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Parkfield jolt triggers quake alert.

After a mild tremor shook the San Andreas fault in central California last week, seismologists issued an alert announcing that the predicted Parkfield earthquake might finally occur. But the three-day alert expired without incident.

The Parkfield segment of the San Andreas has spawned major quakes about every 22 years, with the last shock in 1966. Recognition of this pattern in 1985 led U.S. Geological Survey scientists to predict a 95 percent chance that a magnitude 6 quake would strike the tiny town of Parkfield before the end of 1992.

The quake did not occur last year, but a magnitude 4.7 temblor shook the fault in October 1992, causing California to issue an A-level alert- signifying a one-in-three chance that the quake would hit in three days. This was the first time the state had ever issued a quake alert. Last week's alert, issued after a magnitude 4.8 shock, also proved to be a false alarm.
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Title Annotation:Nov 1993 Parkfield, California, quake-alert proves to be false alarm
Publication:Science News
Article Type:Brief Article
Date:Dec 4, 1993
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