August 18, 2000, Friday
National Desk 'Big Quake' Found Less Likely Along Fault
By SANDRA BLAKESLEE
The earthquake fault considered most likely to bring destruction to the San Francisco Bay area may actually be incapable of generating a large earthquake on its own, scientists say.
New measurements taken from space and from deep within the earth suggest that a 12-mile northern segment of the fault, the Hayward Fault, which runs north from Berkeley to the southern edge of San Pablo Bay, is in constant motion, creeping steadily along its entire width and depth. This process relieves stresses along the fault and prevents large earthquakes.
But this discovery does not mean communities along the fault segment are safe from earthquakes, said Dr. Roland Burgmann, a geophysicist at the University of California at Berkeley who described the finding today in the journal Science. He said there were plenty of places nearby or across San Francsico Bay that could give rise to an earthquake powerful enough to damage buildings in the northern Hayward Fault area. The Hayward Fault is part of the San Andreas Fault system, which runs through California.
''I don't advise anyone to unbolt their water heaters or cancel their earthquake insurance,'' said Dr. Robert Simpson, a geophysicist at the United States Geological Survey in Menlo Park, Calif., who is familiar with the new research.
Dr. Simpson said the new research was notable not simply because of its conclusions but also because it provides one of the first uses of satellites to study the San Andreas Fault as a whole, above ground and underground.
In the past, scientists using surface-based instruments could not discern tectonic forces at work below the surface, said Wayne Thatcher, a physicist at the geological survey. But Dr. Burgmann and his colleagues used a satellite-based technique, interferometric synthetic aperture radar, that can monitor deformities on the earth's surface over a broad region and detect rising bulges or falling depressions in the earth's crust, signs of tectonic activity below.
When pressure is building in a section of fault, Dr. Thatcher said, it will warp the surface in a characteristic way that reveals the location and depth of these areas where pressures are bottled up. When these locked areas, or asperities, accumulate a critical amount of strain, they break apart, resulting in an earthquake.
Dr. Simpson said researchers used to track only five or six major faults in the Bay Area. ''Now we are looking at 17 to 18 segments that stretch over a much wider area'' and have come to realize that they are dynamically interconnected in a deep subterranean maze.
Dr. Burgmann and his colleagues also studied a series of tiny earthquakes, so small they could not be felt by people, along the northern segment of the Hayward Fault. These small quakes have identical signatures on a seismograph, he said, which suggest that the same asperity is breaking apart again and again. Measurements of these microearthquakes help scientists estimate how fast the fault is slipping at its deepest levels, he said.
The Hayward Fault is one of the first fractures in the Bay Area to be studied with these new tools, Dr. Simpson said. The fault is notorious because, when combined with a fracture to the north, the Rodgers Creek Fault, the 100-mile system is estimated to have a one-in-three chance of generating a severe earthquake within the next 30 years, an estimate scientists say remains in force even if the northern segment of the Hayward Fault is creeping.
The southern end of the Hayward Fault, from Fremont to Berkeley, appears to be both creeping near the surface and accumulating strain deeper underground, Dr. Burgmann said. Its last earthquake occurred in 1868 and, until the cataclysmic San Andreas earthquake in 1906, Hayward was called ''the big one.''
There was also evidence that the northern Hayward segment may have ruptured in 1836 but that has since been proved erroneous, he said.
Everyone knows the Hayward Fault is creeping, Dr. Simpson said. Sidewalks are constantly being torn apart and there is a fracture running down the middle of the university's football stadium. But the question was, how much is it creeping and to what depth?
Dr. Burgmann used satellite and microearthquake data collected over the last decade to calculate a relationship between the deformation measured at the surface and the slippage through its entire depth. The mathematical model that best fits the data, he said, indicates that this portion of the fault is creeping at about one-fifth of an inch a year all the way down to where rocks get sticky and soft.
There is no big asperity to cause a big earthquake. Underlying rocks in the area may be softer and more crumbly than rocks in other parts of the Bay Area, he said.
In another experiment, researchers at the geological survey dug trenches at a golf course on the northern Hayward segment and found that the surface damage from 1836 was probably caused by a neighboring fault and not from an epicenter beneath Berkeley.
''The dream we have is to use these methods to learn how fast each fault segment is moving. Where are they slipping and where are they locked. We'd like to make models of real time movies of all the faults in the region,'' he said. In the meantime, people living near the San Andreas Fault should not let their guard down, Dr. Simpson said. Some of the worst damage caused by the 1989 Loma Prieta earthquake, centered south of San Francisco, occurred 50 miles north in the San Francisco marina district and in Oakland across the bay.
Correction: August 21, 2000, Monday
A map on Friday with an article about research on the chances of an earthquake at the Hayward Fault in Northern California misspelled the name of a city along the fault. It is Fremont, not Freemont.
Organizations mentioned in this article:
University of California (Berkeley)
Earthquakes; Research; Satellites
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