SOURCE PARAMETERS OF THE 23 APRIL 1992 M 6.1 JOSHUA-TREE, CALIFORNIA,EARTHQUAKE AND ITS AFTERSHOCKS - EMPIRICAL GREENS-FUNCTION ANALYSIS OF GEOS AND TERRASCOPE DATA

Source parameters of-the M 6.1 23 April 1992 Joshua Tree mainshock and 86 M 1.8 to 4.9 aftershocks are determined using an empirical Green's function methodology. For the aftershocks, deconvolved P- and S-wave spectra are calculated for 126 pairs of closely spaced events recorded on portable GEOS stations; S-wave spectra from the two horizontal com ponents are averaged. The deconvolved spectra are fit by a ratio of om ega-square source models, yielding an optimal (least-squares) corner f requency for both the large and the small event in each pair. We find no resolved difference between the inferred P- and S-wave corner frequ encies. Using the standard Brune model for stress drop, we also find n o resolved nonconstant scaling of stress drop with moment, although we also conclude that detailed scaling systematics would be difficult to resolve. In particular, a weak increase of stress drop with moment ov er a limited moment/magnitude cannot be ruled out. For magnitudes smal ler than M 3 to 3.5, the inferred stress-drop values will be limited b y the maximum observable corner frequency value of 60 Hz. For the main shock, source-time functions are obtained from mainshock recordings at three TERRAscope stations (PFO, PAS, and GSC) using an M 4.3 foreshoc k as an empirical Green's function. The results indicate a fairly simp le, single-pulse source-time function, with clear south-to-north direc tivity and an inferred rupture radius of 5 to 6 km. The deconvolved so urce-time functions are inverted to obtain a finite-rupture model that gives a robust estimate of rupture dimension. Early aftershocks are f ound to lie along the perimeters of regions with high mainshock slip. The inferred mainshock stress-drop value, 56 bars, is within the range determined for the aftershocks. Our derived mainshock source spectra do not show resolvable deviation from the omega-square model.