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

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• 作者：Mandal ; Prantik
• 关键词：intraplate stresses ; topography ; crustal density heterogeneities ; South Indian shield
• 刊名：Tectonophysics
• 出版年：1999
• 出版时间：February 15, 1999
• 年：1999
• 卷：302
• 期：1-2
• 页码：159-172
• 全文大小：772 K

文摘

The occurrences of intraplate earthquakes in the south Indian shieldis attributed to movement along old pre-existing Precambrian shear/weak zones in response tothe perturbation in the local stress regime due to topography, crustal density inhomogeneities andthe 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 southof India, the stresses are estimated a depth of 20 km beneath two study areas lying at9°–16°N and 73°–80°E. A two-layered homogeneouslithospheric model with irregular interfaces of a small amplitude and a flatlithosphere–asthenosphere boundary has been used for the elasto-static stress calculations.The estimated three-dimensional elastic intraplate stresses show that most of the small tomoderate 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 zoneswhere an noticeable change in the stress gradient is observed. However, the occurrences of a fewearthquakes, 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 presenceof weak faults (low material strength) at their focal depths. A marked concentration of stress isestimated beneath the region west of 78°E whilst a subdued level of stresses is observed inthe region east of 78°E. A maximum value of 60 MPa at 5 km depth is calculated beneatha region about 100 km west of Bangalore, whilst the highest value of 64 MPa at 20 km depth isobtained beneath a region near Coimbatore. Based on these stress concentrations, it is suggestedthat the regions near the western end of both Dharwar as well as the south Indian granulite terrainare the potential locales for future earthquakes. The estimated principal stress directions for boththe study areas suggest a dominance of strike-slip and reverse mode of deformation for southIndia. However, the direction of maximum compression axis shows a dominant NE directionnear Bangalore and a variation from N–S to NE near Coimbatore. Analysis of thecomputed stresses indicates that the orientation of principal stresses favors strike-slip movementat the focal depths of the Ongole (1967) and Idukki (1988) earthquakes, but a combination ofreverse and strike-slip deformation at the focal depth of the 1984 Bangalore earthquake. Theseresults are in good agreement with the reported focal mechanism solution for thoseearthquakes.