Friday, August 18, 2000 | Print this story

Chance of Hayward Fault Quake Downgraded, but Risk Still High

By USHA LEE MCFARLING, Times Science Writer

     A stretch of Northern California's most dangerous fault--the Hayward--running underneath densely populated portions of San Francisco's East Bay is far less likely to spawn an earthquake than previously thought, according to a new report being published in today's issue of the journal Science.
     The study shows that a 12-mile section of the fault running north from Berkeley is creeping along steadily, thus avoiding the locking that can build strain and lead to earthquakes. The finding was made using new precision satellite data and recordings of barely perceptible microquakes deep within the Earth.
     While praising the study, earthquake experts were quick to warn that the seismic hazard remains high in the area--because of quakes that could originate on parts of the fault system to the south or north and on other area faults.
     "Folks should not be unbolting their water heaters," said Robert Simpson, a geophysicist at the U.S. Geological Survey in Menlo Park who has studied the fault for a decade.
     "It's false to give the impression that the hazard is lower," added David Schwartz, head of the San Francisco Bay Area Earthquake Hazards Project for the survey.
     The Hayward fault, a major branch of the San Andreas fault system, runs for more than 60 miles through the densely populated East Bay from Fremont through Hayward, Oakland and from goal post to goal post underneath UC Berkeley's football stadium. It runs underneath freeways and aqueducts and close to many Bay Area reservoirs, freeways and hospitals.
     Since the late '80s, the lesser known Hayward has overshadowed the legendary San Andreas as a potential fault hazard: It is considered to be the Bay Area fault most likely to rupture in a major quake in the next three decades. A 1995 study suggested that the financial toll of a major quake on the Hayward fault could be twice that of Northridge and that thousands of people would be injured or killed. "The Kobe quake [in Japan] is a preview of Hayward," Schwartz said.
     While the fault's future is uncertain, its past is murky as well. Geologists divide the fault into two portions: a northern stretch that runs from the Oakland and Berkeley border past El Cerrito, and a southern stretch running from Oakland to Fremont.
     Historical records show that the southern portion of the fault ruptured in 1868, in a quake that was known as "the big one" until the 1906 San Francisco quake. It was long thought that the northern part of the fault had ruptured in a large 1836 earthquake.

     Gold Rush Documents
     But in the mid-1990s, enterprising seismologist Tousson Toppozada of the California Division of Mines and Geology delved into scanty pre-Gold Rush historical records and determined that newspaper reports of a quake on the northern part of the fault in 1836 were wrong. There had been no such quake.
     That finding made the northern stretch of the Hayward seem even more dangerous. If it hadn't ruptured in the recent past, the fault was thought to be overdue for a big quake. So geologists began to study the northern stretch of the fault in earnest.
     To find out when the last big earthquake occurred there, a team of paleoseismologists in 1997 carved a trench over the fault--on the second fairway of El Cerrito's Mira Vista golf course. There, disturbed layers of sand and earth confirmed that the fault had not seen an earthquake for 200 to 300 years.
     That led to more scientific questions. Why hadn't it ruptured in so long? And was it overdue for a dangerous quake?
     That's what Roland Burgmann, the lead author of the current study and a UC Berkeley geologist, hoped to find out. It was known that the surface of the fault crept along at 5 millimeters per year. But such creeping would only prevent earthquakes if it also occurred deep within the fault, miles below the surface, where locking occurs.
     Burgmann's team could only answer the questions using new technology. They compared images of the fault taken from satellites in 1992 and in 1997 to measure how far the ground had moved.
     The satellite technique is precise and able to measure small differences in ground movement over a large area, said Eric Fielding, a geophysicist at NASA's Jet Propulsion Laboratory who helped develop the technology. Previously, geologists had to rely on a smattering of painstakingly gathered individual readings of ground motion and did not have enough observations to see the whole fault in context.
     Burgmann also worked with geologists Robert Nadeau and Thomas McEvilly at Lawrence Berkeley National Laboratory to measure clusters of nearly imperceptible microquakes six miles underneath the surface. These little quakes, which are recorded by seismometers at the surface even though they cannot be felt by people, indicate that small patches of the fault have slipped deep underground and can be used to infer how fast the earth is creeping at those depths.
     "It's like having a creep-meter at depth," said the geological survey's Simpson, who was not involved in the study but said the new technology provided "terrific new tools" for geologists.

     Most Faults Do Not Slip So Easily
     Putting all the data together, Burgmann found that the fault is slipping at about the same rate underground as it is at the surface. "However hard you push it, it will eventually respond by creeping. There's no locking going on," he said. Most faults outside California do not slip so freely. California faults may differ because of the state rock, serpentine. Soft and easily fractured, the greenish rock may lubricate faults, Burgmann said. The work adds to the knowledge about the little understood process of creep and how it alters fault behavior.
     The finding, in preliminary form, has already been used by geologists who assess quake risks.
     Such probabilities are determined by scientific committee, using a mixture of history, science and instinct. They balance the strain caused by the motion of the Earth's plates with the strain that has been released by earthquake and by creep. They hone these estimates using knowledge of historical quakes and theoretical knowledge of fault behavior.
     In October, when the most recent assessment was made, scientists used the new report to downgrade the likelihood of a major quake on the northern stretch of the Hayward fault within the next 30 years from 28% to 16%. The risk for the entire fault system, including the Rodgers Creek fault, which runs north through Napa County, is 32%--the highest in the Bay Area.
     The hazard on the northern section of the Hayward fault did not drop to zero, because the area still could rupture from quakes that originate on other parts of the fault system. "It ain't going to start the quake, but you're sitting between two segments that can," Toppozada said of the northern section of the fault.
     In addition, shaking from distant parts of the fault could cause major damage in Oakland and Berkeley. The Loma Prieta quake of 1989 severely damaged the Bay Bridge, about 60 miles from the epicenter.

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