Deep lower crustal earthquakes in central India: inferences from analysis of regional broadband data of the 1997 May 21, Jabalpur earthquake

Abstract

The only instance of a confirmed deep lower crustal earthquake occurrence in the Indian shield region has been that of the 1938 Satpura earthquake (M 6.3) of central India, reportedly at a depth of about 40 km. Moment tensor inversion of regional broadband waveform data of the 1997 May 21 Jabalpur earthquake (Mw 5.7) confirms yet another such earthquake at about 35 km depth in the central part of the Narmada-Son lineament (NSL) zone. The study is based on a refined velocity model obtained using a traveltime grid search method. A reverse fault mechanism is obtained which, for a palaeo-rift valley zone, indicates the possibility of reactivation of a pre-existing fault under the influence of the ambient stress field due to the India-Eurasia plate collision forces. The occurrence of earthquakes at lower crustal depths, quite unusual for the Indian shield region, indicates a possible causative mechanism related to crust-mantle interaction. Based on the close proximity of the two deep earthquakes and their disposition with respect to the local trend of the central part of the NSL, we suggest a model of stress accumulation due to horizontally elongated or elliptical, possibly serpentinized mafic intrusives in the lower crust, to explain the occurrence of deep earthquakes in the heart of the Indian shield.