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

Harsh Gupta ¹, N. Purnachandra Rao ¹, D. Shashidhar ¹, K. Mallika ¹

Affiliations
1 National Geophysical Research Institute, Council of Scientific and Industrial Research, Uppal Road, Hyderabad-500007, IN

Abstract

On 12 May 2008 an earthquake of magnitude 7.9 devastated the Northwestern Sichuan province of China. This earthquake occurred along the northeast trending Longmenshan fault bordering the Tibetan plateau on the west and the Sichuan Basin on the east. The focal mechanism depicts a thrust fault mechanism with the Northwest dipping fault plane correlating well with the tectonic fault. This plane also hosts a component of right-lateral strike slip consistent with the local tectonics and orients well along the trend of aftershock distribution. Broadly, the Sichuan earthquake is a consequence of the northward convergence of the Indian plate against the Eurasian plate resulting in eastward crustal extrusion of Tibetan plateau accompanied by clockwise rotation of several tectonic blocks of the Eurasian landmass. More specifically, it occurred along the Northeast trending Longmenshan fault, due to over-thrusting of a weak Tibetan crustal block over a mechanically stronger Sichuan Basin. Estimation of p value using about 164 aftershock data provides a value of 0.87, comparable to a value of 1.0 obtained for the Muzaffarabad earthquake in western Himalaya, indicating a normal rate of decay of the aftershocks in the months ahead. We infer that aftershocks of magnitude exceeding 5 may occur for a duration of about 7 months.

Keywords

Earthquake, Seismicity, Tibetan Plateau, Siachun, China.

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Harsh Gupta ¹, D. Shashidhar ², K. Mallika ², N. Purnachandra Rao ², D. Srinagesh ², H. V. S. Satyanarayana ², Satish Saha ², R. T. B. Naik ²

Affiliations
1 National Geophysical Research Institute (Council of Scientific and Industrial Research), Uppal Road, Hyderabad - 500 007, IN
2 National Geophysical Research Institute (CSIR), Uppal Road, Hyderabad - 500 007, IN

Abstract

Earthquake activity is monitored in real time at the Koyna reservoir in western India, beginning from August 2005 and successful short term forecasts have been made of M ∼ 4 earthquakes. The basis of these forecasts is the observation of nucleation that precedes such earthquakes. Here we report that a total of 29 earthquakes in the magnitude range of 3.5 to 5.1 occurred in the region during the period of August 2005 through May 2010. These earthquakes could broadly be put in three zones. Zone-A has been most active accounting for 18 earthquakes, while 5 earthquakes in Zone-B and 6 in Zone-C have occurred. Earthquakes in Zone-A are preceded by well defined nucleation, while it is not the case with zones B and C. This indicates the complexity of the earthquakes processes and the fact that even in a small seismically active area of only 20 km x 30 km earthquake forecast is difficult.

Keywords

Earthquake, Nucleation, Forecast, Koyna Reservoir, Maharashtra.

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