Broadband waveform observation of the 28 June 1991 Sierra Madre earthquake sequence (M_L = 5.8)

The Sierra Madre earthquake of 28 June 1991 occurred at a depth of about 12 km, on the Clamshell-Sawpit fault in the San Gabriel Mountains. The focal mechanisms and seismic moments of the mainshock and 21 aftershocks were determined by combining the waveform and first-motion data. The events were classified into five groups according to the location and waveforms recorded at PAS. Most events located within 5 km west of the mainshock are similar to the mainshock in waveform. The mechanisms thus determined are thrust mechanisms. The stress drop of the mainshock is about 500 bars. Most of the aftershocks have stress drops between 10 and 100 bars. -from Authors