Analysis of similar event clusters in aftershocks of the 1994 Northridge, California, earthquake

[1] We perform waveform cross-correlation on over 14,000 aftershocks of the 1994 Northridge M-W 6.7 earthquake in southern California as recorded by short-period stations of the Southern California Seismic Network (SCSN). Approximately 10-30% of the events belong to similar event clusters, depending upon the similarity criteria that are applied. We relocate events within 218 of these clusters to a relative location accuracy of about 30 m using the differential times obtained from the cross-correlation. These relocated event clusters often show planar features suggestive of faults at depth, and we apply principal parameter analysis to characterize the shape of each cluster and to compute best fitting planes. In several cases, these planes are parallel to the main shock fault plane; however, more generally, the seismicity planes exhibit a wide range of orientations, suggesting complexity in the aftershock faulting. Composite focal mechanisms can be obtained for each cluster by combining the P polarity data from individual events. A comparison of polarity measurement differences within similar event clusters provides constraints on the error rate in the individual focal mechanisms. For some clusters, we are able to resolve the primary versus auxiliary fault plane ambiguity by comparing the computed focal mechanisms with the best fitting seismicity planes. Individual event focal mechanisms are in general agreement with the composite focal mechanisms for the similar event clusters. Events occurring along the main shock rupture plane are mainly thrust, whereas events in the hanging wall are predominately strike-slip.