M 7.1 - Ecuador

Tectonic Summary

The August 12, 2010, M 7.1 Ecuador earthquake occurred as the result of normal faulting at an intermediate depth, approximately 200 km beneath central Ecuador within the lithosphere of the subducted Nazca plate. Focal mechanism solutions indicate that rupture occurred on either a steeply dipping normal fault or a shallowly dipping normal fault. Slip on a fault of either orientation would accommodate the down-dip extension of the Nazca slab that is implied by the normal-component of the faulting solution. At the location of the earthquake, the oceanic Nazca plate moves east relative to the South America plate at a rate of about 70 mm/yr, subducting at the Peru-Chile Trench west of the Ecuadoran coast and sinking into the mantle beneath South America. Like most earthquakes of Ecuador and western South America, this event was caused by strains generated by the ongoing subduction process. This earthquake occurred as normal faulting within a segment of the subducted plate that has produced frequent earthquakes with focal depths of 160 to 200 km beneath the Earth’s surface. A M 6.7 earthquake in 1971 was situated 60 km to the southwest of the August 12, 2010, earthquake at a depth of 170 km.

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. “Deep-focus” earthquakes, those with focal depths greater than 300 km, also occur in the subducted Nazca plate farther east from the Peru-Chile Trench, beneath western Brazil and central Argentina. Earthquakes have been reliably located to depths of about 650 km in this region.

The Peru-Chile Trench is an area that hosts large earthquakes quite regularly. Within 400 km of the August 12th earthquake, there have been six other earthquakes with magnitudes of 7+ over the preceding 40 years. The largest of these events was a M 7.7 earthquake in December 1979, off the coast of northern Ecuador roughly 200 km to the northwest of the August 12th event. That earthquake, associated with thrust faulting on the interface between the Nazca and South America plates, resulted in more than 600 fatalities and 20,000 injuries.

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)