Source parameters of the 23 April 1992 M 6.1 Joshua Tree, California, earthquake and its aftershocks: Empirical Green's function analysis of GEOS and TERRAscope data

Abstract

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 components are averaged. The deconvolved spectra are fit by a ratio of omega-square source models, yielding an optimal (least-squares) corner frequency for both the large and the small event in each pair. We find no resolved difference between the inferred P - and S -wave corner frequencies. Using the standard Brune model for stress drop, we also find no 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 over a limited moment/magnitude cannot be ruled out. For magnitudes smaller than M 3 to 3.5, the inferred stress-drop values will be limited by the maximum observable corner frequency value of 60 Hz. For the mainshock, source-time functions are obtained from mainshock recordings at three TERRAscope stations (PFO, PAS, and GSC) using an M 4.3 foreshock as an empirical Green's function. The results indicate a fairly simple, single-pulse source-time function, with clear south-to-north directivity and an inferred rupture radius of 5 to 6 km. The deconvolved source-time functions are inverted to obtain a finite-rupture model that gives a robust estimate of rupture dimension. Early aftershocks are found 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.