Two large earthquakes occurred in Costa Rica in 1983 and 1991. The source areas are in the vicinity of the triple junction of the Cocos, Nazca, and Caribbean plates, where a large bathymetric feature, the Cocos Ridge, is colliding/subducting beneath the Caribbean plate. The 1983 event occurred in the subduction zone of the Cocos Ridge near the southeastern terminus of the Middle America trench just west of the Panama Fracture zone. The 1991 earthquake took place about 100 km north of the 1983 epicenter and is considered to be a back-thrusting event representing the overthrusting of the North Panama deformed belt over the Caribbean plate. In this region the subduction of the buoyant Cocos Ridge should play an important role in the occurrence of unique seismological and tectonic features, for example, a decrease in seismicity, a cessation of volcanic activity, and a shallowing of the trench.

This chapter summarizes several different seismological studies for the rupture processes and associated seismicity patterns of the 1983 and 1991 Costa Rica earthquakes, attempting to constrain the tectonic structure in this region. The results provide a clearer picture of the mechanical interaction and geometry between the subducting ridge and overriding Caribbean plate than previous hypothetical models schematically illustrated for ridge-trench intersections.