November 24, 2015 Earthquake: Peru-Brazil

Felt (IV) at Arequipa, Lanlacuni Bajo, Pucallpa, Puerto Maldonado and Quillabamba; (III) at Cusco, Juanjui, Juliaca, Lima, Moyobamba, Puerto Inca, Satipo, Tarapoto and Tingo Maria; (II) at San Luis, Peru. Also felt at Calca, Chachapoyas, Huancayo, Huanuco, Santa Maria and Urubamba. Felt (IV) at Arica, Camina, Codpa, Iquique and Poconchile; (III) at Alto Hospicio and Huara; (II) at Putre, Chile. Also felt at Antofagasta. Felt (III) at Rio Branco, Brazil. Also felt at Candeias do Jamari, Manaus and Porto Velho. Felt (II) at Bogota and Itagui, Colombia. Also felt at Armenia, Calarca, Cali, Funza, Girardot, La Calera and Pereira. Felt at Cuenca, Loja, Quito and Yantzaza, Ecuador. Also felt at Cochabamba, El Alto and La Paz, Bolivia. Puerto Maldonado, Madre de Dios, Peru - CDI: 5; Iberia, Madre de Dios, Peru - CDI: 5; Iquique, Tarapaca, Chile - CDI: 4; El Alto, La Paz, Bolivia - CDI: 4 Girardot, Cundinamarca, Colombia - CDI: 4

Tectonic Summary

The November 24, 2015 22:45 UTC M 7.6 earthquake was the first of two similarly sized events near 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.

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 - 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 earthquakes, 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. 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 southeast.

(above ref #13456)