\nThe April 23, 2000, M 7.0 earthquake near Santiago Del Estero, Argentina, occurred as the result of deep normal faulting within the subducted oceanic Nazca plate, more than 600 km beneath South America in northern Argentina. Focal mechanism solutions for the earthquake indicate that rupture occurred on either a near-vertical normal fault or on a very shallowly dipping oblique normal fault within the interior of the subducted Nazca slab. 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 moves eastward with respect to the South America plate, subducting at the Peru-Chile Trench west of the April 23rd earthquake at a velocity of about 71 mm/yr. \n
\n\nEarthquakes 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 April 2000 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.\n
\n\nOver the past century, 66 earthquakes with M 7 or larger have occurred at depths greater than 300 km globally; 3 of these were located in the same region as the April 23, 2000, event. The largest nearby event at these depths was a M 7.3 earthquake in June 1991, a widely felt event less than 150 km to the northwest and 50 km shallower than the April 23rd event, with no reported casualties.\n
\n\nHayes 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)\n
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