23 August 00 | A campus geophysicist assessed movement along the northern Hayward fault and found less chance of a major quake originating on that segment than previously thought.

With the help of radar interferometry and data from global positioning satellites, plus analysis of repeating microquakes six miles below the surface, he and his colleagues concluded that the deep portions of the fault steadily slip at about the same rate as the surface does. This means the rocks deep below the surface aren't locked and building up strain that could be released in a catastrophic quake.

"Our research shows no evidence of locking at any depth, which means the threat from one of our worst hazards, right in our backyard, is much reduced," said RolandBürgmann, assistant professor of geology and geophysics. "However, other hazards - from the southern Hayward fault, the San Andreas fault and other nearby faults - leave the need to build reinforced homes and the need to be prepared just as high as before."

Bürgmann and his colleagues at Berkeley, the Lawrence Berkeley National Laboratory, the Jet Propulsion Laboratory in Pasadena, Calif., and UC Davis reported their findings in the Aug. 18 issue of Science magazine.

The Hayward fault, considered one of the most dangerous faults in California, stretches more than 60 miles from San Pablo Bay in the north to below Fremont in the south, and is a branch of the more famous San Andreas fault that extends much of the length of California. Last year a state-wide team of seismologists estimated a 32 percent chance of a major quake originating somewhere on the Hayward fault in the next 30 years. A major quake is one of magnitude 6.7 or greater.

The segment of the Hayward fault from San Pablo Bay south to the border between Berkeley and Oakland is referred to as the northern Hayward fault, which may connect under the bay with the Rogers Creek fault that runs through Napa County. Until recently, the northern Hayward fault also was ranked high in terms of the chance of a major quake. The latest assessment, that of the U.S. Geological Survey Working Group on California Earthquake Probabilities issued last October, lowered this risk, in part based on preliminary findings supplied byBürgmann's team.

Bürgmann set out several years ago to clarify the confusing history of earthquake activity along the northern Hayward fault. If, as trenching evidence suggests, the northern segment was the site of a major quake sometime between the mid-1600s and the arrival of Spanish colonists in 1776, why hasn't another quake occurred since then,Bürgmann wondered. Perhaps, he thought, the fault slips freely and large quakes do not occur on the northern segment.

"We know the Hayward fault creeps at about 5 millimeters per year at the surface, but we don't know how deep this creep goes,"Bürgmann said. "We decided to use all the data that exists to try to say how deep the creep goes, and whether the fault is locked at depth."

The techniquesBürgmann used to study activity along the fault have just recently become available. Only within the past few years has interferometric synthetic aperture radar from satellites been used to measure ground motion along faults. Thanks to detailed mathematical analysis, it is possible to determine the surface displacement that has occurred between successive orbits of the satellite, even when the orbits are years apart. With data taken in 1992 and 1997 by a pair of European satellites, plus analysis software developed at JPL,Bürgmann was able to determine surface creep within a few millimeters along the northern Hayward fault.

In addition, seismologists at Berkeley and LBNL have just recently discovered that repeating microquakes - quakes too small to be felt but indicative of small patches of the fault suddenly slipping deep underground - can reveal the amount of movement below the surface. This technique was calibrated at a study site on the San Andreas fault near Parkfield, 165 miles south of San Francisco, by Robert Nadeau, a researcher in the Berkeley Seismological Laboratory, and Thomas McEvilly, a professor emeritus of geology at Berkeley. Both are members of the Earth Sciences Division at LBNL.

"They found that some of these microquakes were occurring at exactly the same spot, and that the microquake clusters could be used to infer how fast the fault is creeping near these stuck fault patches deep underground,"Bürgmann said. "We found clusters of repeating microquakes as deep as 6 miles under Berkeley, which is evidence of structural creep far below the surface."

Putting all this information together, he estimated that the northern Hayward fault slips underground at a rate of about 5 to 7 millimeters per year, essentially the same rate as at the surface. The similar rates indicate that the fault is slipping freely without locking, he said.