Big One on its way?

Scientists have long believed that Sonoma County is ripe for a major quake on the Rodgers Creek-Healdsburg Fault that will rip the ground apart by six feet.

With a new study recently out, they are now more convinced than ever, even if they can't say exactly when.

"It's locked and primed," said David Schwartz, a seismologist with the U.S. Geological Survey in Menlo Park. "Looking at the individual segments of faults in the Bay Area, the Rodgers Creek has the highest probability, 17 percent in 30 years."

The computer modeling, released last month, suggests that the 1906 earthquake relieved much of the stress on the Rodgers Creek-Healdsburg, Hayward and Calaveras faults. Those faults run roughly in a line from Healdsburg to Sunol.

"With the modeling, we can see the positive effect of 1906, to relax the region and suppress seismicity," Schwartz said. "That shadow is over."

The new study, released at a San Francisco meeting of seismologists, was written by Schwartz and USGS geophysicist Fred Pollitz.

It looks for the first time at the interaction between the earth's firm upper crust, which is six miles thick, and the hot, fluid rock surface on which the crust floats.

Both play a role in how stress builds and is relieved on the Bay Area's earthquake faults. Stress in the crust changes instantaneously when faults slip. The fluid's effects occur over long periods of time.

"To me, the really important insights this study has shown is it suggests on a physical basis that the stress shadow effect has dissipated, that the areas of highest stress in the Bay Area are in southern Hayward, northern Calaveras and the Rodgers Creek faults," Schwartz said.

"This is not a new message, we have been saying that for awhile, but it is a different way of looking at it," he said. "It reinforces our concern."

There have been three major quakes on the Rodgers Creek-Healdsburg Fault, which runs from San Pablo Bay to Healdsburg, that scientists have been able to identify between the year 1000 and the present. The last one occurred between 1650 and 1776.

The quakes were identified during trenching across the fault in southern Sonoma County. The trenching also shows the earth slipped about six feet, indicating a quake with a magnitude of about 7.

The last major quake on the Hayward Fault was a 6.8 occurring in October 1868.

In contrast, the Loma Prieta quake of October 1989 was a 6.9.

Schwartz said that if not for the 1906 San Francisco earthquake, which measured 7.8, there already would have been another major quake on either the Rodgers Creek-Healdsburg, Hayward or Calaveras faults, or perhaps all three will slip together.

"One model we are considering is a quake starting in San Pablo Bay and going north on the Rodgers Creek and south on the Hayward. That is not far fetched because it happened in Kobe, Japan," Schwartz said.

That Japan quake occurred in 1995, measured 6.9, killed 5,500 people and caused $147 billion in damage.

"Santa Rosa is going to get smashed, and clearly all the towns around it will have damaging effects," Schwartz said.

Schwartz said the 1969 Santa Rosa quakes, which occurred minutes apart and measured 5.6 and 5.7, were just signs the stress is building.

"Time is really running out," Schwartz said. "A whole set of signs points at these faults having the highest stress, and that can be translated into having the highest hazard."

Schwartz said the probability of a major quake on the Rodgers Creek-Healdsburg is 17 percent in the next 30 years, which is a high probability, even if they cannot say when the quake will occur.

"If something doesn't happen in 20 years, that is a distinct possibility, and if it happened tomorrow, that is a distinct possibility," Schwartz said.

You can reach Staff Writer Bob Norberg at 521-5206 or bob.norberg@pressdemocrat.com.