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'Creep' Along Fault Lines May Ease Quake Risk, Geologist Says

Geology: Arthur Sylvester believes the behavior of rock strata under Ventura challenges the premises of a dire report by seismologists.

March 04, 1996|KENNETH R. WEISS | TIMES STAFF WRITER

When Arthur Sylvester measures a growing fold in the Earth's layers beneath Ventura, he sees evidence that the hazard of more Northridge-type earthquakes in Southern California may not be as dire as seismologists predict.

In the eyes of the UC Santa Barbara geologist, the arching strata of rock look like an enormous telephone book, its pages pooching up as it buckles from the tremendous pressure of colliding tectonic plates.

Sylvester has discovered that one section of the bulge has pushed upward at about 2 millimeters a year during a 13-year period free of earthquakes in the Ventura basin.

Despite skepticism from his peers, he believes that his discovery challenges a premise underlying a report that proclaimed the region is overdue for a dozen or more quakes of the magnitude that struck Northridge, or a single, more powerful one.

To reach the report's alarming conclusion last year, a team of seismologists took a stand on a core issue that has long bedeviled scientists attempting to calculate the earthquake risks in Southern California:

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Do the opposing faces of rock along fault lines remain locked in a tight embrace until enough pressure causes them to slip with a violent jolt? Or do they also creep slowly along fault lines undetected?

The team reported no evidence of such "fault creep" in the Los Angeles metropolitan region that would help absorb some of the pent-up stress in the brittle upper crust being squeezed by the Pacific and North American plates.

But Sylvester believes he has found what has eluded his colleagues in the fault-riddled fold of rock called the Ventura Avenue Anticline.

If fault creep is pushing up the fold beneath Ventura and relieving underground strain, he reasons that similar phenomena may be happening elsewhere in Southern California.

"I agree with everybody else that we are due for more earthquakes," Sylvester said. "But if the folding is picking up some of the strain, the impending earthquakes may not be as frequent or as large as they say."

Using surveying data from 1978 and 1991 along Ventura Avenue, Sylvester found that the crest of the bulging rock formation grew 30 millimeters, about 1 1/4 inches, compared with its flanks. There were no significant earthquakes during that 13-year period.

The measurements look at only one 6-mile-wide cross-section of the fold that reaches 4 miles deep and stretches for 30 miles, from Santa Paula to halfway across the Santa Barbara Channel. It's unknown, he said, if any other sections of this Rincon-Ventura fold are growing.

The geologic formation is well known among geologists for its prolific oil and gas wells.

Oil tends to pool in the apex of the rock bent into an A-frame shape. The path of the fold can be spotted by following the oil pumps on land and the offshore oil platforms that extend like a dotted line off Carpinteria.

Given the extensive oil production, some scientists contend that Sylvester's theory is on shaky ground.

"In my heart, I wish he were right, but scientifically, I think he is wrong," said Robert S. Yeats, an Oregon State University geologist who coauthored the study that highlighted the region's heightened quake hazards.

Yeats and others associated with the Southern California Earthquake Center think the anticline's growth could be the result of human rather than natural forces.

Geologists noticed the formation sinking like a deflated balloon in the first half of this century, after millions of barrels of oil were pumped from the ground.

In 1956, California enacted a law requiring oil companies to inject as much fluid and gas back into the ground as they were extracting. The technique worked; it stopped the ground from subsiding.

Scientists now question if the anticline's newly measured growth can be chalked up to rebounding from earlier lost ground or if the oil companies are overcompensating and pumping too much back into the rock strata.

"There is never any spray paint on the bedrock, saying this is a natural phenomenon we are seeing," said Ken Hudnut, a geophysicist with the U.S. Geological Survey in Pasadena. "So we have to infer what is really happening."

Sylvester acknowledges these concerns.

"The Ventura Avenue Anticline is not the best structure to determine if folding is absorbing energy," he said. "Ideally, we should be looking at a fold where they are not drawing out oil or pumping in water."

To test his theory, he is now checking the growth rates with oil company injection logs. He is also floating proposals to examine other folds in the region that have not endured as much human tampering.

Still, Sylvester believes his discovery is significant--a view shared by other earthquake scientists.

"He may be on to something," said Hudnut, who has heard Sylvester explain his theory at two recent symposiums. "I see it as a very open question."

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