Velocity gradients in the northern Indian Ocean inferred from earthquake moment tensors and relative plate velocities

The region in the northern Indian Ocean between the Central Indian Ridge, the Sumatra Trench, and 15°S to 9°N exhibits high seismicity, along with basement and sediment deformation. This area represents a diffuse plate boundary between the distinct and separate portions of the Indian and Australian plates. The spatial distribution of horizontal strain rates in this diffuse plate boundary zone are inverted to obtain the horizontal velocity field associated with the moment release from 61 earthquakes that have occurred over the past 80 years. By minimizing the rate of work within a continuous viscous medium that accommodates Australia's motion relative to India, the complete horizontal velocity gradient tensor within the zone of diffuse deformation is determined. Although most of the observed style and direction of strain can be explained by lithospheric strength differences between the Wharton Basin and Ninetyeast Ridge areas alone, the presence of strike-slip faulting within the Wharton Basin requires a preexisting fabric that favors strike-slip faulting there as a strain mechanism that accommodates, in part, the motion of Australia relative to India. -from Authors