Interseismic strain accumulation along the western boundary of the Indian subcontinent

Despite an overall sinistral slip rate of approximately 3 cm/yr, few major earthquakes have occurred in the past 200 years along the Chaman fault system, the western boundary of the India Plate with the Eurasia Plate. GPS and InSAR data reported here indicate sinistral shear velocities of 8-17 mm/yr across the westernmost branches of the fault system, suggesting that a significant fraction of the plate boundary slip is distributed in the fold and fault belt to the east. At its southernmost on-land segment, near the triple junction between the Arabia, Eurasia, and India Plates, we find the velocity across the Ornach Nal fault is 15.1+13.4+16.9 mm/yr, with a locking depth probably less than 3 km. At latitude 30 degrees N near the town of Chaman, Pakistan, where a M6.5 earthquake occurred in 1892, the velocity is 8.5+6.8 +10.3 mm/yr and the fault is locked at approximately 3.4 km depth. At latitude 33 degrees N and further north, InSAR data indicate a velocity across the Chaman fault of 16.8±2.7 mm/yr. The width of the plate boundary varies from several km in the south where we observe approximately 2 mm/yr of convergence near the westernmost strike-slip faults, to a few hundreds of km in the north where we observe 6–9 mm/yr of convergence, and where the faulting becomes distinctly transpressional. The shallow locking depth along much of the transform system suggests that earthquakes larger than those that have occurred in the historical record would be unexpected, and that the recurrence interval of those earthquakes that have occurred is of the order of one or two centuries, similar in length to the known historical record