Breaking into the plate: A 7.6 Mw fracture-zone earthquake adjacent to the Central Indian Ridge

On 15 July 2003 an extremely large (7.6 Mw) strike-slip earthquake initiated on or near the end of an active transform along the northern Central Indian Ridge. The event propagated away from the plate boundary along the typically inactive fracture zone, with a sense of slip that opposes the active transform slip direction. Seismically and hydroacoustically determined aftershock locations delineate a 210 +/- 25 km long mainshock rupture. The seismic moment and rupture dimensions imply a stress drop of 4.5-6.5 MPa and a mean slip of 3.0 +/- 0.5 m. The largest aftershock (5.6 Mw) occurred on the active portion of a neighboring transform at a distance of similar to160 km, where mainshock-induced static stress changes are predicted to promote failure. Near-axis fracture-zone earthquakes may promote and inhibit ridge-parallel diking along different spreading segments, perhaps contributing to inter-segment variability in the rate and asymmetry of spreading.