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 fault parameters of pre-1963 earthquakes are derived using a grid search for the best fit between observed and synthetic body waveforms, surface wave spectral amplitudes, and polarity and polarization constraints, and location uncertainties are determined using a bootstrap resampling method. 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. Two small events which occurred at the location of the 1928 event in 1980 show focal mechanisms nearly identical to that determined for the 1928 earthquake and apparently reactivated the same fault. The 1928 waveforms can also be fit by a thrust faulting solution with ENE striking fault planes, but a variety of evidence, including the recent strike-slip earthquakes at the same location, suggest the strike-slip solution. These earthquakes are located along the western scarp of the ridge and do not represent reactivation of the Ninetyeast Fracture Zone to the east of the edifice, which was a major plate boundary in the early Cenozoic. 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.

The compressional axes are oriented perpendicular to linear gravity anomalies associated with lithospheric flexure, indicating little change in stress orientation since the onset of folding. Seismic deformation along the diffuse boundary consists of sinistral strike-slip faulting associated with N-S features such as the Ninetyeast Ridge and the 86¿E fracture zone, and thrust or reverse faulting in regions between fracture zones. Assuming transform motion, the approximate slip rate along the northern Ninetyeast Ridge calculated from the summed moments of earthquakes is 2.8 mm/yr. This value is an order of magnitude less than previous estimates, and suggests that at most only a small fraction of the current relative plate motion (~7 mm/yr) occurs along the ridge. The geographic distribution of seismicity suggests that the Wharton Basin east of the Ninetyeast Ridge is nearly as seismically active as the Central Indian Basin to the west. This result, along with observed long wavelength gravity anomalies in the Wharton Basin, indicates that the India-Australia plate boundary intersects the Sumatra Trench along a diffusely deforming region several thousand kilometers long, rather than along a discrete transform zone. ¿ American Geophysical Union 1989