The 23:19 aftershock of the 15 October 1979 Imperial Valley earthquake: More evidence for an asperity

Abstract

The well-recorded strong ground motion data for the 23:19 aftershock of the 15 October 1979 Imperial Valley earthquake provide a good opportunity to study the high-frequency source characteristics and the path effects at near-source distances. The best-fitting point source model has a strike-slip mechanism, N40°W, which is nearly identical to the main event. The estimated stress drop is extremely high, roughly 500 bars, with a triangular time history (0.1, 0.1 sec) but with a moment of 1.0 × 10^(24) dyne-cm. A double-source model found by inversion fits the high-frequency data better but indicates complex faulting: the first source (with strike = N319°E, dip = 42°NE, and slip angle = 165°) has a moment of 0.7 × 10^(24) dyne-cm, the second source (with strike = N324°E, dip = 82°SW, and slip angle = 181°) lies about 0.5 km to the north and has a seismic moment twice that of the first source. Source dimensions appear very small for this amount of energy release. Many of the anomalous behaviors observed at certain stations for the main event are, also, present in the aftershock data. These features are examined in terms of path effects.