November 24, 2015 Earthquake: Peru-Brazil

Felt (IV) at Lanlacuni Bajo, Pucallpa, Puerto Maldonado and Quillabamba; (III) at Huanuco, Juanjui, Juliaca, Moyobamba, Puerto Inca, Satipo, Tarapoto and Tingo Maria. Also felt at San Luis. Felt (IV) at Arica and Cuya; (III) at Camina, Codpa, Poconchile and Quillagua; (II) at Alto Hospicio and Iquique, Chile. Felt at Calarca and Envigado, Colombia and at Gualaceo and Quito, Ecuador. Also felt at Chacao, Venezuela. Pucallpa, Ucayali, Peru - CDI: 5; Iquique, Tarapaca, Chile - CDI: 4; Huánuco, Huánuco, Peru - CDI: 3; San Luis, Lima Provincias, Peru - CDI: 3; Calarca, Quindio, Colombia - CDI: 3

Tectonic Summary

The November 24, 2015 22:50 UTC M 7.6 earthquake was the second 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 1000 km east of the Peru-Chile Trench within the subducted oceanic lithosphere of the Nazca plate. Focal mechanisms indicate rupture occurred on either a north or south-southeast striking, moderately dipping normal fault. 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 22:50 UTC event followed another M 7.6 earthquake (22:45 UTC) by 5 minutes; the events were separated by approximately 55 km horizontally - slightly more than typical location uncertainties of global earthquakes - and just 12 km vertically. The two events also had approximately the same focal mechanism. 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 eastwards 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 km 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 Nov. 24th earthquake, the subducted Nazca plate is again seismically active between depths of about 500 km and 650 km. The deep part of the Nazca plate, in which the Nov. 24th earthquakes occurred, took 10 million years 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 to date was the M 8.3 event that occurred at a depth of 600 km within the subducted Pacific plate beneath the Sea of Okhotsk offshore northeastern Russia in 2013. The M 8.3 Okhotsk earthquake was felt all over Asia, as far away as Moscow, and across the Pacific along the western seaboard of the United States. 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 Nov. 24, 2015 events. The Bolivia earthquake was also broadly felt across the United States.

Over the past century, 91 earthquakes with a magnitude of M 7 or more have occurred at depths greater than 300 km globally; 13 of these were located in the same region as the Nov. 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 Nov. 24, 2015 events was a M 7.0 earthquake in Oct. 1990, 15 km to the southeast.

(above reference #13456)