Dec. 12, 2004

Geologists are recording continuous tremors deep under the San Andreas Fault, according to an article published in this week's edition of Science Magazine.

A cluster of these tremors was observed before a significant quake last September, raising the possibility that they may be precursors of larger events, and therefore a possible predictive tool for future earthquakes. During September, clusters of these deep tremors were measured before a September 28 earthquake measuring 6.0 on the Richter Scale took place in the Parkfield segment of the San Andreas fault.

The tremors are taking place at a depth of 12 to 15 miles beneath the surface of the earth. Normal earthquakes rarely start deeper than 9 miles. The quakes are not noticeable on the surface.

The present cluster starts near the town of Cholame in northern San Louis Obispo Country, 60 miles east of San Simeon. Cholame was the epicenter of the 1857 Fort Tejon quake, which measured 7.8 on the Richter Scale and was the last major quake to strike southern California.

On average, large quakes strike the area roughly every 140 years. It has been 147 years since the Fort Tejon quake.

To see a listing of recent earthquakes in the area, http://quake.wr.usgs.gov/recenteqs/ .

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