Wednesday, September 5, 2012

Costa Rica - A Seismically Active Nation

Today's earthquake in Costa Rica, while quite large at a 7.9 magnitude, is not unprecedented and not unexpected by the world's geoscience community.  Here is some background information on Costa Rica's geology, something that I have seen firsthand.

Costa Rica sits along the boundary of the Cocos and Caribbean Plates as shown on this map:

The Cocos Plate is being subducted under the Caribbean Plate along the Mesoamerican trench and it is this subduction, similar to what exists in Japan, that it responsible for the country's seismic and volcanic activity.  The Cocos and Caribbean Plates converge at a rate of about 75 millimetres per year and the Cocos slab dips under the Carribbean plate at a 45 degree angle.  As the oceanic crust of the Cocos Plate descends into the depths of the crust, temperatures rise and the oceanic crust melts and rises back through the continental crust of Costa Rica, forming a chain of volcanoes including Arenal, the most active of Costa Rica's seven historically active volcanos as shown on this diagram:

When I visited Arenal, a picture-perfect conical volcano, I could see and hear huge boulders frequently erupting from from the volcano and noticed many steam vents along its flank, indicating it was still quite active.  Vulcanologists believe that Arenal is tapping a lower to mid-crustal magma chamber located about 22 kilometres below the surface.  In July 1968, Arenal erupted explosively, killing 78 people and devastating 12 square kilometres.

Subduction has created two staggered mountain ranges, the northernmost is a chain of young, active volcanoes and the southern range is the uplifted and eroded core of an earlier volcanic chain as shown on this relief map:  

The relief of some of these mountains is amazing; Cerro Chirripo has a peak altitude of 12,300 feet and was high enough to support glaciers during the Pleistocene.  Despite the fact that they are located in tropical climates, the summits of both Irazu and Turrailba can experience snow, both peaking at around 11,000 feet.

Costa Rica has a relatively long history of fairly large earthquakes.  The April 1991 magnitude 7.6 earthquake killed 47 people in the Limon area on the Caribbean coast and left 7439 homeless.  This earthquake was accompanied by a 2 metre tsunami which ran up as far as 300 metres.  In January 2009, a magnitude 6.1 earthquake centred 30 miles northwest of San Jose killed at least 20 people in landslides.  This particular quake was a strike-slip event related to subduction of the Cocos Plate.

Subduction of the Cocos plate has been responsible for some very damaging earthquakes.  The most memorable in our lifetime was the 1976 Motagua magnitue 7.5 earthquake in Guatemala that killed 23,000 and injured 76,000.  In January 2001, a magnitude 7.7 earthquake in El Salvador killed 852 people and damaged 150,000 buildings.

Here's hoping that Costa Ricans suffer little disruption from today's geologically significant earthquake.