A large earthquake in the Eastern Pacific on 7 November 2012 is not expected to generate a major tsunami, although early reports from the Pacific Tsunami Warning Center indicated that a small tsunami was recorded, affecting local coastal areas.
According to the United States Geological Survey the tremor was initially recorded at a magnitude of 7.4 (M7.4) and occurred just offshore 24km from the town of Champerico.
Though by no means the largest earthquake of the year (in which two earth tremors of >M8.0 were recorded off Sumatra) Reuters reports that the quake has caused at least 8 casualties as a result of collapsed buildings at the time of writing.
Tectonic Setting of the M7.4 Guatemala Earthquake of November 2012
The USGS initial reports suggested that the quake, whose focus was at a depth of around 38km, occurred just offshore and was the result of reverse, or thrust, faulting along an offshore subduction zone. At this point on the Pacific rim the Cocos tectonic plate is moving roughly north-eastwards relative to central America at a rate of approximately 70-80mm.
The collision of the Cocos Plate with the North and South American plates creates a three-way ‘triple junction’ at which the former is forced downwards beneath the two latter. Such subduction zones are highly seismically active and prone to regular earthquakes which can reach significant magnitudes.
Major Historic Earthquakes in Guatemala
The USGS list of historic earthquakes indicates that Guatemala has experienced at least three ≥M7.0 in recorded history: the same organisation’s map of historic seismicity, however, shows many more large-scale events. And, given the fact that earthquakes are no respecters of national boundaries, it is worth noting that the country has also been affected by large tremors occurring along the length of the subduction zone.
Seismic activity characterises such tectonically dynamic belts and in 2012 alone the Cocos subduction zone has seen a number of significant earthquakes, including two of M7.4 and M6.4 off the coast of Mexico and two of M7.4 and M7.6 off Costa Rica, the latter occurring within the previous two months. Smaller earthquakes have also abounded, as the map of seismicity for the year demonstrates.
Earthquake Fatalities in the Cocos Subduction Zone
Although the region has never experienced a deadly earthquake on the scale of those which have struck in other major subduction zones, such as Japan or Sumatra, previous earthquakes in the region have proved costly in human terms. The most deadly earthquake in the region was that which affected Guatemala in 1976, after which an estimated 23,000 people were reported dead.
The Cocos subduction zone was also the source of the notorious 1985 earthquake which killed around 9,500 people, most of them in Mexico City, some distance inland from the earthquake’s epicentre. Thankfully, however, this year’s earthquakes in the region have resulted in relatively few deaths.
Pacific Tsunami Warning Center. Tsunami warning bulletin. Accessed November 7, 2012.
Reuters. Eight killed in Guatemala quake. (2012). Accessed November 7, 2012.