THE CLOCK IS TICKING SAN ANDREAS QUAKE DUE, STUDIES SHOW.

Byline: Becky Oskin Staff Writer

PASADENA - The San Andreas fault seems to have lost its menace in recent years as powerful earthquakes in Northridge and the Mojave desert have stolen the spotlight from the master fault.

But a series of new studies released today reminds us that the San Andreas, which separates urban California from the rest of the nation, is too powerful to ignore.

Scientists now have an earthquake record for the fault stretching back more than 1,500 years, in Wrightwood, near the east edge of the San Gabriel Mountains.

Geologists have also burrowed in the Earth farther west at Frazier Mountain and toward the fault's southern extent in the badlands near Indio. Their techniques are the same ones geologists in Alaska used to successfully predict the size of this month's magnitude 7.9 quake near Denali National Park.

The results, reported in a special issue of the Bulletin of the Seismological Society of America, confirm earlier findings showing the southern San Andreas is overdue for a large earthquake. And with potential to produce a quake between magnitude 7.5 and 7.9, all of Southern California would shake when if the fault ripped loose.

``We're getting enough data to think we can really confidently characterize (the fault's) activity in the past,'' said Tom Fumal, a research geologist with the United States Geological Survey in Menlo Park. ``It's pointing toward something happening in the near future, and we think it's going to be a large earthquake. It will probably be very damaging, particularly to San Bernardino-Riverside area, which is built so close to fault.''

The San Andreas fault has few equals in the world. Only Turkey, Alaska, China and Sumatra have cracks of comparable size and strength. Here in California, the San Andreas marks the boundary between the North American and Pacific tectonic plates - pieces of the Earth's crust that slide past each other as fast as fingernails grow.

The plates don't slide smoothly - their movement is often compared to rubbing two bricks together.

So bit by bit, tension builds along stuck areas of the San Andreas. Earthquakes release that strain.

The last significant quake on the southern San Andreas fault was in 1857, when the fault broke southward from Parkfield to San Bernardino, about 225 miles. Shaking from the estimated magnitude 8.0 quake lasted one to three minutes.

Understanding how the San Andreas system works will help predict the size and strength of the next quake, necessary for knowing what cities will face the most damage.

The sparse number of San Andreas quakes in recorded history has sent geologists digging trenches across the fault to look for disturbed soil and other clues left by super-sized shakers.

``This is really important because the historic record has two large quakes in 1812 and 1857, then nothing for 145 years,'' Fumal said. ``It's really hard to say what will happen in the future based on one small sample.''

Southern excavation sites reported in the studies range from Frazier Mountain to Thousand Palms and Mecca Hills. Scientists also studied the fault's northern section.

Fumal has placed more than 40 trenches across the San Andreas fault near Wrightwood, where he thinks layers of peat, coal and dirt record more than 30 quakes over a period of more than 3,000 years.

Glenn Biasi, a geophysics professor at the University of Nevada, Reno, correlated 14 quakes in Wrightwood's record with another trench to calculate an earthquake probability for the southern San Andreas fault.

His results: In the next 30 years, the models offer a 25 percent to 46 percent chance there will be a large earthquake or magnitude 7.0 or more. Biasi notes the models involve only two areas in the San Gabriel Mountains, and the San Andreas could still surprise Californians.

Biasi's statistics also show Wrightwood has an average interval of 100 years between earthquakes. Pallett Creek to the north is slightly longer, about 135 years between quakes.

With 145 years since the last big event, Biasi said the interval is ``longer than the average but not remarkably longer. If the thing went 50 more years (without an earthquake) the data wouldn't have to apologize for that.''

Fumal thinks his trenches show the next earthquake would happen south of Wrightwood, an overlap area between San Andreas segments.

``If that hypothesis is true, there are two types of big earthquakes that alternate,'' Fumal said. ``The last one was on northern part of the San Andreas, and the next one would be on southern part.''

And the last quake farther south was about 326 years ago.

``It's pretty likely that, during our lifetime, there's going to be a large earthquake on the southern San Andreas, and almost certainly within the lifetime of our children,'' Fumal said.

The quake's size is less certain. The San Andreas dies out near Indio, and scientists don't agree whether an earthquake would rupture the fault from Wrightwood to its southern limit, producing a huge quake close to magnitude 8.0.

If the next quake rips a shorter distance, the earthquake could be magnitude 7.5, which would heavily damage the San Bernardino-Riverside area and produce moderate damage in Los Angeles.

But a new study on the San Jacinto fault makes geologists question whether a San Andreas quake could jump to the San Jacinto fault and increase its length, as the Alaska and Landers quakes did.

Katherine Kendrick, a geologist with the Pasadena office of the USGS, presents evidence in the special issue showing the San Jacinto fault may take on more strain than the San Andreas' tail end.

``I always thought it was a little optimistic on the part of geologists to tell you where (an earthquake) is going to stop,'' said Lucy Jones, a seismologist and scientist-in-charge at the Pasadena office. Jones was not involved in the San Andreas studies.

``You could use the Alaska quake as great example of why we do this,'' Jones added.

Work by seismologists 30 years ago convinced builders of the Alaskan pipeline to accommodate 20 feet of movement from a Denali fault earthquake. The fault had never had a quake in written history.

The scientists were proved correct Nov. 3, when the fault offset the pipeline 19 feet without spilling a drop of oil.

``Think of what the environmental damage would have been if it ruptured,'' Jones said.

``That didn't happen because of seismology, and all the work presented in these papers allows us to do this the next time for the San Andreas,'' Jones said.