MODELING NEAR-SOURCE EARTHQUAKE GROUND MOTION WITH EMPIRICAL GREEN'S FUNCTIONS (CALIFORNIA)

Modeling near-source earthquake ground motion with empirical Green's functions extends synthesized seismograms to include the full frequency range, absolute amplitude, and duration of observed seismograms. However, the complexity of the source-time function and the difficulty of obtaining empirical Green's functions from strategically located small earthquakes makes modeling exact waveforms difficult.; Digitized three-component records of 52 aftershocks from 25 recording sites distributed throughout the San Fernando Valley are analyzed and used as empirical Green's functions. Recording sites ranged in hypocentral distance from 3 to 33 km. There is a moment threshold of 1.5 {dollar}times{dollar} 10{dollar}sp{lcub}21{rcub}{dollar} dyne-cm for these earthquakes, below which seismograms have a character that is independent of the source-time function. These records are termed empirical Green's functions. Their waveforms are nearly identical when their source events are less than 1.3-2.5 km apart, and they are recorded at distances greater than 8.0 km. They are band limited by the recording site geology, and scale linearly in amplitude only with differences in seismic moment. Empirical Green's functions contain all the waveform information that can be obtained for a recording site/elemental source pair. Thus, they account for all the geological inhomogeneities affecting seismograms at a recording site. It is shown that using empirical Green's functions allows one to exactly simulate the representation relation.; Empirical Green's functions were used in deconvolution to obtain point-source, source-time functions for small earthquakes 1.5 {dollar}<{dollar} M {dollar}sb{lcub}o{rcub}{dollar} {dollar}<{dollar} 16.0 {dollar}times{dollar} 10{dollar}sp{lcub}21{rcub}{dollar} dyne-cm. These were then used along with empirical Green's functions to model finite sources for these events. Well constrained moments and focal mechanism solutions allowed for examination of the effect that source dimension, stress drop, source-time function and rupture velocity have on seismograms. Absolute amplitude, frequency content and energy distribution are modeled at four stations with three components, but exact waveforms are not.; Twenty seconds of seismograms from the 1971 San Fernando earthquake are synthesized. At two of the stations, 10.0 and 15.0 km distant, absolute amplitudes, frequency content and energy distribution are modeled for the full 20 seconds on three components. The third station is at Pacoima dam. Here, some prominent phases and energy distribution are modeled, but absolute amplitudes are not.