Seismicity and tectonics of the Ninetyeast Ridge Area: Evidence for internal deformation of the Indian Plate

Abstract

The Ninetyeast Ridge, far from being ‘aseismic’, has historically been a region of substantial seismicity. Since 1913, 4 magnitude 7 or greater earthquakes (including one with Ms = 7.7) and 10 magnitude 6 events have occurred in this general area. We have determined the mechanisms of several of these earthquakes, which suggest that the Ninetyeast Ridge area is presently a complex zone of deformation within the Indian plate. The northern portion (3°N–10°S) of the ridge is the active seismic zone, where both vertical and strike slip motion occur, while further south the ridge is far less seismic. This transition roughly coincides with a change in the ridge's morphology from irregular en échelon blocks to a smooth flat‐topped high. The strike slip motion is left lateral, which is consistent with the Indian (west) side encountering resistance due to the collision with Asia while the Australian (east) side is subducting smoothly at the Sumatra trench. South of about 9°S the style of deformation differs on the two sides of the ridge. To the east, normal faulting occurs, which may be related to the formation of grabenlike structures. To the west the topography can be interpreted as the result of NW‐SE compression which takes place largely aseismically but is observed for one large earthquake. This signficant intraplate deformation may explain the difficulties that occur in attempts to close the India‐Africa‐Antarctica triple junction using a rigid Indian plate.