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Scientists eye earthquake 'swarm'
SAN FRANCISCO, California (Reuters) -- U.S. seismologists Friday were closely watching a small fault in the Bay Area that has produced more than 100 minor earthquakes since Sunday for any evidence the tiny temblors could herald a bigger quake. Scientists said the earthquakes, known as a swarm because they happen in the same area and are all about the same strength, were clustered on a two mile-long fault that abuts the northern part of the Calaveras fault. The cluster of quakes is happening near the town of San Ramon, which is about 25 miles east of San Francisco near the Calaveras fault. The area has seen earthquake swarms in the past and they have not been precursors to a major quake. Most recently in 1990, a nearby fault produced 350 temblors over six weeks and before that a 1970 swarm included 353 quakes in about four weeks. In neither case did a serious earthquake follow. But officials with the U.S. Geological Survey said the current swarm was of concern because the fault producing it actually touches the Calaveras fault, which is viewed as prone to a bigger quake. "We believe that this swarm, while not significant ...because of the location of this fault across the Calaveras, which is a mighty fault, bears careful monitoring," said Stephanie Hanna, spokeswoman for the U.S. Geological Survey. A 1999 study of earthquake probabilities in the Bay Area predicted that there was a 70 percent likelihood of a magnitude 6.7 or higher earthquake between 2000 and 2030. That study said there was an 18 percent chance the quake would come on the Calaveras fault. David Schwartz, chief of the San Francisco Bay Area Earthquakes Hazards Project, said scientists could not be sure if the small quakes, none of which have reached 4.0 on the open-ended Richter scale that measures earthquake strength, meant a big quake was on the way. But he said there was some evidence that quakes on one fault could trigger quakes on other, nearby faults. "The question is will this have any effect on the Calaveras fault," Schwartz said. "If we were to suddenly see these earthquakes migrate onto the Calaveras fault, that might be a sign that things are changing. We haven't seen that." While the southern and central parts of the Calaveras fault have produced sizable quakes in recent years such as the 6.3 magnitude Morgan Hill earthquake in 1984, the northern end of it has been quiet for decades, Schwartz said. Morgan Hill is about 60 miles south of San Francisco. "The northern part of the Calaveras fault has been recognized as the future site of large earthquake because it is locked, that is both sides of the fault are stationary," Schwartz said. "Like the other faults in the Bay Area, this thing is primed and we don't know if on its own it will go tomorrow or in ten years or in 100 years," he said. In addition, Hanna and Schwartz noted that previous large earthquakes such as the 1989 Loma Prieta quake were preceded by small foreshocks. Loma Prieta is about 70 miles south of San Francisco. The Loma Prieta quake had a magnitude of 7.1, strong enough to knock down part of the Bay Bridge and kill 63 people. Copyright 2002 Reuters. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.
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