Source parameters of the 23 April 1992 Joshua Tree, California, earthquake, its largest foreshock, and aftershocks

Abstract

Abstract Seismic moments, corner frequencies, and stress drops have been calculated for the 23 April 1992 Joshua Tree, California, earthquake, its largest foreshock, and 12 aftershocks using recordings from TERRAscope stations. Source parameters are determined from nonlinear least-squares best fits to ratios of P -wave Fourier amplitude spectra assuming an omega-squared source model. The spectral ratios are the mainshock spectra divided by the foreshock and aftershock spectra. Seismic moments range from 2 × 10 21 to 2 × 10 25 dyne-cm. Corner frequencies range from 0.51 to 4.8 Hz. Aftershock stress drops range from 4.2 to 41 bars with a log-average stress drop of 15 bars. The foreshock stress drop is relatively large, 31 bars, but is within the aftershock stress-drop range. The mainshock stress drop of 203 bars is significantly larger than any of the aftershock or foreshock stress drops.