Structure and Tectonics of the Andaman Subduction Zone from Modeling of Seismological and Gravity Data

The 10 August 2009 Andaman earthquake of Mw 7.5 occurred to the north of the Andaman and Nicobar Islands at 14o N and 93o E which interestingly, coincides with the northern periphery of the rupture of the Sumatra-Andaman giant mega-thrust earthquake of Mw 9.1 that occurred on 26 December 2004. The event was followed by aftershocks with a peculiar vertical distribution at the same location which was earlier devoid of any significant seismicity. Waveform modeling of five of these events recorded by ISLANDS - the broadband seismic network deployed along the Andaman and Nicobar Islands, indicates that the main shock and two of its aftershocks have a normal fault mechanism with shallow focal depths within 18 km while two others have a strike-slip mechanism occurring deeper, down to 26 km. The computed Bouger gravity anomalies in this region indicate the steepest gradient of 1.5 mgal/km exactly centered over this zone of vertical seismic distribution that characterizes a region of lithospheric split or tear which is devoid of a subducting slab. This is in contrast to a clear subduction trend visible in the southern Andaman and Sunda arcs further south, as evidenced by tomographic images. Joint inversion of waveforms of these five events simultaneously, provides the best fitting P wave velocity structure of this region, given by a Moho at a depth of 30 km and a high crustal Vp/Vs ratio of 1.81. We infer an oceanic double crustal column corresponding to a thickness of about 21 km of Burmese crust including a 5 km thick sedimentary column, underlain by a thinner Indian crust which apparently has a thickness of about 9 km, a model that is also confirmed independently by gravity modeling. We interpret the mechanism of shallow normal fault earthquakes as an intra-plate relaxation phenomenon following the buckling of the overriding Burmese plate in the accretionary wedge of the fore-arc basin, in response to the 2004 mega-thrust subduction event. The deeper strike slip events correspond to an intra-plate phenomenon within the subducting Indian lithospheric plate representing left-lateral faulting across the Andaman arc, due to uneven convergence along the subduction front. Such strike-slip movements are seen all over the Indian Ocean diffuse deformation zone and represent strain accommodation in the Indian crust in response to a grosser mechanism of wrench fault tectonics of the Indo-Australian subduction beneath the Burma-Sunda plate.