Fault ‘Creep’ May Cut Ventura Quake Risk, Expert Says

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 pouching up as it buckles from the tremendous pressure of colliding tectonic plates.

Sylvester has discovered that one section of the bulge is pushing upward at a rate of about 2 millimeters a year during an earthquake-free period in the Ventura basin.

Despite skepticism from his peers, he believes 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:

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 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 in that time the crest of the bulging rock formation rose 30 millimeters, about 1 1/4 inches, compared to its flanks. There were no significant earthquakes during those 13 years.

The measurements look at only one six-mile-wide cross section of the fold that is four miles deep and stretches for 30 miles, from Santa Paula to halfway across the Santa Barbara Channel. It is unknown, he said, if any other sections of this 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 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.

Effects of Drilling

Yeats and others associated with the Southern California Earthquake Center think that 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 wonder whether 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 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.”

Indeed, it was USC earthquake geologist James F. Dolan, the principal author of last year’s study that found no evidence of fault creep, who urged Sylvester to look for unexplained growth in folds of the earth.

“To be scientifically honest, it is one possibility that needs to be investigated,” Dolan said.

Scientists have long known of movement along the Hayward Fault and the San Andreas Fault in Central California without accompanying earthquakes. They have been able to measure this creep where these faults break the surface, slowly ripping apart roads, sidewalks and buildings.

But they have found little surface slippage in Southern California. The lingering question is whether the so-called “blind” faults, those hidden deep beneath the surface, can experience creep that could only be measured by the uplift of a fold reaching the surface.

That’s where Sylvester comes in, as one of the nation’s experts in tectonic geodesy, the tedious and highly technical field of precisely measuring changes in the earth’s surface.

For 27 years, he has been doing this kind of surveying, or leveling, with the help of his students at UC Santa Barbara. It can take years or decades for any detectable change in the earth’s surface, unless abruptly altered by a massive quake.

If anyone could have pinpointed such a phenomenon, it was likely to be him, his colleagues said.

And they said it is not surprising that the find would come in Ventura County, a little-studied area despite the extensive strain caused by two mountain ranges pushing closer together at a rapid rate.

Seismologists sometimes joke about Ventura as the black hole of earthquake science because nearly all of the research is focused on the more heavily populated Los Angeles area.

Still, those most familiar with the Ventura basin believe it is among the most earthquake-prone areas in the world.

“It is inevitable that the Ventura basin will be struck by a fairly large earthquake,” said Tom Rockwell, a San Diego State University geologist. “Whether it is tomorrow or 200 years from now is anybody’s guess.”

Rockwell, one of Dolan’s coauthors and an expert on the Ventura Avenue Anticline, notes that the Ventura basin has not had a major quake in at least 200 years.

He is not sure if Sylvester, who was once his professor, is correct in his theory about the anticline’s growth taking up accumulated strain. If he is right, then how much pent-up energy is being released?

Sylvester ponders the same question.

Effect Uncertain

“We don’t know how much of that stored energy might be released in a fold,” Sylvester said. “If it is 100%, then we will never have a big earthquake in the Ventura area. If it is 10%, then we will have a big one, as we might expect.”

If folds are absorbing some strain, they would only reduce a small fraction of overall buildup of seismic stress in Southern California, Dolan said.

“Even if Art [Sylvester] is 100% correct, we are only dealing with one component of the seismic hazard,” Dolan said.

Allan Lindh, a seismologist with the U.S. Geological Survey in Menlo Park, has long suspected that some strain gathering in Southern California has been taken up by enlarging folds.

He points to Armenia as a geologically similar area where that has been happening. It’s also a place where scientists can check their theories against church records of quakes dating back 2,000 years.

The problem with studying Southern California, he said, is that the earliest records are merely 200 years old.

“When you don’t have a long historic record, it is really putting together a jigsaw puzzle to figure out what’s going on,” Lindh said. He has had reservations about whether the Dolan study correctly assembled all of the pieces.

“If Art [Sylvester] has found a new piece that doesn’t fit, it is really important.”