The 11th August 2009 Andaman earthquake (Mw 7.6): A plausible cause of its genesis

The occurrence of a strong sub-oceanic earthquake of Mw 7.6 at Centroid depth of about 20 km in the North Andaman Arc (Latitude: 14.096N; Longitude: 92.926) at 01:25:39 IST has created panic in the most parts of eastern and northern India. The genesis of this mainshock is very much intriguing from scientific point of view due to following reasons: i) The occurrence of the 11th August 2009 North Andaman Mainshock hypocenter (Mw 7.6) is related to the 2004 Sumatra - Andaman megathrust tsunamigenic mainshock (Mw 9.3) [here referred to as SAMTE 9.3] rupture zone that falls in very complicated tectonic settings of the North Andaman Arc where subduction complex and accretionary prism exist very near to the North Andaman trench. ii) The 11th August 2009 North Andaman Mainshock hypocenter (Mw 7.6) falls in the area of earlier demarcated seismically active zone by Geological Survey of India during extensive aftershock monitoring (2004 - 2005) of the SAMTE 9.3 as reported through a series of publications by GSI scientists e.g., Mishra et al., 2005 (GSI Special Publication); Mishra et al. 2006 (Bulletin Seismological Society of America Special Issue, U.S.A); Mishra et al., 2007 (Seismological Research Letters, U.S.A); Mishra and Ghosh, 2008 (Specl. Issue, Indian Minerals). The aftershocks of the 2004 mega-thrust earthquake also located at depths varying from 12 km to 20 km (sP-phase relocation method, Zhao and Mishra et al., 2002; Mishra et al., 2003) where the 11th August 2009 mainshock earthquake (Mw 7.6) rocked.It is, inferred that the 11th August 2009 Andaman earthquake occurred in the previous ruptured zones of the 2004 SAMTE 9.3. iii) The mainshock hypocenter is confined to the zone where north Andaman trench starts bending, which in turn indicates a stressed zone that might have triggered brittle failure by normal faulting. The brittle failure through normal faulting represents a down deep tension, which may be related to the tearing of the subducted Indian plate as that of 1933 Sanriku Oki earthquake (Mw 8.7) near the Japan trench, which also occurred due to tearing of the Pacific Sea Slab near the Japan Trench (Kanamori, 1976, Tectonophysics; 1977, BSSA). It is also inferred that dehydration of the subducting Indian plate may have facilitated the intra-plate sub-oceanic Andaman earthquake (Mw 7.6) on the August 11, 2009, which did not generate tsunamis because of very less amount of slip during faulting. Several pieces of evidence in support of tearing of Indian plate will be presented in the Fall Meeting of the AGU 2009.