Significant trends related to the slab seismicity and tectonics in the Burmese arc region from Harvard CMT solutions

Abstract

A careful examination of 50 centroid-moment tensor (CMT) earthquake focal mechanisms in an east-west depth section in the Burmese arc region reveals some interesting trends. The P-axis azimuths of most of the mechanisms in the region of the east-dipping Indian lithospheric slab are, on average, oriented NNE. This indicates that the principal compressive stress direction in the Burmese arc region is commensurate with the direction of convergence of the Indian plate with respect to the Eurasian plate, rather than with respect to the Burmese plate. Another interesting observation is that the strike-slip and thrust type focal mechanisms along the slab do not coexist, but are distinctly segregated, the predominantly strike-slip type occurring in the upper half of the slab down to a depth of about 90 km, and the thrust type in the lower hair, below that depth level. The former possibly indicates shearing of the Indian plate past the Burmese plate, along the zone of contact, which is inferred down to a depth of 90 km. The latter is suggestive of a NNE-directed stress on the lower portion of the subducted slab, as it is dragged against the asthenosphere by the Indian plate.