M 7.7 - 79 km ESE of Kh?sh, Iran

Tectonic Summary

The April 16, 2013, M 7.7 earthquake east of Khash, Iran, occurred as a result of normal faulting at an intermediate depth in the Arabia plate lithosphere, approximately 80 km beneath the surface in southeastern Iran. Focal mechanism solutions indicate that rupture occurred on either a southwest or east-striking, moderately dipping normal fault. Slip on a fault of either orientation would accommodate the down-dip extension of the Arabia slab that is implied by the normal-component of the faulting solution. Regional tectonics are dominated by the collisions of the Arabia and India plates with Eurasia; at the longitude of this event, the Arabia plate is converging towards the north-northeast at a rate of approximately 37 mm/yr with respect to the Eurasia plate. Arabia plate lithosphere is subducted beneath the Eurasia plate at the Makran Coast of Pakistan and Iran, and becomes progressively deeper to the north. The subducted Arabia plate is seismically active to depths of about 160 km.

The frequency of moderate and large earthquakes within the subducted Arabia plate is not high compared with similar events in some other subducted plates worldwide, but several earthquakes have occurred within this slab in the region of the April 16th event over the past 40 years, including a M 6.7 event 50 km to the south in 1983. In January 2011, a M 7.2 earthquake occurred approximately 200 km to the east, in a similar tectonic environment to that of the April 16th earthquake.

Earthquakes like this event, with focal depths between 70 and 300 km, are commonly termed “intermediate-depth” earthquakes. Intermediate-depth earthquakes represent deformation within subducted slabs rather than at the shallow plate interface between subducting and overriding tectonic plates. They typically cause less damage on the ground surface above their foci than is the case with similar-magnitude shallow-focus earthquakes, but large intermediate-depth earthquakes may be felt at great distance from their epicenters.

Hayes et al. (2016) Tectonic summaries of magnitude 7 and greater earthquakes from 2000 to 2015, USGS Open-File Report 2016-1192. (5.2 MB PDF)