Argentina

Argentina presents a geologically diverse seismic landscape shaped primarily by the varying geometry of the Nazca Plate subducting beneath the South American Plate along the Peru-Chile Trench to the west. While the trench itself lies in Chilean territory, the seismic consequences extend far into Argentine territory through a combination of interface coupling, crustal shortening in the Andes, and the unique phenomenon of flat-slab subduction. Beneath the Sierras Pampeanas of central Argentina — roughly between latitudes 27° and 33° South — the Nazca Plate subducts at a near-horizontal angle (flat-slab or shallow subduction) before plunging into the mantle at greater depths. This flat slab carries seismicity and crustal deformation approximately 700 kilometres east of the trench, generating earthquakes in the Argentine Precordillera, Sierras Pampeanas, and even the San Juan and Mendoza regions far inland. The Andean Principal Cordillera of western Argentina, from Jujuy in the north to Neuquén in the south, is crossed by numerous active thrust and reverse faults whose westward dipping planes generate compressional earthquakes. Argentina's southern Patagonian region sits above a different subduction geometry and is more sparsely populated, with lower overall earthquake risk to infrastructure. The actively extending back-arc region of northwestern Patagonia generates extensional faulting. The Buenos Aires metropolitan area, on the far side of the stable South American craton from the Andes, has very low seismic hazard.

Argentina's most seismically active region is the San Juan-Mendoza area of the western Cuyo, which has suffered multiple catastrophic earthquakes. The 26 January 1944 San Juan earthquake (magnitude 7.4) devastated the city of San Juan, killing 10,000–12,000 people — over 10 percent of the city's population — and destroying 90 percent of urban buildings. The disaster triggered a massive national relief campaign, during which a young army colonel named Juan Perón built political support that eventually brought him to the presidency. The 23 November 1977 San Juan earthquake (magnitude 7.4) caused additional deaths and damage in the same region. The 1929 Mendoza earthquake (magnitude 6.3) caused significant damage in the wine capital. The 1861 Mendoza earthquake (estimated magnitude 7.0) killed 12,000 people and destroyed colonial Mendoza, prompting rebuilding of the city with wider streets and lower-profile construction. In northern Argentina, the 1948 Juan Fernández earthquake (magnitude 7.8) caused damage in Tucumán. The Salta region experiences periodic M5–6 events from the Andean thrust belt. The southern Pampas occasionally registers felt events from the deep subducted slab, a seismic source at depths of 100–200 kilometres.

The geological underpinning of Argentina's seismicity involves the progressive construction of the Andean orogen through thrust and fold belt development driven by subduction. The Precordillera of San Juan and Mendoza is a fold-and-thrust belt that has been tectonically active since the Miocene, built on top of old carbonate platform sequences that have been shortened by compressional tectonics. Paleoseismic trenches across Precordillera faults have revealed evidence of multiple surface-rupturing earthquakes in the Holocene, confirming the active nature of this zone. The flat-slab subduction geometry beneath central Argentina is a relatively recent phenomenon in geological time, beginning approximately 10 million years ago as the Juan Fernández Ridge on the Nazca Plate reached the trench and began to be subducted, suppressing the asthenospheric wedge and forcing the slab to flatten. The [[aftershock]] sequences of major Argentine earthquakes in the Cuyo region are well-monitored by the INPRES national seismological institute, which operates the national network and prepares seismic hazard maps for building code purposes. [[seismic-wave]] propagation studies have documented the efficiency of wave transmission through the stable South American craton east of the Andes, explaining why magnitude 7+ Andean events are occasionally felt as far as Buenos Aires.