HR: 16:00h
AN: S24A-01    [Abstracts]
TI: Subduction Zone Geometry and Pre-seismic Tectonic Constraints From the Andaman Micro- plate Region.
AU: * Earnest, A
EM: earnest@cmmacs.ernet.in
AF: Centre for Earth Science Studies, Akkulam, Trivandrum, Kerala, 695031, India
AU: * Earnest, A
EM: earnest@cmmacs.ernet.in
AF: CSIR Centre for Mathematical Modelling and Computer Simulation, NAL Belur Campus, Wind Tunnel Road, Bangalore, KAR 560037, India
AU: Freymueller, J T
EM: jfreymue@gi.alaska.edu
AF: Geophysical Institute, University of Alaska, Fairbanks, AK 757320, United States
AU: Rajendran, K
EM: kusala_05@yahoo.com
AF: Centre for Earth Science Studies, Akkulam, Trivandrum, Kerala, 695031, India
AU: Rajendran, K
EM: kusala_05@yahoo.com
AF: Indian Institute of Science, CNR Rao Circle, Bangalore, KAR 560 012, India
AU: C. P, R
EM: rajendran_cp@yahoo.com
AF: Centre for Earth Science Studies, Akkulam, Trivandrum, Kerala, 695031, India
AB: The 2004 Sumatra–Andaman mega-thrust rupture broke along the narrow fore-arc sliver boundary of the Indo- Burmese collision. Earlier events of 1679 (M~7.5), 1941 (M 7.7), 1881 (M~7.9) and 2002 (Mw 7.3) generated spatially restricted ruptures along this margin. Spatio-temporal analysis of the pre-seismic earthquakes showed dense seismicity in the back-arc region but negligible activity towards the trench. The hypocentral distribution highlights the shallow subduction at the northern segment, which becomes steeper and deeper to the south. The pre-earthquake stress distribution, inferred from the P and T-axes of earthquake faulting mechanisms, represents the compressional fore-arc and extensional back-arc stress regimes. Shallow NNE-SSW under- thrusting and NNW-SSE opening up of the marginal sea basin stresses were observed and this trend changes to NE-SW to N-S at intermediate depths. We collected three epochs of campaign mode GPS data along the arc from May 2002 to September 2004. These observations show nearly pure convergence along the Andaman trench prior to the earthquake. During this period the GPS sites moved westward relative to India at ~5.5 mm/yr, consistent with the earlier results. Along arc GPS velocity vectors suggest that the Andaman trench is part of a purely slip partitioned boundary, with the strike- slip component of the India-Sunda relative plate motion being taken up on the transform fault in the Andaman Sea or on the West Andaman Fault, and the convergent component on the Andaman trench. Although near normal convergence was observed, it sampled only a fraction of a possible full Andaman microplate convergence velocity, because elastic deformation from the locked shallow megathrust caused displacements toward the overriding plate, that is, away from India. Based on the Indian plate velocity and Andaman spreading rates, this component amounts to ~85% of the pre-seismic convergence. These geodetic velocities represent the present day geologic deformation rates and have been used to build an empirical relation for replenishment time required for megathrust occurrence. For Indo/Andaman convergence rates obtained from this study, renewal rates of ~630-1100 years are observed. Assuming the convergence represents the lower limit of the deformation happening there, which may actually vary over the entire seismic cycle, a value of ~1000 years may be appropriate for a 2004 type megathrust earthquake recurrence along the Andaman – Nicobar margin.
DE: 6929 Ionospheric physics (1240, 2400)
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
DE: 8150 Plate boundary: general (3040)
DE: 8164 Stresses: crust and lithosphere
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Seismology [S]
MN: 2007 Fall Meeting