.5 to 0.050 sec at M7.5. Magnitu de dependence of kappa may be interpreted as evidence of nonlinearity for typical California sites subjected to strong ground motion. Compar isons of our empirical source spectra for California to corresponding spectra for eastern North America suggest that the spectral amplitudes are similar in the two regions for low-frequency motions (f < 2 Hz fo r M5.5, f < 0.5 Hz for M7.5), for equivalent crustal conditions. The e astern events appear to have enhanced high-frequency near-source ampli tudes relative to the California events; this is particularly pronounc ed for large-magnitude earthquakes.