Abstract

The Karakorum earthquake of 13 February 1980 is one of the largest events that has occurred beneath the Karakorum range in recent time. It involved oblique thrusting at an intermediate depth. Focal depths determined from forward modeling and moment-tensor inversion of teleseismic P and SH waves give similar results (90 ± 4 km). The scalar double-couple moment of 1.27 × 1018 N-m was obtained from the moment-tensor inversion. Estimates of stress drop based on this moment value are about 0.3 to 1 MPa (3 to 10 bars). The compressional stress orientation, inferred from the focal mechanism, is approximately parallel to the strike of the underthrusting Asian continental lithosphere and may reflect detachment of the Asian crust as it plunges beneath the Karakorum. Alternatively, the T axis of the focal mechanism, oriented parallel to the northward-subducting Indian continental lithosphere, indicates down-dip extension and implies that slab pull is important. The thrust-type mechanism of the Karakorum event contrasts greatly with the normal-type intermediate-depth event reported previously and suggests a complex state-of-stress beneath the Karakorum range.

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