M7.0 earthquake 26 km NNE of El Hoyo, Argentina

Felt (V) at Santiago del Estero and (III) in Cordoba, Corrientes and La Rioja. Also felt at Rosario and San Juan. Felt (III) at Campinas, Brazil. Also felt at Brasilia, Londrina, Marilia, Nova Friburgo, Rio Negro and Sao Paulo. Felt (III) at Mejillones and Pozo Almonte; (II) at Antofagasta, Arica, Coquimbo, Huara, La Serena and Vina del Mar, Chile. Also felt at Calama, Chillan, Quilpue and Valparaiso.

The January 1, 2011, M 7.0 earthquake near Santiago Des Estero, Argentina, occurred as the result of deep normal faulting, approximately 570 km beneath northern Argentina within the subducted lithosphere of the oceanic Nazca plate. Focal mechanism solutions indicate that rupture occurred on either a steep, southeast-striking fault or on a shallow, north-striking 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 moves east relative to the South America plate at a velocity of about 70mm/yr. The Nazca plate is overridden by the South America plate at the Peru-Chile Trench, west of the Chilean coast, and sinks into the mantle beneath South America. This earthquake occurred as normal faulting within a segment of the subducted plate that has produced frequent earthquakes with focal depths of 500 km to 625 km beneath the Earth’s surface. In the past quarter century, two other events of M 7+ have occurred in the segment of the subducted Nazca plate that lies beneath northern Argentina. Neither produced recorded damage or casualties.

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 January 2011 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, 82 earthquakes with a magnitude of M 7+ have occurred at depths greater than 300 km globally; 4 of these were located in the same region as the January 1st event. The largest nearby event at these depths was a M 7.3 earthquake in June 1991, just 20 km to the west and 20 km shallower than the January 1st event.

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)