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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

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.

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Sylvester's theory may rekindle a scientific debate that was popular a decade ago over the growth of another formation known as the Palmdale Bulge.

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 a-seismic creep where these faults break the surface, slowly ripping apart roads, sidewalks, buildings.

But they have found little or no 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 the past 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 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 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 released?

Sylvester ponders the same question.

"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 the overall buildup of seismic stress in Southern California, Dolan said. Most of the region's quake hazard comes from faults that break the surface, he said, not those hidden deep underground.

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

Allan Lindh, a seismologist at 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."

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