M 7.6 - 155 km WNW of Iñapari, Peru

Tectonic Summary

The November 24, 2015 (22:45 UTC), M 7.6 earthquake was the first of two similarly sized events near the Peru-Brazil border in southeast Peru. Both earthquakes occurred as the result of normal faulting at a depth of approximately 600 km, almost 1,000 km east of the Peru-Chile Trench within the subducted oceanic lithosphere of the Nazca plate. Focal mechanism solutions indicate that rupture occurred on either a north- or south-southeast-striking, moderately 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 earthquakes, the Nazca plate subducts to the east under the South America plate at a velocity of about 69 mm/yr.

The 22:45 UTC event preceded another M 7.6 earthquake (22:50 UTC) by 5 minutes; the events were separated by approximately 55 km horizontally—more than typical location uncertainties of global earthquakes—and just 12 km vertically. The two events also had approximately the same focal mechanism solution. The latter earthquake was almost certainly triggered by the earlier event. Seismologists sometimes refer to a pair of similarly sized earthquakes that occur at nearly the same time and location as an earthquake “doublet.”

As it descends eastward from the Peru-Chile Trench off the west coast of Peru, the Nazca plate is seismically active down to depths of about 200 km. Between depths of 200 and 500 km, where the Nazca plate subducts beneath eastern Peru, very few earthquakes are produced. Beneath Peru and Brazil in the border region near the November 24th earthquakes, the subducted Nazca plate is again seismically active between depths of about 500 and 650 km. The deep part of the Nazca plate, in which the November 24th earthquakes occurred, took 10 Myr or more to descend from the point at which it initially thrust under the South America plate.

Earthquakes that have focal depths greater than 300 km are commonly termed “deep-focus” earthquakes. Deep-focus earthquakes cause less damage on the ground surface above their foci than similar-magnitude shallow-focus earthquakes, but large deep-focus earthquakes may be felt at great distance from their epicenters. The largest recorded deep-focus earthquake was a M 8.3 event that occurred at a depth of 600 km within the subducted Pacific plate beneath the Sea of Okhotsk offshore of northeastern Russia in 2013. The M 8.3 Okhotsk earthquake was felt all over Asia, as far away as Moscow, and across the Pacific Ocean along the western seaboard of the United States (though at distant locations, individuals reporting having felt the event were likely very favorably situated for the perception of small ground motions). Prior to 2013, the largest recorded deep-focus earthquake was a M 8.2 event that occurred at a depth of 630 km within the subducted Nazca plate near the northern Bolivian border in 1994, approximately 500 km southeast of the November 24, 2015, events. The M 8.2 Bolivian deep-focus earthquake in 1994 had similarly been reported by individuals in North America at great distance from the epicenter.

Over the past century, 90 earthquakes of M 7+ have occurred at depths greater than 300 km globally; 13 of these were located in the same region as the November 24, 2015, events. The largest nearby event at these depths was the aforementioned M 8.2 Bolivia earthquake. The most recent large event in the immediate vicinity of the November 24, 2015, events was a M 7.0 earthquake in October 1990, 15 km to the southeast.

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)