Intraplate stress distribution induced by topography and crustal density heterogeneities beneath the south Indian shield, India

Abstract

The occurrences of intraplate earthquakes in the south Indian shield is attributed to movement along old pre-existing Precambrian shear/weak zones in response to the perturbation in the local stress regime due to topography, crustal density inhomogeneities and the dominant plate tectonic force in south India (i.e. an assumed ridge compression of 30 MPa). To obtain a causal relationship between these stresses and pre-existing weak zones in the south of India, the stresses are estimated a depth of 20 km beneath two study areas lying at 9°–16°N and 73°–80°E. A two-layered homogeneous lithospheric model with irregular interfaces of a small amplitude and a flat lithosphere–asthenosphere boundary has been used for the elasto-static stress calculations. The estimated three-dimensional elastic intraplate stresses show that most of the small to moderate earthquakes in southern India, such as Malabar Coast (1828, intensity VII), Malabar (1858, intensity VI), Coimbatore (1900, intensity VIII), Shimoga (1975, M=5), Idukki (1988, M=4.5) and Wadakkancheri (1994, M≥4), have occurred in the zones where an noticeable change in the stress gradient is observed. However, the occurrences of a few earthquakes, such as Bellary (1843, intensity VII), Ongole (1967, M=5), Bangalore (1984, M=4.6), and Tirupattur (1984, M=4.5), may be attributed to the presence of weak faults (low material strength) at their focal depths. A marked concentration of stress is estimated beneath the region west of 78°E whilst a subdued level of stresses is observed in the region east of 78°E. A maximum value of 60 MPa at 5 km depth is calculated beneath a region about 100 km west of Bangalore, whilst the highest value of 64 MPa at 20 km depth is obtained beneath a region near Coimbatore. Based on these stress concentrations, it is suggested that the regions near the western end of both Dharwar as well as the south Indian granulite terrain are the potential locales for future earthquakes. The estimated principal stress directions for both the study areas suggest a dominance of strike-slip and reverse mode of deformation for south India. However, the direction of maximum compression axis shows a dominant NE direction near Bangalore and a variation from N–S to NE near Coimbatore. Analysis of the computed stresses indicates that the orientation of principal stresses favors strike-slip movement at the focal depths of the Ongole (1967) and Idukki (1988) earthquakes, but a combination of reverse and strike-slip deformation at the focal depth of the 1984 Bangalore earthquake. These results are in good agreement with the reported focal mechanism solution for those earthquakes.