A new technique, called the method of the principal parameters (Ebblin and Michelini, in press), to infer the orientations of active fault planes of an aftershock sequence has been applied to the sequence that followed the Coalinga, California, earthquake of 2 May 1983 (ML = 6.7).

The method is based on observed clustering in time and space. Clustering is a characteristic feature of aftershock sequences, and it suggests interdependence of the events. It follows that the spatial locations of time-successive foci may provide additional information about the geometries of the rupturing fault system. The method involves sliding of a temporal window of a fixed number of foci along the sequence and estimating the eigenvalues and eigenvectors of a spatial matrix for each window-set. This matrix can be interpreted as an ellipsoid which is fitted through the foci. During the earthquake sequence, the different trends of the seismicity pattern can be isolated by selecting and averaging the greatly flattened ellipsoids.

In the case of the Coalinga sequence, the trends are generally consistent with one of the focal planes obtained from the fault-plane solutions given in earlier published studies. The method appears to offer a simple way to infer average active fault geometries in complex areas from hypocentral locations only.

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