El Salvador, along with the rest of Middle America (a region comprising mainly Mexico and Central America), is one of the most seismologically active regions on earth, situated atop three of the large tectonic plates that constitute the earth's surface. The motion of these plates causes the area's earthquake and volcanic activity.

Most of Central America and the Caribbean Basin rests on the relatively motionless Caribbean Plate. The Pacific Ocean floor, however, is being carried northeast by the underlying motion of the Cocos Plate. Ocean floor material is relatively dense when it strikes the lighter granite rocks of Central America, the ocean floor is forced down under the land mass, creating the deep Middle America Trench that lies off the coast of El Salvador. The subduction of the Cocos Plate accounts for the frequency of earthquakes near the coast. As the rocks constituting the ocean floor are forced down, they melt, and the molten material pours up through weaknesses in the surface rock, producing volcanoes and geysers.

North of El Salvador, Mexico and most of Guatemala are riding on the westward-moving North American Plate that butts against the northern edge of the stationary Caribbean Plate in southern Guatemala. The grinding action of these two plates creates a fault, similar to the San Andreas in California, that runs the length of the valley of the Rio Motagua in Guatemala. Motion along this fault is the source of earthquakes in northernmost El Salvador.

El Salvador has a long history of destructive earthquakes and volcanic eruptions. San Salvador was destroyed in 1756 and 1854, and it suffered heavy damage in the 1919, 1982, and 1986 tremors. The country has over twenty volcanoes, although only two, San Miguel and Izalco, have been active in recent years. Violent eruptions are rare. From the early nineteenth century to the mid1950s , Izalco erupted with a regularity that earned it the name "Lighthouse of the Pacific." Its brilliant flares were clearly visible for great distances at sea, and at night its glowing lava turned it into a brilliant luminous cone.

Data as of November 1988