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Forces in Mexico Unrelated to San Andreas Fault : No Greater Potential for California Quake Seen

September 25, 1985|DAVID SMOLLAR

The geological forces along the Pacific Ocean floor that were responsible for last Thursday's massive earthquake in Mexico are different from those at work in California along the San Andreas fault.

And, scientists say, the occurrence of the Mexican quake, involving a different section of the Earth's crust, has not increased or decreased the potential for quakes in California. While the basic physical mechanism is the same in both cases--slabs of the crust rubbing against each other--the geometry is different.

In the case of Mexico, two plates of the crust are moving vertically against each other. The oceanic Cocos plate is being pushed down into the earth's mantle beneath the Mexican mainland, which is part of the continental American plate.

As seismologist Christopher Scholz of the Lamont-Doherty Geological Laboratory of Columbia University explained, the two plates rub together and stick for long periods until strain builds to a point where slippage--and thus an earthquake--takes place.

Similar Action

A similar vertical action--called a subduction zone in scientific terminology--is taking place off Japan and is responsible for quakes such as the massive 1923 temblor in Tokyo that killed more than 100,000 people, mainly because of ensuing fires.

The under-thrusting in Mexico between the Cocos and American plates ends south of the mouth of the Gulf of California, Scholz said.

To the north, a separate oceanic plate--the Pacific--begins to rub horizontally against the American plate, setting in motion the strains that account for activity in California along the boundary line, which is the San Andreas fault. When the friction builds up to a certain point, slippage occurs horizontally and the energy released becomes a quake, as in San Francisco in 1906.

Activity along the Mexican quake zone has no connection with the forces on the San Andreas, Scholz said.

A third type of geological result--the upthrusting of a region with quake activity--happens when one continental plate thrusts under a second continental plate, as occurs in the Himalaya Mountains of Asia.

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