The Earthquake: A Battle of Plates

Even as Guatemala was struggling to recover from its awesome earthquake, geophysicists were trying to determine its cause. Their explanation: a battle between the gigantic plates that make up the lithosphere, or crust of the earth (TIME cover, Sept. 1).

Geophysicists theorize that these plates move like rafts on the partially molten material that surrounds the earth's liquid core. Most earthquakes take place where the plates meet and either slide past or dive beneath or ride up over each other. The majority of quakes--and volcanic eruptions--in Central America are caused by the movement of the Cocos Plate, a section of the Pacific floor that tends to move northeastward and slides beneath Central America at a deep oceanic trench just off its west coast (see map).

The Guatemala quake appears to have been caused by the movement of the two plates opposite the Cocos--the northern portion of the Americas Plate, which carries Mexico, the U.S. and Canada and generally moves in a westerly direction, and the Caribbean Plate, which carries part of Central and South America and moves toward the east, relative to its neighbor. This movement is slow, perhaps no more than 1 1/2 in. a year. But the strains created as these two huge masses slide against each other are enormous. For at least 200 years, there has been no major movement where the plates meet in Guatemala. Two weeks ago, with a titanic jolt, the Americas Plate slipped to the left, moving as much as 3 ft. westward along a rupture of more than 90 miles.

The break occurred along a generally east-west line known as the Motagua Fault and measured 7.5 on the Richter seismological scale (the 1972 quake that leveled much of Managua, Nicaragua measured 6.3). The sudden movement may have answered a question that has been bothering geophysicists for years. Earth scientists have never been sure just where the Guatemalan section of the boundary between the North American and Caribbean plates lies. Now they have an idea.

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