MW 4.9 earthquake of 21 August, 2014 in Kangra region, Northwest Himalaya: Seismotectonics implications

Abstract

A moderate earthquake (ML 5.0, MW 4.9) occurred in the Kangra region of Himachal Pradesh on 21 August, 2014 (08:11UT). The earthquake attains significance as it occurred immediately northeast of the source zone of the large Kangra earthquake of 1905 (MW 7.8), the source mechanism of the later earthquake remains a subject of debate. Further, the seismic event is nucleated in a narrow block, where intense clustering of small and moderate events has been indicated by earlier campaign mode surveys. Taking advantage that recent earthquake is well recorded by a regional BBS network, the source parameters are estimated and interpreted to provide fresh constraint on the elements of seismotectonic model. The earthquake with its epicenter at 32.34°N and 76.52°E is seated on the Panjal Thrust, boundary between Panjal imbricate zone and the Chamba nappe. The travel time inversion and wave form inversion technique suggest focal depth no more than 5km. A stress drop of 92 bars and a source radius of 1.1km. The moment magnitude (MW) for this event is calculated as 4.9. The anomalous high value of stress drop compared to the generally low values prevailing over much of the Kangra–Chamba region indicates that the earthquake of 21 August 2014 is rooted in crustal blocks characterised by strong material strength to support accumulation of high strain during the earthquake building cycle. Fault plane solution of the event under study suggested that source fault is striking 323° with dip of 34°. The rake value of 105° indicated thrust movement with a very negligible right lateral movement. Synthesis of source parameters, especially the shallow focal depth, strike paralleling the Panjal thrust/MBT with steep dip, favour the seismotectonic model where moderate magnitude earthquake of 21st August 2014 is a typical example caused by the reverse fault displacement on the Panjal thrust fault. In the compressive regime of the Kangra Himalaya, such movements are facilitated by the stresses resulting in response to the southward sliding of the Chamba nappe and thus activating the contact zone between the nappe and Panjal imbricate zone.