Seismic Network Data from Quetta, Pakistan: The Chaman Fault and the Fault Related to the 30 May 1935 Earthquake
Abstract
Preliminary results from a telemetered seismic network in the Quetta area indicate seismic activity along the Quetta fault that ruptured during the 30 May 1935 earthquake (M = 7.5) and along the Chaman fault. However, the seismicity is concentrated in sections of these faults contiguous to, but not included in, the latest major rupture of each fault. The epicenter and the surface effects of the 1935 Quetta earthquake suggest that the associated rupture propagated northward from Kalat to Quetta where it was clearly associated with the Chiltan fault. A number of well-located, shallow (0-15 Km deep) hypocenters fall within the section of this fault-zone just north of Quetta. A composite fault-plane solution indicates left-lateral strike slip for the Quetta fault. The last known large rupture on the Chaman fault occurred on 20 December 1892 and it is recognized from a b u t latitude 30.3" northward. No instrumental or felt earthquakes are reported for the section oi the Chnman fault south of this latitude until 1975. Since then, the record of the Chaman fault consists of 3 of the 4 teleseismic epicenters (3 October 1975, Ms 6.7 and M 6.4; 16 May 1978, Ms 5.9) and most of the epicenters from the network data (June to November 1978) in a 2o square centered at Quetta. This increase in seismicity in the southern portion of the Charnan fault warrants careful consideration as a premonitory signal. The Chaman fault-zone south of the 1892 break is a series of subparallel left-lateral strikeslip faults which include the Quetta fault. Seismicity in this section is high, but it does not include any major event (M > 7.5). A comparison between this section of the Chaman fault system and the San Andreas fault system south of the big bend is suggested.
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