M 7.1 - 105 km E of Cruzeiro do Sul, Brazil

Tectonic Summary

The June 20, 2003, M 7.1 near Amazonas, Brazil, occurred as the result of deep oblique normal faulting approximately 558 km beneath western Brazil, several hundred kilometers 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 earthquake, the Nazca plate subducts to the east under the South America plate at a velocity of about 68 mm/yr.

As it descends into the mantle from the Peru-Chile Trench off of the west coast of Peru to beneath eastern Peru, the Nazca plate is seismically active to depths of about 170 km. Between depths of 170 and 530 km, where the Nazca plate subducts beneath eastern Peru and western Brazil, very few earthquakes are produced. Beneath western Brazil in the region of the June 20th earthquake, the subducted Nazca plate is again seismically active between depths of 530 and 650 km. The deep part of the Nazca plate, in which the June 20th earthquake 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 prior to this June 2003 earthquake was a M 8.2 event that occurred at a depth of 630 km within the subducted Nazca plate beneath South America near the northern Bolivian border in 1994. A larger event has since occurred—namely the M 8.3 earthquake 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 Sea of 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). 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, 70 earthquakes with magnitudes of 7+ have occurred at depths greater than 300 km globally; 12 of these were located in the same region as the June 20, 2003, event. The largest nearby event at these depths was the aforementioned M 8.2 Bolivian earthquake, the largest global deep-focus event to date.

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)