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Three major earthquakes occurred within 4 years between the Bolan Pass and Quetta. The first of these near Sharigh Mw 6.8 24 August 1931, was followed by the Mach Mw 7.3 earthquake 27 August 1931. The third and largest earthquake was the Mw 7.7 30 May 1935 event, which destroyed 90% of Quetta and caused 35,000 deaths. All three earthquakes lie within the 150-km-wide zone between the Asian and Indian plates, a region bounded to the west by the Chaman fault and the Indus plain to the east.
The 1931 Sharigh earthquake was clearly responsible for triggering the Mach earthquake, but the geometrical relationship between these two events is obscure. The Mach earthquake reduced east-west stresses on the decollement/ramp system, and circumstantial evidence suggest that accelerated down-dip creep may have reduced fault-normal stresses that were eventually responsible for triggering the Quetta earthquake 3.5 years later.
The installed continuous operating GPS receiver at Quetta reveals a SW motion of Quetta region relative to India Plate at approximately 7 mm/yr. The observed rate, is puzzling in that Quetta lies in east of Chaman fault and its rate relative to India would therefore be expected to be much lower. For example, measurements of a point near Las Bela, less than 5 km east of the Ornach Nal fault to the south indicate that it moves less than 2±1 mm relative to India. This suggests that whereas the the Ornach Nal system near the Makran coast may in part be creeping close to the surface, the shear component of plate boundary slip must be distributed far more broadly at the latitude of Quetta.
Accordingly an array of GPS measurements has been planned to determine the details of this distributed shear. Whereas we note that our observed signal in Quetta may in part be caused by afterslip following the Quetta earthquake, the observed signal suggests that as much as 75% of the sinestral shear signal at the plate boundary remains west of Quetta suggesting that other prominent strike slip faults are vulnerable to future slip.
The present study shows there is a high potential for recurrence of such events. Potential threat to the existing built up environment of Quetta has increased by tenfold increase in population since the 1935 earthquake both in Quetta and in surrounding villages. Despite the implimentation of earthquake resistant design in post earthquake reconstruction, much recent construction is of very poor quality with an un-reinforced masonry workmanship built on alluvium, which is prone to local amplification of surface waves. The present study not only confirms that the region continues to be seismically active but that the region vulnerable to future earthquakes extends thoughout the thrust and fold belt, and that interaction between thrust faults to the east and strike slip faulting to the west are coupled in ways that may potentially provide methods to forecast future seismicity near populations centers in the province.
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