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Earthquakes: how big will they get?

When seismologists want to assess the threat of large earthquakes, they often use the number of small tremors as a guide. But a new analysis of past jolts suggests this practice overestimates the risk of big shocks.

Scientists have long thought that the size and frequency of earthquakes are linked together by a rule, which holds that magnitude 4 quakes occur in a given region roughly 10 times as often as magnitude 5 quakes and 1,000 times as often as magnitude 7 quakes. Using this relationship, seismologists assumed they could estimate the hazard of major earthquakes from the number of smaller quakes in a region.

Researchers at Columbia University's Lamont-Doherty Geological Observatory in Palisades, N.Y., decided to test this rule by analyzing a revised catalog of the earthquakes that have occurred around the world this century. In the Jan. 2 NATURE, Javier F. Pacheco, Christopher H. Scholz and Lynn R. Sykes report that larger quakes do not follow the patterns set by smaller ones. Shocks above magnitude 6 or so occur less frequently than the number of smaller tremors would suggest.

The researchers propose that this discrepancy stems from a difference in the way large and small tremors grow. Earthquakes occur only in the upper portion of the crust, where rock is brittle enough to fracture. Rock lying below that level is too hot and ductile. When a large earthquake starts, the fracture may spread horizontally for hundreds of kilometers, but the same crack can move only a few tens of kilometers, downward before it hits the ductile zone. In weak earthquakes, however, the fault fracture remains small, so it can spread equally in the vertical and horizontal directions.
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Publication:Science News
Date:Jan 18, 1992
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