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Upland quake: taste of things to come?

Upland quake: Taste of things to come?

Nothing publicizes seismic hazard better than an earthquake, as all seismologists know. True to form, the magnitude 5.5 temblor that hit Upland, Calif., 30 miles northeast of Los Angeles, on Feb. 28 has now focused attention on faults in that area that could potentially generate quakes similar in size to the Loma Prieta shock that shook northern California last October.

The Upland quake struck just south of the San Gabriel mountains on a north-south-trending fault that splits the Cucamonga fault to the east and the Sierra Madre fault to the west. Geologic evidence indicates the Cucamonga has been active during the past several thousand years, spawning earthquakes about every 600 to 700 years. Geologists don't have a firm date for the last large quake on the Cucamonga, but they think it might have occurred 600 to 700 years ago, says geologist Douglas M. Morton of the U.S. Geological Survey in Riverside, Calif.

Last month's Upland shock followed smaller quakes on the same fault in 1988 and on the Cucamonga last year. This sequence may signal that stress in the Cucamonga has reached a point where it could soon generate a large earthquake, on the order of magnitude 6.5 to 7.2, which would likely wreak major damage in this developing region. On the other hand, it may foreshadow nothing for the Cucamonga, says Morton.

Even more uncertain is the hazard from poorly defined faults to the west of the Upland temblor, many of which disappear from view beneath the sediment-filled valley. Geologists know this area must be absorbing stress caused by motion between the North American and the Pacific plates, but they don't know which faults take up the slack and whether such faults move by generating large earthquakes or instead creep without producing quakes. The recent activity offers some hope to Morton and others who have decided to examine this region in closer detail. Seismic information from the earthquake and its aftershocks may help fill in some important details about the faults in that area, he says.
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Title Annotation:earthquake in Upland, California
Publication:Science News
Date:Mar 17, 1990
Words:348
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