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Interplate coupling and temporal variation of mechanisms of intermediate-depth earthquakes in Chile

SEISMOLOGICAL LABORATORY CALIFORNIA INSTITUTE OF TECHNOLOGY, PASADENA, CALIFORNIA 91125

Abstract

We investigated the temporal variation of the mechanism of large intraplate earthquakes at intermediate depths in relation to the occurrence of large under-thrusting earthquakes in Chile. Focal mechanisms were determined for three large events (1 March 1934: M = 7.1, d = 120 km; 20 April 1949: M = 7.3, d = 70 km; and 8 May 1971: M_W = 7.2, d = 150 km) which occurred down-dip of the great 1960 Chilean earthquake (M_W = 9.5) rupture zone. The 1971 event is down-dip compressional: (strike) = 12°, (dip) = 80°, and (rake) = 100°. The 1949 earthquake focal mechanisms is = 350°, = 70°, and = –130°. The data available for the 1934 event are consistent with a down-dip tensional mechanism. Thus, the two events which occurred prior to the great 1960 Chilean earthquake are down-dip tensional. Published fault plane solutions of large intermediate-depth earthquakes (28 March 1965 and 7 November 1981) which occurred down-dip of the Valparaiso earthquakes of 1971 (M_W = 7.8) and 1985 (M_W = 8.0) are also down-dip tensional. These results suggest that before a major thrust earthquake, the interplate boundary is strongly coupled, and the subducted slab is under tension at intermediate depths; after the occurrence of an interplate thrust event, the displacement on the thrust boundary induces transient compressional stress at intermediate depth in the down-going slab. This interpretation is consistent with the hypothesis that temporal variations of focal mechanisms of outer-rise events are due to changes of interplate coupling.

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