New Insight into the Subducted Indian Plate beneath Central Myanmar Based on Seismic Activity and Focal Mechanisms Analysis

Abstract

Abstract Myanmar is governed by the oblique subduction of India beneath the Burma microplate. Accurate earthquake location and focal mechanism data are crucial for understanding the detailed geometric features and dynamics of this subduction zone. We located 1433 subduction-related earthquakes and analyzed the focal mechanisms of 114 events using seismic data recorded by a recently deployed dense array in central Myanmar. The precisely located earthquakes show an east-dipping Wadati–Benioff zone beneath the Burma microplate at latitudes of 20.5°–25.0° N. In general, the subducting Indian plate imaged by seismicity has a dip angle of ∼18°–25° down to a depth of 80 km and then abruptly steepens to ∼33°–45°. The average thickness of the east-dipping seismic layer is ∼15 ± 10 km in the shallow-intermediate depth range (10–80 km) and gradually becomes ∼21 ± 10 km thick. By comparing the seismicity with the seismic velocity models revealed by receiver function and local tomography, intermediate-depth earthquakes appear to be mainly confined to the Indian lower continental crust at depths of 40–100 km. These earthquakes may be triggered by partial eclogitization reactions of the subducted lower continental crust. Stress inversion analysis of focal mechanism results grouped into 40–75, 75–90, and 90–130 km indicates that the principal compressional axes are essentially along the margin between the India and Burma microplate, implying resistance to the northward movement of the Indian plate. The principal tensional stress orients approximately east–west with its plunge systematically increasing with depth, implying the deformation may be caused by the negative buoyancy of the previously subducted oceanic slab.