Magnitude 7.1 AMAZONAS, BRAZIL

Tectonic Setting
Deep earthquakes, such as this 550 km deep (340 miles deep) shock beneath western Brazil, seldom cause damage because of their distance from the earth's surface. They occur only in oceanic lithosphere that has been subducted into the Earth's mantle. The June 20 earthquake occurred within the Nazca plate, which currently underthrusts the South American plate at the Peru-Chile Trench, along the west coast of South America. As it descends from the west coast of Peru to eastern Peru, the Nazca plate is seismically active down to depths of about 170 km. Between depths of 170 km and 530 km, the Nazca plate beneath eastern Peru and western Brazil produces very few shocks. Beneath western Brazil in the region of the June 20 earthquake, the subducted Nazca plate is again seismically active between depths of 530 km and 650 km. The deep part of the Nazca plate, in which the June 20 earthquake occurred, took 10 million years or more to descend from the point at which it initially thrust under the South American plate. The largest earthquake known to have occurred in the subducted Nazca plate was the Bolivian earthquake of June 9, 1994, which had a magnitude of 8.2.