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

Abstract

Summary 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.

2-D Crustal thermal structure along Thuadara-Sindad DSS profile across Narmada-Son lineament, central India

“…It is believed to have originated during the middle to late Archaean period and has influenced the deposition of Neoproterozoic Vindhyan sediments to its north and Gondwana sediments to its south (West 1962;Radhakrishna 1989). The unusual features of NSL compared to other parts of the Indian shield are the occurrence of deeper earthquakes, namely the 1938 Satpura earthquake (21 • 32 N, 75 • 50 E) and the 1997 Jabalpur earthquake (23 • 5 N, 80 • 2 E) with focal depths (> 35 km) in the lower crust while focal depths of most of Indian shield earthquakes are confined to 10 km (Mukherjee 1942;Rao et al 2002;Rajendran and Rajendran 1998;Gahalaut et al 2004). Several geological and geophysical studies have been carried out to understand the evolution of NSL and its influence on tectonic framework of central India.…”

Pseudotachylyte as field evidence for lower-crustal earthquakes during the intracontinental Petermann Orogeny (Musgrave Block, Central Australia)

“…However, in contradiction to such models that limit brittle deformation exclusively to the upper crust, seismicity is also recorded in the lower crust in almost all collisional settings, e.g. the Alps (Deichmann and Rybach, 1989;Singer et al, 2014), the Himalayas (Jackson, 2002b;Jackson et al, 2004), the Tien Shan (Xu et al, 2005), the Central Indian Shield (Rao et al, 2002), and the North Island of New Zealand (Reyners et al, 2007).…”

Estimation of Source Parameters for the Aftershocks of the 2001 Mw 7.7 Bhuj Earthquake, India

“…Given this heat-flow variability, a temperature of 300°C could be reached anywhere from 6-25 km depth. Generally, deep earthquakes (25-40 km) such as the 1997 Jabalpur event in India, the1988 Saguenay event in Brazil, the 1979 Brome event in Australia occurred in areas of low heat flow (< 50 mW/m 2 ) (RAO et al, 2002). MANGLIK and SINGH (1998), while studying the thermomechanical structure of the central Indian shield, have also stated that the occurrence of deep crustal earthquakes at 35-37 km would require the surface heat flow to remain lower than 48 mW/m 2 .…”