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Accurate Relative Earthquake Hypocenters Reveal Structure of the Burma Subduction Zone

Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, U.K.

N. D. Selby

AWE Blacknest, Brimpton, Reading, Berkshire RG7 4RS, U.K. neil{at}blacknest.gov.uk

R. Heyburn and M. P. Searle

Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, U.K.

^* Present address: BMT Isis, Ltd., 210 Lower Bristol Road, Bath BA2 3DQ, U.K.

Present address: AWE Blacknest, Brimpton, Reading, Berkshire RG7 4RS, U.K.

Online Material: Origin time, m_b, and hypocentral location for the 81 Burma earthquakes.

The Burma arc links the Himalaya to the Andaman–Sumatra trench, and comprehension of the geological features in the region is vital to understand the active tectonic processes in the area. Subduction on the Sumatra trench produced the devastating 2004 Sumatra earthquake (M_w9.0), but the mechanisms accommodating relative India–Sundaland motion in Burma are still unknown. Previous seismological studies of the area use only earthquake catalog hypocenters, so structural details of subducted material remain poorly understood. Here, accurate relative hypocenters for 81 earthquakes are estimated using first arrivals picked from regional and teleseismic recordings. Depth determination is improved using measured travel-time differences between P onsets and depth phases (pP and sP). The results clearly illustrate a slab 21°–25° N dipping 25° at 40–80 km deep, 40° at 80–120 km deep, and 60° at 120–160 km deep, and its strike follows the Indo–Burman ranges. At latitudes >25° N earthquakes become more shallow, and a diffuse picture of hypocenters with an east–west lateral discontinuity at depth is observed, indicating lateral deformation of the slab.

Previous studies suggest there is a transition at 90 km deep from shallower strike-slip earthquakes to deeper thrust-type events. Here, accurate depth estimates, combined with confirmation of two Global Centroid Moment Tensor solutions using body-wave modeling, shows that thrust and strike-slip earthquakes both occur deeper than 100 km in the Burma arc.