The October 20, 2011 Mw 5.1 Talala earthquake in the stable continental region of India

Abstract

The Talala (Sasangir) area in the Saurashtra region of Gujarat, western India, is experiencing tremors since 2001. The swarm type of earthquake activity in 2001, 2004, and every year from 2007 onward has occurred after the monsoon and lasted 2–3 months each time. In 2007 some 200 shocks (largest Mw 5.0) and in 2011 about 400 shocks down to M1 are well recorded with 1–2 km location error. The focal depths are about 2–10 km and shocks are accompanied by blast-like subterranean sounds. The epicenter (21.09 N 70.45E, focal depth: 5 km from location program, 3 km from MTS) of the October 20, 2011 mainshock occurred about 12-km WNW of Talala town or 8-km SSW of the 2007 M w 5.0 earthquake epicenter. The epicentral trends deciphered from local earthquake data indicate two ENE trends (Narmada trend) for about 50 km length and a conjugate 15-km-long NNW trend (Aravali trend). The focal mechanisms by moment-tensor analysis of full wave forms of two 2007 events of Mw 4.8 and 5.0 and the 2011 event of Mw 5.1 indicate rupture along either of the two trends. The ENE trends follow a gravity low between the gravity highs of Girnar mounts. Seismic reflections also indicate a fault in the area named Girnar Fault. Most of Saurashtra region including the Talala area is covered by Deccan Trap Basalt forming plateaus and conical ridges. There is no major fault within Saurashtra Peninsula though it is believed to have major faults along the boundaries that are non-seismic. The intensity of the October 20, 2011 Talala earthquake is estimated to be 6.5 in MM scale while isoseismals of 6, 5, and 4 and felt distance give Mw 5.1 based on Johnston’s 1994 empirical regressions. The source parameters of the 2011 Talala earthquake are estimated using data from 14 broadband seismograph stations. Estimated seismic moment, moment magnitude, stress drop, corner frequency, and source radius are found to be 1016.6 N-m, 5.1, 1.6 MPa, 1.3 Hz, and 2,300 m, respectively. The b and p values are obtained to be low, being 0.67 and 0.71, respectively. PGA of 35 cm/sec2 is noted and the decay rate of acceleration has been estimated from strong motion data recorded at 5 stations with epicentral distances ranging from 32 to 200 km.