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LAMONT-DOHERTY GEOLOGICAL OBSERVATORY OF COLUMBIA UNIVERSITY, PALISADES, NEW YORK 10964
Abstract
Data from a local array of 12 short-period seismic stations located near the Tarbela reservoir in the lesser Himalayas of northern Pakistan are analyzed to estimate the anomalous thickness of the crust at the boundary between the colliding Indian and Eurasian subcontinents in the western Himalayas and the Baluchistan arc. Pn and Sn travel times indicate high upper-mantle velocities (8.50 ±0.35 km/sec for Pn at distances up to 1,500 km) combined with a thick crust near the continental plate boundary. Beneath the Himalayas the crust may be 97 ±15 km thick, which suggests it has been doubled in thickness. Local evidence indicates that this thickening occurs by imbricate underthrusting of the Indian shield.
In the Indo-Gangetic basin a heretofore unrecognized continuous zone of seismicity was found to strike parallel to the Himalayas about 200 km south of the Himalayan main boundary thrust. This zone rarely contains events with large magnitudes (m > 5); but since it traverses centers of dense population (e.g., Delhi and Lahore) and straddles potential sites of planned nuclear power plants, it may constitute a serious seismic hazard. In the Himalayas and the Baluchistan arc a comparison is made of the pattern of locally determined seismicity with that of events located as teleseisms. Local network data cover a period of about one year and magnitudes m > 2, whereas teleseismic data cover a period of 14 years and magnitudes m > 4, based on recordings of the World Wide Standard Seismograph Network (WWSSN) as reported by the U.S. Geological Survey. Both data sources show similar patterns. The main difference is that teleseismic events with large magnitudes fall mostly on major faults, whereas locally recorded events with small magnitudes occur more scattered and often fall off major faults. Several seismic gaps in the present seismicity along the Himalayas and the Baluchistan arc are identified. At some gaps the corresponding fault segment appears to be at a low stress level because it has recently undergone a major stress relief, while in other gaps stresses may be high and, hence, the fault may be in preparation for a major stress relief in the not too distant future.
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