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. We invert the spatial distribution of horizontal strain rates in this diffuse plate boundary zone to obtain the horizontal velocity field associated with the moment release from 61 earthquakes that have occurred over the past 80 years. In the inversion, the three-dimensional rotation vector function W is determined at all corner points (i.e., knot points) of a grid such that the predicted strain rate values match the observed strain rates within the grid areas in a least squares sense.

The rotation vector of Australia relative to India, defined by the earthquake data alone, is 10.1 ¿S¿9.1¿, 81.5 ¿E¿5.4¿, 0.1¿¿0.06¿/m.y. (1 sigma error), which is slightly different in location from the expected vector estimated using seafloor magnetic anomalies and transform fault trends (3.2 ¿S, 75.1 ¿E, 0.288 ¿/m.y.)<DeMets et al., 1994a,b>. By minimizing the rate of work within a continuous viscous medium that accommodates Australia's motion relative to India <DeMets et al., 1994a>, we determine the complete horizontal velocity gradient tensor within the zone of diffuse deformation. The viscosity tensor within all of the deforming areas can be specified. When all the regions are free to strain in any direction and have the same viscosity (uniform isotropic viscosity), the predicted strain pattern is either pure extension or pure compression, which is in contrast to the style of earthquake deformation east of Chagos where both thrust and strike-slip mechanisms are observed. However, when the isotropic viscosity of regions within the Wharton Basin is increased relative to the isotropic viscosity levels of areas along the Ninetyeast Ridge, the plate motion is accommodated by combined thrust and strike-slip styles of strain along the Ninetyeast Ridge and within the eastern Central Indian Basin. This predicted strain that accommodates plate motion is in agreement, both in direction and style, with the earthquake-related strains in these regions.

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. To first order, the directionof principal strain rates, the style of strain, and the spatial distribution of strain rates from the earthquakes are in accord with the accommodation of the expected plate motion, whereas across the plate boundary zone the rates from 80 years of seismicity average out to at most 60% of the total strain rate. ¿ American Geophysical Union 1995