Studying the Sequence of the April 17, 2003 Delingha Earthquake (ML=6.7) by Regional Moment Tensor Inversion

On April 17, 2003, a strong earthquake of M_L = 6.7 occurred to the northwest of the Delingha City, Qinghai Province, northwest China. The epicenter (37^o33¡_N, 96^o27¡_E) lies in the Zongwulong Mountain, where the Dachaidan-Zongwulong Mountain fault (DZMF) zone runs through. In this study, we analyzed the focal mechanism of the main shock and several strong immediate aftershocks by regional waveform moment tensor inversion. We collected a comprehensive regional waveform data set that includes the broadband waveforms from the China Digital Seismic Network, the GSN, and a portable broadband seismic network deployed in the near regional distance to the earthquake at the time. One very broadband station equipped with the STS-2 seismometer and 24-bit digital data logger was only 80km from the epicenter, which gave tremendous constraint on the mechanism of the aftershock sequence (Fan and Wallace, 1991; Dreger and Helmberger, 1993). Our results show the dominance of the high angle thrust faulting striking NWW-NW. The distribution of the fault plane solutions reflects the turning and/or branching of the DZMF zone in the area, which has been confirmed by the field geological survey.