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

Abstract

Abstract.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 19The first a& third mainshocks (Mo = 1.3xl0 and 8.4xl0 N m, respectively) showed almost identical senses of motion, a combination of reverse and left-lateral strike-slip motion, on parallel strands, striking Nl5°W, that were separated by a large rightstepping en yghelon offset. The second mainshock (Mo = 4.0xl0 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

Interaction between adjacent left-lateral strike-slip faults and thrust faults: the 1976 Songpan earthquake sequence

“…Thus we used the east-west horizontal principal axis in formulating our model for the computation that follows (Table 2). We assumed the Poisson's ratio of the rock was 0.164, which was used by Jones et al [6] in the wave form modeling. We used the Yang's modulus for a sandstone (7942 MPa) in our model.…”

“…The source parameters of Songpan earthquake sequence (Table 1) we used in this study are from the published work of Jones et al [6] , which were derived from both the relocation and wave form modeling methods. We assume a rectangle faulting surface for each major event.…”

“…The fault model uses the parameters from Jones [6] . White arrow incdicates the rake and the black arrow indicates the north.…”

“…The 1976 Songpan earthquake sequence is not very far (~300 km) from the 1973 Luhuo earthquake, and both events have some similarities, such as that they both occurred on left-lateral strike-slip faults and small dipping faults exist between these strike-slip faults. During the 1976 Songpan earthquake sequence, the second event occurred on a thrust fault which is followed by another left-lateral strike-slip the next day [6] . We also notice that 7 months after the main shock of the 1973 Luhuo earthquake another left-lateral strike-slip earthquake (M s = 5.8) also occurred on the Yushu-Garzê fault [6] .…”

“…During the 1976 Songpan earthquake sequence, the second event occurred on a thrust fault which is followed by another left-lateral strike-slip the next day [6] . We also notice that 7 months after the main shock of the 1973 Luhuo earthquake another left-lateral strike-slip earthquake (M s = 5.8) also occurred on the Yushu-Garzê fault [6] . Besides the difference in the magnitude, the 1976 Songpan earthquake sequence and the 1973 Luhuo earthquake sequence share a similar pattern of leftlateral, thrust, and left-lateral strike-slip events, and the left-lateral strike-slip faults are close to parallel.…”