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

We investigated the rupture geometry of two M-w similar to 4,5 earthquakes (April 26, 1997, and April 27, 1997) that occurred on the western edge of t he aftershock zone of the 1994, M-w 6.7, Northridge earthquake. Both events have thrust mechanisms with a steep plane dipping similar to 75 degrees SE and a shallow plane dipping similar to 45 degrees NE. An empirical Green's function deconvolution followed by a waveform inversion was used to determ ine the slip distribution of the two events. The inversion results show tha t the steep plane fits the data slightly better than the shallow plane. The background seismicity (aftershocks of the 1994 Northridge event) in the ep icentral 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 similar to 1.4 km N70 degrees 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 observation s suggest that the steep plane is the fault plane of both events. The two e vents 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 r upture area for both the April 26 and 27 events is similar to 1 km(2) With a stress drop of at least 20 to 30 bars.