Yesterday's earthquake off the west coast of Costa Rica occurred along a subduction zone where the Cocos Plate is dipping beneath the Caribbean Plate. A remnant of the Farallon Plate, like the Juan de Fuca Plate off the Pacific Northwest and the Nazca Plate off South America, the Cocos Plate lies along the west coast of southern Mexico and Central America. At its western edge, this Plate is both forming and diverging from the Pacific Plate along the East Pacific Rise, a mid oceanic ridge; shoved eastward, it is forced to subduct beneath the more buoyant Caribbean Plate. As it dips toward the mantle, the leading edge of the Cocos Plate melts, producing a chain of volcanoes east of the subduction trench (along the west coast of Mexico and Central America).
Subduction zone earthquakes generally result from a sudden slippage of the overriding plate; in this case, the western edge of the Caribbean plate was pulled down by friction with the dipping Cocos Plate and suddenly rebounded upward. Since subduction zones most often occur along coastlines, this upward motion of the overriding plate (usually a continental plate margin) also displaces the overlying seawater, potentially triggering a tsunami.
Fortunately, yesterday's magnitude 7.6 quake did not spawn a tsunami though its tremors were felt from Nicaragua to Panama. The strongest earthquake to strike Costa Rica in more than twenty years, it will almost certainly be followed by a series of aftershocks as pressure is transferred down the fault lines. In some cases, such subduction quakes also reignite volcanoes that rise along the adjacent coast. Those who live along subduction corridors thus face the triple threat of earthquakes, volcanism and tsunamis.
