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08/30/91

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Stanford geophysicists join in Bay Area deep-fault study

STANFORD -- The U.S. Geological Survey's research vessel S.P. Lee will make nightly expeditions on San Francisco Bay Sept. 4 through 19, bouncing sound waves off faults deep in the earth to sensitive receivers deployed underwater and as far as 70 miles away on land.

The goal is to create the first three-dimensional map of the San Andreas, Hayward and associated faults. The study may answer major questions about how the faults are connected, and may lead to a more accurate assessment of the earthquake hazards in the Bay Area.

For example, the map may indicate whether the 1989 Loma Prieta earthquake could have transferred stress from the San Andreas to the Hayward fault, bringing that fault closer to producing a major earthquake.

It also may shed light on the statistical pattern of "pairing" of earthquakes in the Bay Area. Over the past 150 years, according to the Geological Survey, there has been a pattern of large to moderate earthquakes occurring on opposite sides of the Bay within two to six years of each other.

The cooperative study will be conducted by scientists from the Geological Survey, Stanford, Lawrence Berkeley Laboratory and the University of California-Berkeley. It is led by Jill McCarthy, a USGS geophysicist who received her doctorate at Stanford in 1987.

Stanford geophysics Prof. Simon Klemperer, working with a dozen students and volunteers, will collect the land portion of the data, recording the seismic signals at 28 land arrays spread along the fault lines. In all, the 28 stations will cover a 6,000-square-mile area.

The research vessel will tow an array of chambers that will release pulses of compressed air -- "like bursting a balloon underwater," Klemperer said. The resulting sound waves will travel as deep as 15 to 20 miles beneath the surface, reflecting back from earthquake faults and rock layers.

A network of 100 underwater receivers, anchored to river and bay floors, will pick up the the sound waves reflected back relatively near the path of the ship. The land receivers will record the waves that bounce at an angle, in some cases almost horizontal to the surface of the earth.

Because of the physics of wave propagation, the horizontal signals picked up at a distance on land will in some cases be stronger than the near-vertical signals picked up by the underwater receivers.

The sensitive acoustic receivers, called geophones, will measure sound velocity and log the data on small computers. Satellite clocks will ensure that timing on all parts of the system is synchronized to the millisecond so the sound velocity can be calculated.

Each velocity represents a different type of rock or sediment reflecting the sound.

The ship will cross a series of fault lines as it moves from the mouth of the Sacramento River through Suisun, San Pablo and San Francisco bays, and out the Golden Gate.

As the ship moves, the receivers will record ground motion due to the arriving soundwaves on first one side, then the other side of each fault.

"From as far north as Salt Point (Sonoma County) on the San Andreas fault, we'll be able to 'see' the ship move from the North American tectonic plate to the Pacific tectonic plate," Klemperer said.

When the data are analyzed, geologists should be able to gauge for the first time how deep the San Andreas and Hayward faults penetrate. They will be able to tell whether the faults are vertical all the way to the base of the earth's crust, 19 miles below the surface. Or, the faults may flatten out 6 to 12 miles beneath the surface.into a horizontal fault zone that would connect the major and minor faults.

Klemperer and his students are placing their 28 arrays of land-based instruments along the San Andreas, Hayward, Calaveras and Antioch faults.

The sensitive instruments were delivered on Thursday, Aug. 29, in nine pallet-loads air freighted to Klemperer by the Incorporated Research Institutions for Seismology (IRIS), on behalf of the National Science Foundation. The instruments had been deployed in seismological experiments in Alaska and Newfoundland, and will be shipped on to seismologists in Seattle as soon as the San Francisco experiment is completed.

The instrument center for this shared seismologists' resource is Columbia University's Lamont-Doherty Geological Observatory. Beginning in October, Stanford and the USGS at Menlo Park will administer a second national instrument center for IRIS instrumentation.

The seismic reflection project is funded by the National Earthquake Hazard Reduction Program, the National Science Foundation and the participating institutions. Stanford has contributed $40,000 of its own funds to the study, using grants from the university's Office of Technology Licensing, from the Dean and Dorothea McGee Fund, and from the School of Earth Sciences.

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