Geoseismological study of the Ziarat (Balochistan) earthquake (doublet?) of 28 October 2008

摘要

On 28 October 2008, two main earthquakes with similarfocal depth (10 km), identical magnitude (Mw = 6.4)and focal mechanism solutions (right-lateral strikeslip)shook the town of Ziarat and surrounding areasin Sulaiman fold-and-thrust belt of NE Balochistan,Pakistan. Named here as Ziarat doublet earthquake(ZDE), it occurred in a remote and poorly accessibleregion in the Chiltan Mountains. ZDE was accompaniedby a foreshock of mb 5.2. It killed more than 300 peopleand caused widespread destruction. An intensity ofVIII (MMI scale) has been assigned at the epicentrelocation. Epicentral distribution of 200 aftershocksindicates a clear NW–SE seismicity pattern, contraryto the general seismicity trend of the Sulaiman Beltwhich is sparse and scattered. It further shows thatmore than one tectonic subdivisions of the fold belthave experienced instability. Focal depths indicatethat most activity is confined to a narrow depth range(5–10 km). The NNW oriented right-lateral strike-slipUrghargai Fault seems to be the cause of this seismicactivity, as suggested by the focal mechanism of theZDE and depth distribution of the aftershocks. Threepossibilities can be postulated regarding the causativefault: (i) one single fault being the real cause of doublet,i.e. the point source model of earthquakes; (ii)more than one fault reactivated and (iii) failure ofseparate parts of a single fault. A number of landslidescaused by active faulting and ground shakinghave been observed in the villages of Kant, ShadiKhan and Warchum near Ziarat through field studiesand satellite images.