Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Sushil Gupta ¹, S. K. Arora ²

Affiliations
1 Seismology Section, Reactor Safety Analysis (RSA) Group, Nuclear Power Corporation, Vikram Sarabhai Bhavan, Anushaktinagar, Mumbai - 400094, IN
2 National Institute of Rock Mechanics, Champion Reefs, Kolar Gold Fields (KGF)-563 117, IN

Abstract

The best available data from different compilations have been consolidated and homogenised for the Indian peninsular shield. These historical and instrumental data of 410 years (1586 - 1995) have been subjected to statistical completeness test to determine time - intervals in which the data sets are complete in different ranges (classes) of magnitude. The analysis reveals that the data are complete for the last about 40 years, 60 years and 120 years from present in the magnitude cIasses 4.0≤M<5.0, 5.0≤M<6.0 and M≥6.0 respectively. The two significant parametric constants of seismicity (a and b values) obtained by regression analysis of these data sets turn out to be 4.59 and 0.94 respectively that are typically indicative of a moderately active zone such as the continental interior of southern India is indeed so. Based on this deduction, the recurrence time-interval for a Latur type earthquake (M 6.3) in the Indian shield region works out to be close to 21.5 years. The magnitude of Design Basis Earthquake (DBE), assumed to occur not more than once in 50 years in the peninsular shield region, works out to be close to 6.7.

Keywords

Earthquake, Magnitude Class, Data Completeness, Seismicity Parameters, Recurrence Time-Interval, Peninsular India.

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Manoj Mukhopadhyay ¹, S. K. Arora ²

Affiliations
1 Department of Applied Geophysics, Indian School of Mines, Dhanbad - 826 004, IN
2 Seismology Division, Bhaba Atomic Research Centre, Chembur, Mumbai - 400 085, IN

Abstract

The Western Indian shield is seismically active as shown by both historical seismicity records and recently acquired instrumental data. Moderate to large earthquakes have occurred in the region but they do not clearly correlate with known tectonic features or faults. Current seismicity for this part of the shield for the period 1977-97. as evidenced from Gauribidanur Seismic Array (GBA) data, is found to be largely restricted to an area of about 80 km radius which accounts for almost 85 percent of the activity. The area is referred to as the Surat-Daman Seismic Zone (SDSZ) which has a general correspondence to the regional pattern of seismicity over the Western Indian shield established from both instrumental and non-instrumental data for a longer time span. The Indian shield crust is known to exhibit a sharp reduction in crustal thickness from 36 km below the Cambay Rift to about 20 km below the SDSZ within a distance of about 70 km. The thin shield crust is dissected into several faulted blocks that arc deformed to various degrees; some of these crustal blocks are seismically active. These features suggest that seismic activity in the region possibly relates to the fossil trace of the Khambat Plume along the Western continental margin near the Gulf of Cambay. The fossil plume appears to be located at least 100 km eastward onland. Bouguer anomalies over the fossil plume are largely positive with amplitudes upto 130 mgal, rendering further support to the presence of anomalous crust in the region. Available stress data from focal mechanism solutions indicate compressive stress in east-west to NNE-WSW direction. This suggests that stresses responsible for rift tectonics are not significant at the present stage; rather, the plume tectonics seems to be regionally predominant.

Keywords

Seismicity, Tectonics, Fossil Plume, Gulf of Cambay.

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