Abstract

The Liyang earthquakes of 1974 and 1979 were two moderately sized but destructive events in eastern China. The first shock (Ms = 5.5) occurred on 22 April 1974 in a densely populated area, and the second one (Ms = 5.7) took place on 9 July 1979 near the same location. The epicenters were located near the intersection of two faults and inside a late Mesozoic to late Tertiary sedimentary extensional basin. In this article, long-period P and SH waveforms between 2° and 90° and short-period teleseismic P waveforms were used to determine strike-slip source mechanisms with large reverse-faulting components that occurred for both events. These mechanisms are significantly different from earlier determined solutions reached through P-wave first motions, which found strike-slip and normal-slip components for the two events. The solution of the 1974 event, obtained from this study, strikes 126°, dips 71°, and slips 33°. The focal depth, seismic moment, and stress drop are determined to be 12 km, 8.4 × 1023 dyne-cm, and 41 bars, respectively. The mechanism of the 1979 event strikes 41°, dips 64°, and slips 147°. The focal depth, seismic moment, and stress drop are found to be 7.5 km, 1.6 × 1024 dyne-cm, and 152 bars, respectively. Seismic data and local observations suggest that the 1974 event ruptured a WNW-striking fault. For the 1979 event, the fault plane is difficult to resolve. The observed oblique reverse faulting is relatively rare for major earthquakes in eastern China and probably represents a reactivated and inverted fault motion beneath a structural basin under the present-day E-W to ENE-WSW compressional stress field.

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