A major earthquake that struck off Costa Rica today is the largest recorded in the country’s history. The magnitude 76 earthquake (M7.6), which occurred on the coast close to the town of Hojancha and some 87 miles from the country’s capital San Jose, was originally rated as M7.9 but later revised downwards by the United States Geological Survey (USGS).
Despite the scale of the earthquake, early reports indicate that limited damage occurred in the surrounding area. Though it occurred onshore, the tremor was of sufficient magnitude for a tsunami warning to be issued for coastal areas of Costa Rica, Nicaragua and El Salvador: the warning was still in effect at the time of writing.
Tectonic Setting of Costa Rica
Central America is highly tectonically active, its west coast forming part of the so-called Pacific Ring of Fire (a seismically and volcanically-active zone which surrounds the Pacific Ocean). Whereas much of the Ring is comprised of plate boundaries, the Central American section is complicated by the juxtaposition of several plates with a volcanic complex (the Central American volcanic arc) forming the isthmus of Central America itself.
The eastward movement of the Cocos tectonic plate forces it beneath the largely static Caribbean plate, along whose western edge the Central American volcanic arc (which includes the country of Costa Rica) is located. The subduction of this plate creates a deep offshore ocean trench (the Central American Trench) and as the crust is dragged downwards, friction is generated and earthquakes occur.
The volcanic front itself is a product of the subduction process: with depth, pressure and temperature increase, melting the subducted rock. This, being less dense than its surroundings, then rises upwards to reach the surface as a line of volcanoes broadly parallel with the subduction zone.
Earthquake History of Costa Rica
Much of the west coast of South America is characterised by subduction of tectonic plates and is thus prone to significant earthquake activity. In the light of this, it is perhaps surprising that the September 2012 tremor is the joint largest on record in the country. Although the USGS notes that “Cocos subduction has been responsible for a tremendous amount of historically damaging earthquakes in Central America,” its listing of significant historic earthquakes for Costa Rica contains just five earthquakes with a magnitude of M6 or greater since 1910.
Costa Rica Quakes
Though this listing may not be complete, it is nevertheless instructive to note that these major earthquakes have caused relatively few casualties in Costa Rica itself, compared with significant earthquakes elsewhere in Central and South America. At the time of writing, the country would once again appear to have been spared any serious damage as a result of its geological setting.
BBC News Online. Powerful earthquake rocks Costa Rican capital San Jose. Accessed September 5, 2012.
Carleton College Science Education Research Center. Central America Tectonic Setting. Accessed September 5, 2012.
Pacific Tsunami Warning Center. Tsunami Warning bulletin. Accessed September 5, 2012.
USGS. M7.6 – 10km NE of Hojancha, Costa Rica. Accessed September 5, 2012.
USGS. Historic earthquakes listing. Accessed September 5, 2012.