Quake Theories Have Their Faults : Experts Are Having Second Thoughts About Prevailing Assumptions Concerning the Region’s Seismic Risk

The ponderous grinding of the tectonic plates underlying Southern California generates enormous energy. Los Angeles, perched on the Pacific Plate, moves inexorably north, while San Francisco, sited on the North American Plate, slides south. When the fractured rocks on which they rest slip past each other, the result is the sudden upheaval of an earthquake.

The problem is that there have been too few major earthquakes to dissipate all the accumulated strain, according to earthquake experts at Caltech, the U.S. Geological Survey and the University of Southern California.

Now, some researchers are voicing second thoughts about the earthquake risk facing Southern California in a debate that highlights scientific uncertainties over the region’s future.

In a challenge to prevailing assumptions, UCLA scientists are suggesting that the region could be subject to very rare earthquakes, greater than magnitude 8.0 and more severe than any known temblor in the region for centuries.

Such a temblor, up to 100 times more powerful than the 1994 Northridge earthquake, could occur on any of the major faults in Southern California, including the fault systems underlying Los Angeles itself.

However, these same experts say that the region should expect far fewer major temblors of magnitude 6.5 or greater, contrary to what many experts predict.

UCLA earthquake expert David D. Jackson says that a rare “monstrous” quake between magnitude 8.0 and 8.5 might occur every several thousand years, when the region’s many small, segmented faults link up to channel more energy than they might normally be able to unleash separately.

But this scenario is at odds with more conventional theories. Other researchers argue that in order to release the energy that has accumulated, the region should expect a dozen or more moderate, but still devastating quakes in the coming decades.

To round out the scientific quandary, a few researchers speculate that the region’s pent-up tectonic energy may vent as harmlessly as steam from the whistle of a tea kettle, in thousands of tiny, barely measurable micro-quakes, called seismic creep.

The debate over how to gauge Southern California’s seismic hazards, conducted in recent months at geophysical meetings, university seminars and in peer-reviewed journal papers, hinges on how scientists interpret the confusing evidence of the region’s earthquake potential. The answers have far-reaching implications in a state still reassessing its construction and engineering practices in the knowledge that more earthquakes--of unknown size, magnitude and duration--are a certainty.

“It is important not just for Southern California, but for other regions,” said USGS earthquake expert Ross Stein.

In his work, Jackson is challenging a fundamental assumption in the prevailing theory of how earthquakes are generated.

Many experts believe that the size of an earthquake--as well as the amount of energy it may release and the distance the opposing rock faces may slip--is limited by the length of the fault. Many long faults, such as the San Andreas, are broken into segments by changing characteristics of the rock or by twists and turns in the fault. The energy of an earthquake rupture--traveling along a fault at roughly the speed of sound--is halted by the end of a segment like a car at a stop sign.

The smaller the segment, so the theory goes, the smaller an earthquake it can generate. According to this view, the dozens of active faults in Southern California are broken into so many segments that the largest earthquake possible in the Los Angeles Basin may be no larger than about magnitude 7.0. That is why some experts believe that it may take a number of Northridge-sized earthquakes to release the accumulated strain.

Jackson disagrees. He contends that scientists have placed too much faith in the idea that fault segments limit the power of an earthquake.

“My preferred explanation of all this is that earthquakes don’t obey the stop signs at the end of these segments,” Jackson said. “These barriers can play a role in controlling the size of an earthquake, but they are not always effective.

“They are like buttons on a shirt. If you give it a yank, you might pop one button and have a one-button earthquake. If you yank hard enough you might pop all the buttons,” he said. “We have to allow for the fact that there may be some huge earthquakes.”

To resolve the conflicting theories, geophysicists plunge into complicated computer programs to model the Earth’s behavior and geologists lower themselves down narrow bore holes seeking the region’s buried earthquake history. Other specialists scrutinize satellite measurements of the region or gauge readings like fortunetellers seeking the meaning hidden in tea leaves.

The geological record of past earthquake behavior is sketchy at best. Even large temblors such as the recent 6.7 Northridge earthquake and the 7.1 Loma Prieta quake quite probably left no geologic traces that future scientists could detect, despite the widespread damage they caused, experts said.

In the same way, many other earthquakes may have eluded detection, making it impossible for researchers to assemble an accurate historical record of the region.

Even when scientists can witness a quake in progress, they have no clear way to determine how it may affect nearby faults. Computer models are no better than the assumptions with which they are programmed.

“We are kind of blind,” said geophysicist Steve Ward at UC Santa Cruz. “We do our best with what we have.”

The debate is no less passionate for its mild, academic tone.

“It is definitely intense . . . cordial but intense,” said James Dolan, an earthquake geologist at the Southern California Earthquake Center.

In their effort to account for the untapped earthquake energy in Southern California, experts such as Dolan and his colleagues argue that in coming decades the region may be jolted by as many as a dozen moderate but still devastating quakes like Northridge.

Indeed, a group of earthquake experts recently calculated that there is an 80% chance of a magnitude 7.0 or greater quake striking some part of Southern California within 30 years.

No one knows why earthquake activity has picked up in the region recently.

USGS geophysicist Ruth A. Harris and others say that Southern California only now is emerging from a lull caused by the “seismic shadow” of earlier large earthquakes. Those quakes may have inhibited earthquake activity for decades at a stretch by “relaxing” adjacent networks of faults in the region.

“It might have relaxed a lot of [potential] earthquakes, including in the L.A. Basin,” she said. “It is possible that the 1857 Fort Tejon quake and the 1952 Kern County quake have suppressed the normal rate of quakes.”

As for Jackson’s theories, other respected experts acknowledge their validity, but vehemently disagree over his estimate of the region’s earthquake potential.

“People may have been too rigorous about segmenting faults,” Dolan said. “It is very poorly understood and that is a valid complaint. We have to hold open the possibility that multiple fault segments can rupture at the same time.

“[But] if you wanted to get into the 8.5 earthquake range, you are talking about rupturing every fault in Southern California, which most of us think is simply not possible,” he said. “In terms of pushing, as he is, for the possibility of extremely rare but very large earthquakes . . . there is intense debate about that in the community.”

Stressful Situation

Many experts believe there have not been enough earthquakes to relieve the pent-up seismic stress caused by the lateral movememt of the tectonic plates underlying Southern California.

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The accumulating stress may be relieved by:

* Many moderate-sized earthquakes in the next several decades.

* A single, extremely rare but very severe earthquake every several thousand years.

* Constant micro-quakes almost too small to detect.

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The System at Fault

Tectonic energy is generated as the Pacific Plate, on which Los Angeles is situated, inches north while the abutting North American Plate, on which San Francisco is sited, slides slowly south.

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Setting the Trigger

Satellite sensors and precision laser systems measure the tectonic stress building up in Southern California, with the area of highest strain shown in red and the lowest strain shown in blue.