Stress Orientations and Driving Forces in the Indo-Burma Plate Boundary Zone

Abstract

ABSTRACT The Indo-Burma range (IBR) is the forearc of an oblique subduction zone where the Indian slab obliquely converges with the Burma microplate. In this study, we consider earthquake focal mechanisms to help elucidate the tectonic behavior of the region. Toward this end, we examine the spatially variable stress orientations across the IBR and in the downgoing plate to better understand the associated plate driving forces and subduction dynamics. We combined earthquake focal mechanisms from 1 January 1950 until 31 December 2019 from a number of available catalogs and published studies, and divided the total of 189 events into spatial subdomains before performing stress inversions to document the spatial variability of the stress tensor. Generally, the maximum principal stress (S1) is oriented approximately north–south in all domains, subparallel to the subducting slab. The intermediate principal stress (S2) is plunging westward at variable angles. In contrast, the minimum principal stress (S3) is plunging to the east roughly following the dip of the subducting plate, indicative of down-dip tension in the slab. We do not observe a significant variation of the S1 orientation with depth, suggesting that north–south compression is due to the slab pushing northward through the mantle. Due to the well-defined slab-dip-parallel S3 direction for the megathrust and upper plate catalogs, the primary driving force of this subduction region is likely to be a net slab pull. Based on the seismic activity of the region, the consistency of focal mechanism-based stress and regional tectonic driving forces, as well as the S3 evidence for slab-pull forces, the possibility that the IBR can produce large earthquakes cannot be ruled out.