Historical seismicity and implications for diffuse plate convergence in the northeast Indian Ocean

Large historical earthquakes provide constraints on the geographical extent and kinematics of the slow, diffuse plate boundary extending from the Central Indian Ridge to the Sumatra Trench. The March 9, 1928 Ninetyeast Ridge (M 7.7) event shows left lateral strike-slip faulting along a plane approximately coincident with the orientation of the Ninetyeast Ridge. The January 23, 1949 (M 6.75) and March 22, 1955 (M 7.0) Wharton Basin earthquakes, although only 350 km apart, show different strike-slip and thrust faulting mechanisms. All earthquakes show a regionally consistent pattern of horizontal compressional stress, which changes from NW-SE in the Wharton Basin to N-S in the Central Indian Basin. Assuming transform motion, the approximate slip rate along the northern Ninetyeast Ridge calculated from the summed moments of earthquakes is 2.8 mm/yr. -from Authors