Source parameters for small events associated with the 1986 North Palm Springs, California, earthquake determined using empirical Green functions

Using small events as empirical Green functions, which include path, site, and instrument effects on the P waveforms, source parameters were estimated for 25 ML 3.4 to 4.4 events associated with the 1986 North Palm Springs earthquake. The static stress drops ranged from 3 to 80 bars, for moments of 0.7 to 11 × 1021 dyne-cm. There was a spatial pattern to the stress drops of the aftershocks which showed increasing values along the fault plane toward the northwest compared to relatively low values near the hypocenter of the mainshock. The highest values were outside the main area of slip, and are believed to reflect a loaded area of the fault that still has an higher level of stress which was not released during the main shock. The relatively high stress drop of a preshock in 1983, located close to the hypocenter of the main shock, is also interpreted as evidence that stress drops of magnitude 3 and 4 events can be used as indicators of the stress level in a particular area.