Odds of Big One slipping in East Bay

UC Berkeley study: Quake threat lower

August 18, 2000

By William Brand

BERKELEY -- A ground-breaking new satellite study, led by University of California, Berkeley researchers, shows the threat of a major earthquake on the northern segment of the Hayward fault between Oakland and San Pablo Bay is much less than has long been predicted.

The two sides of the fault, extending northward along the hills beginning at the Oakland-Berkeley city line, are slipping past each other constantly -- easing the stresses that cause big quakes, the researchers said.

Besides satellite radar images, researchers also used a sensor network to examine the almost constant stream of tiny "microquakes" miles below the surface of the fault.

Results of the project have already been used to lower an official earthquake risk estimate for the northern Hayward.

But don't unbolt your water heaters and cancel your earthquake insurance yet, says Roland Burgmann, lead researcher. "Just because the northern Hayward is slipping doesn't make it safe."

The southern Hayward fault -- from Oakland to Fremont -- and the Rogers Creek fault to the north beneath the Bay apparently are locked, not slipping very much. Stresses that trigger major earthquakes are building, said Burgmann, a UC Berkeley assistant professor of geology and geophysics.

Last year, a state seismology team estimated there was a 32 percent chance of a major quake of 6.7 on the Richter scale or higher somewhere on the Hayward in the next 30 years. More than 1 million people live within the danger zone.

To make matters worse, the Hayward is an offshoot of the mighty San Andreas fault that caused the 1906 San Francisco earthquake. And for that matter, the East Bay is surrounded by a web of earthquake faults.

A large earthquake miles away can have a great effect locally, Burgmann said.

Burgmann and colleagues at UC Berkeley, Lawrence Berkeley National Laboratory, the Jet Propulsion Laboratory in Pasadena and UC Davis reported their findings today in Science magazine.

The study also appears to answer a question raised by a pioneering trenching study conducted two years ago on the northern Hayward in the middle of the Mira Vista golf course in El Cerrito. The trench showed disturbances in soil strata and other clues indicating there was a great quake on the northern Hayward sometime between 1640 and 1776, when Spanish colonists arrived.

The trenching study indicated there hasn't been another quake on the northern Hayward since -- and Burgmann's team wondered why.

Their study shows that the northern Hayward creeps at about 5 millimeters a year on the surface and at a similar rate deep in the earth, easing stressors believed to trigger a major quake.

The Burgmann study analyzed sophisticated images taken of the fault in 1992 and 1997 by two European satellites, ERS-1 and ERS-2. Using mathematical analysis software developed at JPL, Burgmann was able to compare the older and new radar interferometry images to measure surface creep between the two sides of the faults down to a few millimeters.

He backed up his measurements by comparison with images taken by Global Positioning Satellites.

At the same time, seismologists at UC Berkeley and Lawrence Berkeley lab discovered that tiny quakes deep below the surface indicate the fault sides slipping. Researcher Robert Nadeau said the technique was developed in recent research his team did last year on the San Andreas fault near Parkfield.

"At Parkfield, we were able to infer how fast stresses below the surface were being loaded and how fast they were being unloaded by these small slippages," Nadeau said. "It was a new piece of information about a fault miles below the surface.

"So we decided to look at the northern Hayward and sure enough, we found slipping. Our instruments here aren't as good as at Richfield. But we've certainly got enough information to say that indeed, very deep down, the northern Hayward fault is slipping just like it is on the surface and we have a rough idea of how fast it is slipping."

Earthquake hazards are created when sides of a fault lock together instead of slipping by each other. The surface areas of the fault can continue to slip, but deep down in the fault, the pressure to slide builds. Finally, the blockage slips, causing a major earthquake.

Burgmann said he got interested in earthquake slippage when the Loma Prieta stuck in 1989. "I was a graduate student at Stanford and was in the Geology Building which was damaged pretty heavily.

"We got up from the floor and said we've got to do something. I got my roommate to drive us to where the earthquake happened, but we went to the wrong place."

The idea of looking at these giant fractures in the earth's crust that cause so much havoc fired Burgmann's imagination. A few months later he volunteered to participate in an early satellite project using Global Positioning Satellite surveys.

He has stayed with it, through his doctorate at Stanford, a few years at UC Davis and now at Berkeley. "Using this combination of methods, we can see where slippage occurs and we can see the patches that are locked and eventually turn into large earthquakes," he said.

"We still can't pin earthquakes down in time -- when the next one will happen -- but we can narrow it down."

Bob Simpson, a geophysicist with the U.S. Geological Survey in Menlo Park, said both the satellite and deep fault studies are exciting new tools for earthquake study.

"They're proposing that a 20 kilometer (12 mile) stretch of the northern Hayward fault -- as best they can tell -- is creeping from top to bottom," Simpson said. "That's good news, but remember there are many faults in the region that pose a threat."