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

Basab Mukhopadhyay ¹, Anshuman Acharyya ¹, Manoj Mukhopadhyay ², Sujit Dasgupta ¹

Affiliations
1 Geological Survey of India, Central Headquarters, 27 J. L. Nehru Road, Kolkata - 700016, IN
2 Department of Geology & Geophysics, King Saud University, P.O. Box 2455, Riyadh 11451, SA

Abstract

An extraordinarily strong and persistent earthquake swarm (Andaman swarm 2005) originated in the Andaman back-arc following the aftershock sequences of the 26 December 2004 Sumatra earthquake. The swarm (n = 651, mb_max= 5.9) came mainly in two phases: January 26-31 and Feb. - Aug. 2005, in an area of size 90 x 40 km², at the centre of which lies a broad bathymetric depression and high gravity zone. The swarm demonstrates a complex faulting series, initially the strike-slip motion followed by normal faulting in repetitive sequences, whose representative fault planes orient at high angle to the regional faults. The swarm character as well as the distribution of stress-axes and their correlation to tectonic features lends speculation for formation of a nascent rift segment in NW-SE direction at the doorstep of the Sewell Seamount. The swarm has given rise to 21 episodes of rifting activities of variable time extent within 26 -31 January 2005. The r-t plots corresponding to the swarm data, modelled with variable hydraulic diffusivity (D) values 4, 6, 8 and 10 m²/s, suggest for excess pressure front from ascending magmatic fluid. This eventually heralded the rifting; causing pore pressure perturbations that propagated in accordance with known diffusion parabolic equations.

Keywords

Andaman Swarm 2005, Focal Mechanisms, Pore Pressure, R-T Plot, Nascent Rift.

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