The September 7, 2013 M 6.6 earthquake near the west coast of Guatemala occurred close to the triple-junction between the Cocos, Caribbean, and North America plates. The depth and style of faulting of the earthquake indicate that slip likely occurred on a nearly horizontal or nearly vertical fault within the subducting Cocos plate, rather than on the overlying thrust interface.  The “nearly horizontal” and “nearly vertical” possibilities correspond to two alternative, “conjugate,” faults that would produce very similar radiation of seismic waves.  Determining which of the two, conjugate, possible fault-planes actually corresponds to the causative fault will require additional research focused specifically on this issue.  At the latitude of this event, the Cocos plate moves towards the north-northeast with respect to the North American and Caribbean plates at rates of approximately 79 mm/yr and 67 mm/yr, respectively .

The broad scale tectonics of the southwestern coast of Central America are dominated by the northeastward subduction of the Cocos oceanic plate beneath the North America and Caribbean plates. Thrust- and normal-type earthquakes are a common occurrence in southwestern Central America and adjacent Mexico, with events occurring within the subduction zone - on the megathrust interface and within the subducting plate - and also in the overriding plates. Over the past 40 years, 27 events of M 6.0 or greater have occurred whose epicenters were within 300 km of the September 2013 event. Events of note in this region during this period include earthquakes on November 2012 (M 7.4) offshore of Guatemala, which caused around 40 fatalities; September 1993 (M­ 7.2) offshore of Chiapas, Mexico, which killed one person; and December 1983 (M 7.0) offshore of Guatemala. The Guatemala earthquake of February 4, 1976, occurred on the Motagua fault in the boundary-region between the Caribbean and North American plate well to the east of the September 7, 2013 epicenter.  The 1976 earthquake killed approximately 23,000 people.