Hi All,

Here's a SF Chronicle news story about the geology of the coastal area at Santa Cruz. This week should bring a list of geo stories with the AGU convention in progress. Its the norm..Petra

Santa Cruz may be earthquake hot spot

David Perlman, Chronicle Science Editor Thursday, November 22, 2001

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Scientists studying the geology of the Santa Cruz coast have determined that major quakes could strike the region at least three times more frequently than earlier calculations suggested.

Using sophisticated techniques to establish the ages of the coast's stepped terraces, researchers at the University of California at Santa Cruz and the Lawrence Livermore National Laboratory estimate that such powerful quakes occurred about every 160 to 270 years.

The most recent was the deadly Loma Prieta quake, which struck with a magnitude 6.9 on Oct. 17, 1989. It was centered in the Santa Cruz mountains.

But the "recurrence interval" is only an average, so the new estimate doesn't mean that Santa Cruz and the Bay Area have that long before another comparable quake hits. In fact, a major quake could strike anytime along the region's San Andreas Fault, as earthquake experts continually emphasize.

A study in the current issue of the journal Geology, by Lesley Perg and Robert Anderson of UC Santa Cruz and Robert C. Finkel of the Livermore lab, describes how they determined the approximate frequency of ancient quakes in the region.

From the air over Monterey Bay, the flat-topped green hills along the Santa Cruz coast resemble a giant's staircase, mounting nearly 750 feet from the ocean to the mountains.

Known to geologists as marine terraces, the hills have been created by a combination of seismic activity and the ocean's retreat and advance over the millennia, Perg and Anderson explain.

During ice ages, glaciers cover the land, and the ocean retreats. At the same time, the land rises millimeter by millimeter, uplifted by a succession of powerful earthquakes. When sea levels rise again at the end of each ice age, the waves carve the land into steplike cliffs.

To determine the ages of these terraces, the scientists used a technique known as cosmogenic dating. It is based on the fact that cosmic rays from outer space constantly bombard Earth and can turn ordinary elements into their radioactive versions.

In this case, traces of radioactive beryllium and aluminum in quartz grains from ancient sands in the marine terraces serve as atomic clocks to measure their age.

Studying marine terraces at Wilder Ranch State Park near Santa Cruz, Perg found that the youngest one, more than 100 feet feet above the ocean, was about 58,000 years old. And the oldest terrace, uplifted nearly 750 feet above sea level, is about 212,000 years old, she estimates.

Perg calculates that the terraces have been uplifted by earthquakes during tens of thousands of years at a rate of 1.1 millimeter -- only a few hundredths of an inch -- a year.

While that seems extraordinarily slow, it is actually much faster than scientists had previously suspected, Perg says -- meaning that large earthquakes must have been striking the region at least three times more frequently than earlier estimates.

E-mail David Perlman at dperlman@sfchronicle.com.