RUPTURE DISTRIBUTION OF THE 1977 WESTERN ARGENTINA EARTHQUAKE

Teleseismic P and SH body waves are used in a finite-fault, waveform i nversion for the rupture history of the 23 November 1977 western Argen tina earthquake. This double event consists of a smaller foreshock (M( 0) = 5.3 x 10(26) dyn-cm) followed about 20 s later by a larger main s hock (M(0) = 1.5 x 10(27) dyn-cm). Our analysis indicates that these t wo events occurred on different fault segments: with the foreshock hav ing a strike, dip, and average rake of 345 degrees, 45 degrees E, and 50 degrees, and the main shock 10 degrees, 45 degrees E, and 80 degree s, respectively. The foreshock initiated at a depth of 17 km and propa gated updip and to the north. The main shock initiated at the southern end of the foreshock zone at a depth of 25 to 30 km, and propagated u pdip and unilaterally to the south. The north-south separation of the centroids of the moment release for the foreshock and main shock is ab out 60 km. The apparent triggering of the main shock by the foreshock is similar to other earthquakes that have involved the failure of mult iple fault segments, such as the 1992 Landers, California, earthquake. Such occurrences argue against the use of individual, mapped, surface fault or fault-segment lengths in the determination of the size and f requency of future earthquakes.