Seismicity and plate deformation below the Andaman arc, northeastern Indian Ocean

Abstract

Abstract The seismic activity originating below the Andaman arc-Sea region is generally discernible into fore- and back-arc seismic zones which are traceable for nearly 1500 km in a N-S direction at the junctures between the Indian, Burma and SE Asia plates. The fore-arc seismicity displays an east-dipping (40–55°) Benioff zone upto about 200 km focal depths. Details of the Benioff zone, in correspondence to the observed gravity field, are discussed in four N-S sectors, which suggest some significant variations in the configuration of the Benioff zone. The back-arc seismicity affects only the top 40–45 km of the lithosphere below the Andaman Sea, where the back-arc spreading ridge splits the volcanic arc. Stress distribution and faulting due to earthquakes below the Andaman-West Sunda arc are studied here using 68 focal mechanism solutions. Their most significant results are: low-angle thrust events occur along the upper edge of the descending Indian plate, downdip tensional events have steeply dipping ( ⩾ 60°) nodal planes, and normal faulting takes place in most parts of the Benioff zone along moderately dipping (30–45°) planes. Downdip compressional events (high-angle reverse fault, nodal plane dip > 60°) or reverse faulting along moderately dipping (30–45°) nodal planes also occur below the Andaman arc. The compressive earthquakes dominate the shallower level of the subducting slab, and the tensional stress observed locally in north part of the Andaman Sea may be an outcome of the weak coupling between the descending and overriding plates. Generally, a more or less complete sequence of faulting i.e., thrusting below the trench, normal faulting below the fore arc, and strike-slip motion along the inner edges of the fore arc characterize the Andaman-West Sunda arc. In the southern Andaman region, a rather oblique convergence between the Indian Ocean and the SE Asia plates is needed to explain the existence of a somewhat contorted Benioff zone, in which, compressional stress dominates in deeper lithosphere. Oceanward, the Ninetyeast Ridge also impinges on the subduction zone in this region. Left-lateral shear motion along the east margin of the Ninetyeast Ridge is further inferred by the results of focal mechanism solutions.