M 7.5 - 39 km NW of Yurimaguas, Peru

Tectonic Summary

The September 26, 2005, M 7.5 northern Peru earthquake occurred as the result of normal faulting at an intermediate depth, approximately 115 km beneath the surface within the subducted lithosphere of the Nazca plate. Focal mechanism solutions indicate that rupture occurred on either a north- or south-striking, moderately dipping normal fault. Of these two possible fault orientations, finite-fault modeling of globally recorded seismic data is more consistent with slip on the north-striking fault. At the location of the earthquake, the Nazca plate moves to the east relative to the South America plate at a velocity of about 70 mm/yr, subducting at the Peru-Chile Trench, to the west of the Peruvian coast and the September 26th earthquake. The earthquakes of northern Peru and most of western South America are due to strains generated by this ongoing subduction; at these latitudes, the Nazca plate is seismically active to depths of about 650 km. This earthquake occurred in a segment of the subducted plate that has produced frequent earthquakes with focal depths of 100 to 150 km.

Large intermediate-depth earthquakes are reasonably common in this section of the Nazca slab, and four other M 7+ events have occurred within 300 km of the September 26th earthquake over the past two decades. A M 7.2 earthquake in 1997, located at a similar depth but 150 km to the north of the September 26th earthquake, caused only slight damage. Two smaller earthquakes in the early 1990s, a M 5.5 and M 6.9, less than 100 km to the southwest but much shallower, caused hundreds of injuries.

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)