Seismotectonic Analysis of the 7 October 2021 Mw 5.9 Harnai Earthquake, Pakistan

Abstract

ABSTRACTAn earthquake of moderate magnitude 5.9 occurred northeast of Harnai (Baluchistan), Pakistan, on 07 October 2021. This event caused several deaths and injuries due to the collapse of many mud houses in the epicentral area according to the Provincial Disaster Management Authority (PDMA). The event occurred along the Quetta syntaxis, which is one of the most seismically active zones of the country. The source mechanism of the mainshock based on the moment tensor inversion technique was determined using seismic data from the local network. Stress patterns and active tectonics within the Harnai area were investigated on the basis of recent event and previously available focal mechanism solutions. The 2021 earthquake source mechanism was oblique slip with 12 km focal depth and appears to be associated with the Harnai–Karahi fault zone. Interestingly, the nodal plane oriented in the east–west direction having a high dip corresponds to reverse faulting, whereas the low dip nodal plane trending northwest–southeast-depicts a strike-slip mechanism. The upper crustal stress field and current tectonic deformation in this region are governed by the oblique convergence between the Indian and Eurasian plates along the Chaman fault zone marking the transform plate boundary. The local tectonic stress pattern anomalously supports the low dip nodal plane of the determined fault plane solution representing transpressional environment in which originally produced reverse fault is being reactivated with a dominantly strike-slip sense of movement. In this area, two stress regimes, strike-slip and thrust mechanism, were identified based on homogenous stress patterns. The recent Harnai earthquake occurred in a region where thrust-faulting environment due to horizontal principal compression (SHmax) directed in north-northwest–south-southeast act as first order stress owing to the collision of the Indian and Eurasian plates. The SHmax direction of other subset areas has a similar orientation but different stress ratio (R′), which depicts strike-slip faulting. The presence of a rigid (Katawaz) block along the plate boundary, which impedes the Indian plate motion, causes the second-order stresses that result in strike-slip sense of movement in the pre-existing reverse faults. The same stress seems to be responsible for bending of the existing faults to produce en echelon structures within the Sulaiman range and generates connecting faults in the form of a bookshelf model.