Strong quake rattles San Juan

A magnitude-6.4 earthquake struck near San Juan, Argentina, this week, just days after the city commemorated a destructive earthquake three quarters of a century earlier.

By Carlos Costa, Ph.D., Departamento de Geología, Universidad Nacional de San Luis

Citation: Costa, C., 2021, Strong quake rattles San Juan, Temblor,

A magnitude-6.4 earthquake struck Argentina’s west-central province of San Juan around midnight on Monday, January 18, 2021. The epicenter has been preliminarily located at 28 miles (45 kilometers) southwest of San Juan city, the provincial capital town (population ~ 500 thousand). The event left no fatalitities, despite it’s “very strong” maximum intensity, as reported in San Juan city by the national seismological survey (INPRES). Five people were injured and 30-40 others were evacuated from damaged buildings.

map with terrain hillshade. Red, orange, and yellow circles represent earthquake locations
The magnitude-6.4 quake struck about 28 miles (45 kilometers) southwest of San Juan.


Field surveys to evaluate damage are still in progress. Despite its shallow depth — 4-5 miles (6-8 kilometers) — this event was widely felt in central-western Argentina; light shaking was felt in cities such as San Luis (200 miles/322 kilometers) and Córdoba (275 miles/443 kilometers). Numerous aftershocks of magnitude-4.9 and smaller have so far nucleated in the epicentral area at depths ranging from 4 to 7 miles (7 to 12 kilometers).

Only ground failures due to shaking in the foundations of a paved road (Route 40) have so far been observed. The epicenter is located in an uninhabited, rugged intermountain depression and surface deformation has not been reported after field checks.

Complex geology

The earthquake was located in a complex area of interacting thrust (compressional) faults oriented approximately north-south (Ahumada and Costa, 2009; Perucca et al., 2013). The area is part of a larger “fold and thrust belt” — a section of folded and faulted crust resulting from compression between two tectonic plates. This area, known as the Precordillera, formed during the latest stage of Andean mountain building resulting from subduction of the Nazca tectonic plate beneath South American plate (Ramos et al., 2002, and references therein).

Map of south America with Nazca Plate and South American Plate labeled.
The Nazca Plate subducts beneath the South American Plate.


The moment tensor — a 2D representation of the orientation of fault slip — reported by the INPRES, U.S. Geological Survey and other international agencies suggests either a northeast-southwest or northwest-southeast oriented fault surface with a significant strike-slip component. This is inconsistent with the orientation of the known active faults in the area. Thus, the mainshock and aftershocks, although consistent with a general east-west shortening, do not easily illuminate the structure at depth. The mainshock may have occurred in a transfer zone or ramp between thrusts, but more and refined data are necessary to shed light on this topic.

Past earthquakes also indicate complex structures

The relationships between the main recorded events and crustal structures have proved to be difficult to unravel in this region. Two historic events have produced surface ruptures in the area. The 1944 magnitude-7.0 earthquake and the 1977 magnitude-7.4 earthquake, both occurring in areas of compresion generated sub metric fault scarps (Kadinsky-Cade et al., 1985; Rockwell et al., 2014). Oddly, in the latter event, the surface ruptures were generated by normal (tensional) faults. Thus, it is suspected that these surface deformations do not represent the main slipping fault, but instead result from secondary ruptures.

A magnitude-6.8 earthquake, at 6-7 miles (10-12 kilometers) depth; Alvarado and Beck., 2006) struck in 1952 very close to the area of this January 18, 2021 event. In that case, no surface ruptures or deformation were reported. These issues lead some to assume that the threshold magnitude for a shallow crustal earthquake to generate primary surface ruptures in these compressive settings must be larger than magnitude-7.0 (Costa, 2005; Costa et al., 2020).

The January 18th event took place three days after the city commemorated the 77th anniversary of the destructive 1944 event which caused more than 10.000 fatalities and led to the complete reconstruction of San Juan city.

Further Reading

Alvarado, P., Beck, S., 2006. Source characterization of the San Juan (Argentina) crustal earthquakes of 15 January 1944 (Mw 7.0) and 11 June 1952 (Mw 6.8), Earth Planet. Sci. Lett., 243(3–4), 615–631, doi:10.1016/j.epsl.2006.01.015.

Ahumada, E., Costa, C., 2009. Antithetic linkage between oblique Quaternary thrusts at the Andean front, Argentine Precordillera. J. South Am. Earth Sci. 28, 207–216.
Costa, C., 2005. The seismogenic potential for large earthquakes at the southernmost Pampean flat-slab (Argentina) from a geologic perspective. Proceedings 5th International Symposium on Andean Geodynamics, 211-214.

Costa, C., Alvarado, A., Audemard, F., Audin, L., Benavente, C., Bezerra, F., Cembrano, J., González, G., López, M., Minaya, E., Santibañez, I., García, J., Arcila, M., Pagani, M., Pérez, I., Delgado, F., Paolini, M., Garro, H., 2020. Hazardous faults of South America; compilation and overview.

Kadinsky-Cade, K., Reilinger, R., Isacks, B., 1985. Surface deformation associated with the November 23, 1977, Caucete, Argentina, earthquake sequence, J. Geophys. Res., 90(B14), 12,691–12,700, doi:10.1029/JB090iB14p12691.

Ramos, V.A., Cristallini, E.O., Pérez, D.J. 2002. The Pampean flat-slab of the Central Andes. Journal of South American Earth Sciences 15, 59-78.

Rockwell, T., Ragona, D., Meigs, A., Owen, L., Costa, C., Ahumada, E., 2014. Inferring a thrust-related earthquake history from secondary faulting: A long rupture record of La Laja fault, San Juan, Argentina. Bull. Seismol. Soc. Am. 104, 269–284.