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Tiny Movements Ease Fault Risk in East Bay Pressure builds up less in northern Hayward segment David Perlman, Chronicle Science Editor |
Friday, August 18, 2000
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Using the latest satellite technology, scientists have found evidence that the northern segment of the dangerously unstable Hayward Fault is steadily slipping and is far less likely to generate a major earthquake than researchers had thought. The conclusion offers scant comfort, however, for people living or working around the densely populated East Bay fault. Earthquake experts continue to warn that a major temblor on any of the Bay Area's faults is bound to shake the land everywhere -- with casualties possibly in the thousands and billions of dollars in property damage. A research team headed by Roland Burgmann of the University of California at Berkeley has found that the rocks six miles deep within the northern segment of the Hayward Fault are constantly slipping on both sides of the fault in a motion seismologists call ``aseismic creep.'' Repeated clusters of tiny, imperceptible ``microearthquakes'' detected beneath Berkeley add more evidence that the two sides of the fault's northern segment are not ``locked'' deep underground. Rather, they slip past each other, and that steady slip releases seismic strain that builds up on the fault so the strain is unlikely to snap suddenly in a single violent quake, Burgmann and his colleagues find. The scientists are publishing a report in the journal Science today that describes in detail the evidence they have gathered from radar observations by two European Space Agency satellites and by the U.S. Global Positioning System, a network of 24 satellites in orbits 12,000 miles high. The Hayward Fault cleaves the Bay Area for more than 60 miles, stretching from San Pablo Bay in the north to southern Fremont near the Santa Clara County line. Scientists divide the fault into two segments whose seismic histories differ significantly. The southern segment between Oakland and Fremont was last hit by a major quake in 1868. But recent trenching on a golf course in El Cerrito shows that the last big quake occurred on the northern segment much earlier -- sometime between the mid-1600s and the arrival of Spanish colonists in 1776. Puzzled by the northern segment's long absence of large quakes, Burgmann used a new technique called ``synthetic aperture radar interferometry'' to gather data from the European satellites as they passed overhead in 1992, and again five years later. The data detected evidence of extremely subtle changes in the earth's surface between the two sets of observations. Those changes are measured in millimeters -- only a few fractions of an inch -- but with the aid of complex mathematical computations they provide powerful evidence of changes occurring deep within the earth. The extraordinarily precise measurements, he said, could well be used to understand the behavior of the region's other major faults and to reveal portions of faults long considered seismically dangerous that may be less likely to produce major quakes. Burgmann's satellite radar observations, combined with data from the Global Positioning System and the record of ``microearthquakes,'' show that the two sides of the Hayward Fault zone, down to a depth of seven miles, are slipping past each other at slightly more than two- tenths of an inch a year along a 12-mile stretch of the fault's northern segment. NORTHERN FAULT NOT LOCKEDThat steady motion, however small, establishes that the northern fault segment is not ``locked'' deep underground as it is on the southern segment, and therefore unlikely to pose a threat. ``However, other hazards -- from the southern Hayward Fault, the San Andreas, and other nearby faults -- leave the need to build reinforced homes and the need to be prepared just as high as before,'' Burgmann said. Scores of small earthquakes constantly rattle the Bay Area, and the slow motion of ``aseismic creep'' can easily be seen in offset curbstones and cracks in the pavements of many East Bay communities along the Hayward Fault -- and also, as science students like to note --in the cracked walls of the University of California's Memorial Stadium in Berkeley. Early in its research, Burgmann's team provided data to the earthquake hazard working group headed by geophysicist David Schwartz of the U.S. Geological Survey. Last year, Burgmann's data caused the group to lower the probability of a major quake striking on the Hayward Fault's northern segment from 28 percent to 16 percent within the next 30 years. But there has always been a mystery about the northernmost end of the Hayward Fault, in part because scientists have not yet traced it down beneath the bottom of San Pablo Bay. Just on the other side of the bay, the Rodgers Creek Fault runs northward into Sonoma County as far as Santa Rosa. Many earthquake researchers believe that the two faults must be linked, and last year's working group set the probability at 32 percent for a major devastating quake on the two faults together -- the highest threat for any fault region in the study. PART OF SAN ANDREASAll the faults that lace the Bay Area are, in fact, smaller branches of the great San Andreas Fault zone, which runs for more than 600 miles from Point Delgada on the far Northern California coast all the way down to the southern San Bernardino region. The San Andreas marks the edges of two great moving slabs of the earth's crust where the Pacific plate far beneath the ocean has been grinding slowly northward against the North American continental plate for millions of years, distorting the crust and generating seismic hazards on land that never cease. Burgmann's team included scientists from the Lawrence Berkeley National Laboratory, NASA's Jet Propulsion Laboratory in Pasadena and UC Davis.
NORTHERN HAYWARD FAULT POSSIBLY LESS DANGEROUSScientists from UC Berkeley have found that rocks deep within the northern segment of the Hayward Fault are in a constant motion called ``aseismic creep.'' Because the two sides of the north-ern segment of the fault are not ``locked,'' their creeping motion eases the seismic strain on the fault so it cannot build up and snap suddenly in a single violent quake.
E-mail David Perlman at perlmand@sfgate.com. |
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