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12/17/91

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Stanford geophysicists probe deep structure of Bay Area earthquake faults By Jennifer Howard and Janet Basu

Are the earthquake faults of the San Francisco Bay Area linked miles below the Bay?

Is stress transmitted from one fault to the next - and if so, how?

Could transmitted stress explain the apparent "pairing" of major quakes in the region during the 1800s and early 1900s - and does this mean an East Bay shock is due to follow the San Andreas fault-based earthquake of 1989?

In this most-studied of earthquake regions, surprisingly little is known about interconnections among the faults visible at the surface, even less about what those faults look like at depth.

Now, thanks to a massive seismic mapping experiment led by the U.S. Geological Survey in September, geophysicists have gathered the data that will make up the first three-dimensional profile of the Bay Area's geologic layers, 19 miles down to the "Moho" - Mohorovicic discontinuity at the base of the earth's crust.

Preliminary results from the Bay Area Seismic Imaging Experiment (BASIX) were presented Dec. 10 at the American Geophysical Union meeting in San Francisco.

The experiment was "like doing a brain scan of the earth," said geophysics Prof. Simon Klemperer, who led Stanford's contribution to the collaborative effort.

Led by Jill McCarthy (Stanford PhD '86) of the U.S. Geological Survey, the project took advantage of the fact that the navigable waters of San Francisco Bay overlie parts of the region's major faults. Over a period of two weeks, the geological survey research vessel S.P. Lee traversed the bay, sending out sound signals that were bounced off geologic layers beneath the earth. The signals were picked up by hundreds of land- and water-based seismic receivers, positioned over 6,000 square miles.

Planned and executed in less than a year, BASIX was a collaboration among a team of scientists from USGS, Stanford, the University of California-Berkeley, Lawrence Berkeley Laboratory and Pennsylvania State University. Co-principal investigators were McCarthy of USGS, Klemperer of Stanford, Thomas McEvilly of UC- Berkeley and Kevin Furlong of Penn State.

Stanford's contribution was to deploy more than 1000 geophones in 28 arrays specifically aligned to profile strands of major faults. The sensitive geophones, flown in especially for the experiment, are capable of picking up sounds as faint as the rustle of wind in leaves or the passing of a curious snake. Arrayed in groups of six, they can distinguish faint reflections of acoustic signals from the distant ship. Klemperer, Prof. Emeritus George Thompson and a crew of graduate students and volunteers tramped the vineyards of the Napa Valley and the redwoods of Big Basin looking for quiet locations for the instruments. They mobilized 4 tons of equipment, laid out and retrieved 10 miles of cable, and drove more than 10,000 miles to collect their data.

The specially placed land arrays, buoyed underwater microphones and the region's 200 permanent Calnet seismic stations picked up sound waves from the S.P. Lee. Differences in the time it took signals to reach a receiver signaled different layers in the earth's crust: faults show up clearly because rock layers are different on each side of the fault.

Early results are tantalizing. There is a hint of an offset in San Pablo Bay that may be a so-far-unsuspected earthquake fault. However, it will take at least a year to compare and analyze the massive amounts of data collected by the BASIX instruments and construct a full profile of the Bay Area's geologic structure.

McCarthy said that among the facts the team hopes to learn are how thick the earth's crust is in this region, and the composition of its lower layers.

Why isn't this basic information already known? Klemperer explained that traditional seismic experiments, using dynamite detonated underground, are expensive and impractical in the crowded, noisy Bay Area. Most current information about the region's faults comes from seismic recordings during earthquakes, which only disturb the top 6 to 8 miles of crust. If the BASIX profile is as detailed as expected, geophysicists may make marine-based surveys of other regions such as New York's Hudson River or the Chesapeake Bay.

One major question the scientists expect to be able to answer was posed by Penn State's Furlong: Did the grinding and overlapping of the earth's tectonic plates create horizontal links between the region's major faults?

The prevailing assumption is that the San Andreas, Hayward and other major faults are vertical all the way down to the Moho. Furlong's hypothesis holds that the region's western faults (the San Andreas and San Gregorio) may turn a horizontal fault and connect the 6 to 12 mile depth. This horizontal fault may extend beneath the bay and connect with the eastern faults (the Hayward, Calaveras and Antioch), finally slanting down through the Moho to the earth's underlying mantle.

If the faults are only vertical, some stress would be transmitted between them via the hot lower crust. If they are also horizontally connected, stress transfer could be much greater. This underground linking of faults could explain why major Peninsula or East Bay earthquakes in 1836, 1865, 1892 and 1906 were followed within two to six years by major quakes (magnitude 6.5 or greater) on the opposite side of the bay.

"The beauty of this experiment is its ability to answer simply the questions that determine earthquake hazard," Klemperer said. He cautioned, however, that it will not be able to predict with certainty whether strain might have been transmitted from the San Andreas to the Hayward faults during the 1989 Loma Prieta earthquake.

The BASIX study is being funded by the National Earthquake Hazards Reduction Program, USGS, the National Science Foundation and the participating institutions. Stanford contributed $40,000, using grants from the university's Office of Technology Licensing, the Dean and Dorothea McGee Fund and the School of Earth Sciences.

Jennifer Howard is a science writing intern with Stanford News Service

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