Source parameters of the Gonghe,Qinghai Province,China,earthquake from inversion of digital broadband waveform data

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Source parameters of the Gonghe, Qinghai Province, China, earthquake from inversion of digital broadband waveform data

An earthquake of M _s=6.9 occurred in Gonghe County, Qinghai Province, China on April 26, 1990. This earthquake was followed by three larger aftershocks of M _s=5.5 on May 7, 1990, M _s=6.0 on Jan. 3, 1994, and M _s=5.7 on Feb. 16, 1994, consecutively. The moment tensors of these earthquakes as function of time were obtained by the technique of moment tensor inversion in frequency domain. The results inverted indicate that these earthquakes had a very similar focal mechanism of predominantly reverse faulting on a plane striking NWW, dipping to SSW. The scalar seismic moments of these earthquakes are M ₀=9.4×10¹⁸ Nm for the M _s=6.9 event, 8.0×10¹⁶ Nm for the M _s=5.5 event, 4.9×10¹⁷ Nm for the M _s=6.0 event and 2.9×10¹⁷ Nm for the M _s=5.7 event, respectively. The results inverted also show that the source processes of these events were significantly different. The main shock had a very complex process, consisting of two distinct sub-events with comparable sizes. The first sub-event occurred in the first 12s, having a seismic moment of 4.7×10¹⁸ Nm, and the second one continued from 31s to 41s, having a seismic moment of 2.5×10¹⁸ Nm. In addition, a much smaller sub-event, having a seismic moment of about 2.1×10¹⁸ Nm, may exist in the interval of 12 s and 31 s, In contrast, the source processes of the three aftershocks are quite simple. The source time function of each of aftershocks is a single impulse, suggestting that each of aftershocks consists of a mainly uninterrupted rupture. The rise times and total rupture durations are 4 s and 11 s for the M _s=5.5 event, 6 s and 16 s for the M _s=6.0 event and 6 s and 13 s for the M _s=5.7 event, respectively.