Fine structure of the rupture zone of the April 26 and 27, 1997, Northridge aftershocks

We investigated the rupture geometry of two M w ∼4.5 earthquakes (April 26, 1997, and April 27, 1997) that occurred on the western edge of the aftershock zone of the 1994, M w 6.7, Northridge earthquake. Both events have thrust mechanisms with a steep plane dipping ∼75°SE and a shallow plane dipping ∼45°NE. An empirical Green's function deconvolution followed by a waveform inversion was used to determine the slip distribution of the two events. The inversion results show that the steep plane fits the data slightly better than the shallow plane. The background seismicity (aftershocks of the 1994 Northridge event) in the epicentral region shows the existence of a north dipping fault plane, similar to that of the 1971 San Fernando earthquake. However, the spatial trend of the two April 1997 events and their aftershocks reveals tightly clustered seismicity on a steep plane dipping south. Relative relocation of the April 26 and 27 events shows that the April 27 event ruptured ∼1.4 km N70°E of the April 26 event and at a slightly shallower depth, i.e., almost along strike and on a steep plane dipping south. These observations suggest that the steep plane is the fault plane of both events. The two events ruptured on a plane which is almost perpendicular to the trend of the regional background seismicity. Thus the seismogenic structure beneath the Transverse Ranges exhibits complexity on scales of a few kilometers. The rupture area for both the April 26 and 27 events is ∼l km 2 with a stress drop of at least 20 to 30 bars.