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Published September 1984 | Published
Journal Article Open

Focal mechanisms and aftershock locations of the Songpan earthquakes of August 1976 in Sichuan, China


The precursory swarm, three mainshocks (M = 7.2,6.7, 7.2), and aftershocks of the Songpan earthquakes have been reanalyzed using both local and teleseismic data. The three mainshocks of this sequence occurred on the Huya fault over a 7-day period. Relocations of the aftershocks using local arrival times show that three fault strands were activated during this sequence. Each mainshock occurred on a separate strand, each one south of the strand activated in the previous mainshock, and the aftershock zones of each mainshock appear to abut rather than overlap. Fault plane solutions determined by matching teleseismic P waveforms at World-Wide Standard Seismograph Network stations with synthetic seismograms are consistent with the observed aftershock zones. The first and third mainshocks (M_0 = 1.3 ×10^(19) and 8.4 × 10^(18) N m, respectively) showed almost identical senses of motion, a combination of reverse and left-lateral strike-slip motion, on parallel strands, striking N15°W, that were separated by a large rightstepping en echelon offset. The second mainshock (M_0 = 4.0 × 10^(18) N m), occurred in this offset on a fault at a steep angle (∼125°) to the other two strands and showed almost pure reverse motion. Differences in the orientations of the slip vectors of the three mainshocks show that the first mainshock increased the normal and shear stresses on the fault segment that moved in the second mainshock and that the second mainshock decreased the normal stress on the fault segment activated by the third mainshock. These changes in normal stresses may have given rise to the longer time between the first and second events (5 days) as compared with the time between the second and third events (30 hours). A precursory swarm that preceded the Songpan sequence by 3 years occurred in a volume that surrounded the northernmost part of the planar aftershock zone. The time between the start of the swarm and the mainshocks and the magnitude of the largest event in the swarm are similar to those seen for precursory swarms in Soviet Central Asia.

Additional Information

© 1984 by the American Geophysical Union. Received September 13, 1983; revised March 5, 1984; accepted March 30, 1984. This research was conducted under the auspices of the protocol for scientific exchange between the State Seismology Bureau (SSB) (of the People's Republic of China) and the U.S. Geological Survey (USGS) (of the United States of America). The research was supported by the USGS under Earthquake Hazard Reduction Act contract USGS 1408000120620 and by the SSB. We would like to thank the Institute of Geophysics of the SSB for their hospitality in Beijing and especially Xu Shaoxie for his help in arranging this project. We would also like to thank the Sichuan Provincial Seismology Bureau, especially Li Xinghai and Hong Xiangming, for their support and hospitality during the 2 months of work in Chengdu. We also thank Bill Bakun for supplying us with the HP-85 computer for use in Chengdu and Gerardo Suarez for supplying advice, help, and programs for the synthetic seismograms. Mary Ann Luckman and Kazuko Nagao drafted the figures. Paul Richards and David Simpson critically reviewed the manuscript. Lamont-Doherty Contribution Number 3669.

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August 19, 2023
August 19, 2023