On Wednesday 5 September 2012, slightly after 8.40 am local time (slightly after 2.40 pm GMT), the Costa Rican province of Guanacaste suffered an Earthquake measured by the United States Geological Survey as a Magnitude 7.6 quake at a depth of 40.8 km. This is reported to have caused considerable damage to buildings, and while the level of injuries reported has on the whole been light so far, two people are reported to have died of heart attacks triggered by the quake, and it is quite possible that more deaths or serious injuries may emerge. A tsunami warning was initially released by the NOAA Pacific Tsunami Warning Center, but this has now been removed.
Map showing the areas which suffered the most severe shaking during the 5 September 2012 Earthquake. Damage to buildings is quite likely inside the areas outlines in green and (particularly) yellow. USGS.
Costa Rica lies on the southern margin of the Caribbean Plate; to the south of the country the Cocos Plate, which underlies part of the eastern Pacific Ocean) is being subducted under the Middle American Trench, passing under Central America as it sinks into the Earth's interior. This is not a smooth process, and the plates often stick together until the pressure builds up enough to force them to shift suddenly, causing Earthquakes. As the Cocos Plate sinks deeper if is partially melted by the friction and the heat of the Earth's interior. Some of the melted material then rises up through the overlying Caribbean Plate, fueling the volcanoes of Central America.
Diagram showing the passage of the Cocos Plate beneath Costa Rica (not to scale). Carleton College.
See also Large Earthquake off the south coast of El Salvador, Earthquake swarm strikes southern California, Seismic activity on Mount Cumbal, Colombia, Strong Earthquake to the south of Panama and Earthquakes on Sciency Thoughts YouTube.
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