Major Earthquake Due to Hit Southern California, Study Says

Get ready for the Big One.
John Roach for National Geographic News
June 21, 2006

About 300 years of pent-up stress in southern California is sufficient to trigger a catastrophic earthquake on the San Andreas Fault system, according to a new study.

The San Andreas Fault marks the boundary between the North American and Pacific tectonic plates. Tectonic plates are pieces of the Earth's outer crust that jostle about like constantly moving pieces of a jigsaw puzzle.

The fault is notorious for major earthquakes, including the 1906 earthquake that reduced the San Francisco Bay Area to piles of smoldering rubble.

But the 100-mile (160-kilometer) southern section of the fault, which runs south from San Bernardino to the east of Los Angeles and San Diego, has remained eerily quiet for nearly three centuries.

Now, scientists believe, the fault is ready to rumble.

"It is fully charged for the next big event," said Yuri Fialko, a geophysicist at the Scripps Institution of Oceanography in La Jolla, California.

"When the event will occur, we cannot tell," he continued.

"It could be tomorrow or 20 years from now, but it appears unlikely the fault can take another few hundred years of slow strain accumulation."

Fialko reached this conclusion after studying the fault system with radar-equipped satellites and global positioning systems (GPS).

He reports the finding in tomorrow's issue of the journal Nature.

Mary Lou Zoback is a research geologist with the U.S. Geological Survey in Menlo Park, California.

She said Fialko "really exploits the true power" of modern satellite radar and GPS equipment to reach a conclusion with a familiar message.

"The conclusion … seems to concur with previous interpretations that the southern San Andreas Fault is in a late stage of its earthquake cycle—meaning it's likely to go," she said.

San Andreas Fault

Fialko used a technique called interferometric aperture radar (InSAR), a set of satellite-based radar systems that are able to detect 1-millimeter (0.04-inch) displacements in the Earth's crust.

The satellite data, combined with ground-based GPS measurements, allowed Fialko to calculate how much the North American and Pacific plates slip past each other on the San Andreas Fault—about 1 inch (25 millimeters) a year.

But the plates are locked together along the southern section of the fault. As a result, the plate motion builds up underneath the crust as stress.

Over the past 300 years, energy equivalent to 20 to 26 feet (6 to 8 meters) of slip has accumulated below the locked part of the southern San Andreas Fault, according to Fialko.

"This is an indication the fault is storing significant elastic strain, and the amount of strain is equivalent to a major earthquake," he said.

According to Zoback, the amount of strain Fialko's calculations show is at the high end of estimates obtained from other research using different methods.

"That's like being ten months pregnant—past all reasonable estimates of when it should go," she said.

Warning Signs?

Zoback adds that an increase in earthquake activity in southern California over the past several decades may indicate the main fault is ready to rupture.

A similar increase in earthquake activity occurred on the northern section of the fault in the 70 years prior to the 1906 San Francisco earthquake, she notes.

"We may be seeing something similar," she said.

Moderate temblors—including 1994's magnitude 6.7 Northridge earthquake in Los Angeles—may be a sign that tectonic stress in the region is reaching a breaking point, according to Zoback.

And if the Big One hits, what are the consequences?

Unlike the northern section of the San Andreas Fault, which slices through the middle of the heavily populated San Francisco Bay Area, the southern section of the fault passes through mostly uninhabited desert, Zoback says.

However, computer simulations show that a southern California earthquake that ruptures toward the north could be devastating. The energy from an earthquake is focused in the direction of the rupture, Fialko explains.

For an earthquake of magnitude 7.5 to 8 that ruptures east of San Diego toward San Bernardino, the "shaking in Los Angeles would be extremely strong and extensive," he said.


June 22, 2006 - Sydney Morning Herald, Reuters

The southern end of the San Andreas fault near Los Angeles, which has not had a major rupture for more than 300 years, is under immense stress and could produce a massive earthquake, a new study said today.

But exactly when that quake will take place cannot be predicted, the scientist who conducted the study said in an interview with Reuters.

"The fault is accumulating stress at a high rate, but this does not suggest that a rupture is imminent," said Yuri Fialko, of the Scripps Institution of Oceanography at La Jolla, California. "When the quake will happen nobody knows."

His study - published in the British journal Nature - found that, given average annual movement rates in other areas of the fault, there could be enough pent-up energy in the southern end to trigger a cataclysmic jolt of up to 10 meters.

"This is new evidence that tells us the same story that we have known for a while," said Scott Brandenberg, a professor at the University of California, Los Angeles school of engineering. "It's a reminder that we need to be ready for it when it happens."

Fialko said his data taken by satellite was more complete than in previous studies because measurements were taken every 20 meters instead of at ground stations 10 kilometers apart.

"The observed strain rates confirm that the southern section of the San Andreas fault may be approaching the end of the interseismic phase of the earthquake cycle," he wrote in Nature.

A sudden lateral movement of seven to 10 meters would be among the largest ever recorded.

The fault is the 1290-kilometre-long geological meeting point of the Pacific and the North American tectonic plates

According to the US Geological Survey (USGS), the earthquake that destroyed San Francisco in 1906 was produced by a sudden movement of the northern end of the fault of up to 6.4 meters.

Fialko said there had been no recorded movement at the southern end of the fault since the dawn of European settlement in the area.

He said this lack of movement correlated with the predicted gaps between major earthquakes at the southern end of the fault of between 200 and 300 years.

"The longer you wait, the higher the likelihood of rupture," he said.

Ken Hudnut, a scientist at the USGS, said experts had known since 1988 that the southern section of the San Andreas fault was the most likely source of a "Big One".

"We think recurrence is on a level of a few hundred years and the last one was a few hundred years ago, but we don't understand earthquakes well enough to predict when they will happen," he said.

Fialko's study found that elsewhere on the fault there were average slippage rates of up to a few centimeters a year that prevented the build-up of explosive pressure deep underground.

When these became blocked and then suddenly broke free they produced tremors or earthquakes of varying intensity depending on the movement that had taken place before and the duration of the blockage.

USGS said the most recent major earthquakes in the northern and central zones of the San Andreas fault were in 1857 and 1906.


Big One may hit as soon as today
San Andreas study cites pent-up stress

With more than 300 years of stress building on the San Andreas Fault, Los Angeles is long overdue for the "Big One," according to new research published this month in the journal Nature.

The research by the Scripps Institution of Oceanography is among the most detailed yet and confirms a USC study that found pent-up stress along the southern end of the fault means it could rupture at any moment.

"All this data suggests that the fault is ready for the next big earthquake - but exactly when the triggering will happen and when the earthquake will occur we cannot tell," said Yuri Fialko, an associate professor at the Institute of Geophysics and Planetary Physics at Scripps.

"It could be tomorrow or it could be 10 years or more from now."

Fialko's research, using satellite and other data tracking, found the risk of a large earthquake - magnitude 7.0 or greater - may be increasing faster than researchers had previously believed on a 100-mile stretch of the fault southeast of San Bernardino. That section last erupted in 1690.

Experts say such a large earthquake could be 70 to 80 times more powerful than the 1994 Northridge Earthquake - and would be strongly felt throughout much of Southern California, including Los Angeles County.

"It depends on how the earthquake ruptures," said Debi Kilb, a Scripps seismologist. "If it goes from south to north, all the energy will be directed to the north, in which case Los Angeles will get hit pretty substantially."

Ken Hudnut, a geophysicist at the U.S. Geological Survey in Pasadena, said simulations have found that thousands of people would be killed if such a blast of energy hit L.A.

"The damages would be immense," Hudnut said. "The long-term impact in terms of business disruption would be quite significant."

Local emergency preparedness officials say the L.A. area is probably the best-prepared region in the nation, but earthquake experts still note that possibly thousands of buildings throughout the county could collapse in such a large earthquake.

Thomas Heaton, a professor of engineering seismology at the California Institute of Technology in Pasadena, said officials have done a good job retrofitting unreinforced brick buildings and bridges, but that he is "very unsatisfied" with steps taken to retrofit thousands of poorly reinforced concrete buildings built before 1974.

"There may be some very important flaws in many of these older buildings," Heaton said. "My guess is that a quarter to half of the older concrete buildings are in a state that I would not be comfortable having any of my family residing or working in those buildings.

"There is another issue that most of our steel-frame buildings have deficient welds in their joints and some simulations we have been doing indicates that makes them considerably prone to collapse. And no one is doing anything about that."

Since Hurricane Katrina, Hudnut said, earthquake experts have been studying what kind of damage the "Big One" would cause to infrastructure the public relies on for food, water and other necessities trucked into L.A. across the fault line.

"A big San Andreas event would almost certainly do damage to the lifeline structures we count on," Hudnut said. "We are reliant on engineering having been done properly."

James Lott, executive vice president of the Hospital Association of Southern California, said the state's already overcrowded hospitals are not ready to deal with such a disaster.

"Our hospitals are operating year-round with an average daily census approaching 85 percent," Lott said. "And so the numbers would be overwhelming."

Lott said California hospitals are required to meet seismic standards starting in 2008 that would keep them up and running in a magnitude-7.0 or -8.0 earthquake - but that price tag is at least $45 billion.

"The problem, of course, is that there is no funding for that," Lott said. "Many hospitals are proceeding on track to meet the new requirements, but that's years down the road."

In his research, Fialko found evidence that the southern San Andreas is mostly locked and continues to accumulate significant amounts of strain.

He calculated the rate at which the fault is moving and estimated the pace of plate movement at the fault at about an inch per year. That means that during the past 300 years the fault has accumulated about six to eight meters of slip "deficit" that will be released in a future earthquake.

If all the stress is released in one earthquake, it would result in a magnitude-8.0 earthquake, about the size of the 1906 San Francisco Earthquake that resulted in 3,000 deaths.

Michael Brooks, acting administrator for the county Office of Emergency Management, said police, firefighters and other first responders throughout the county are trained and prepared for such a large earthquake.

"Los Angeles County is well-prepared for a disaster, whether it be moderate or major," Brooks said. "Does that mean the response will be perfect? Probably not.

"But particularly in light of (Hurricane) Katrina, we went back and looked at our emergency response plans to look for gaps. All the responders have been working very hard to ensure we address those gaps."

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Experts say SoCal overdue for Big One
Quake could cause billions in damages
By Alicia Chang Associated Press
LOS ANGELES - New earthquake research confirms the southern end of the San Andreas fault near Los Angeles is overdue for a Big One.

The lower section of the fault has not produced a major earthquake in more than three centuries. The new study, which analyzed 20 years of data and is considered one of the most detailed analyses yet, found that stress has been building since then, and the fault could rupture at any moment.

"The southern section of the fault is fully loaded for the next big event," said geophysicist Yuri Fialko of the Scripps Institution of Oceanography in La Jolla.

Predicting exactly when that might happen, however, is beyond scientists' ability.

The analysis was published in today's issue of the journal Nature.

Experts have estimated that a quake on the southern San Andreas of magnitude 7.6 or greater could kill thousands of people in the densely populated greater Los Angeles area and cause tens of billions of dollars in damage.

It was the 800-mile San Andreas fault, which runs down California like a scar, that caused the 1906 San Francisco earthquake that led to about 3,000 deaths.

But scientists know very little about the 100-mile dormant southern segment, which slices through Southern California from San Bernardino, east of Los Angeles, to near the Mexican border.

The section last popped in 1690, producing an estimated 7.7 magnitude quake, but caused little injury or damage because hardly anyone lived there at the time.

Using satellite radar and global positioning data, Fialko measured the movement of the southern San Andreas between 1985 and 2005. Small movements along a fault can relieve strain. Calculating those subtle motions allows scientists to figure out how much strain is building up.

Fialko found that the southern end of the fault has shown little movement and that significant strain is building up. The fault's slip rate, or average annual movement, was measured to be about an inch a year - similar to previous estimates.

Surprisingly, Fialko found that the two sides of the southern San Andreas behaved differently, with one side showing more flexibility than the other.

This could help scientists understand potential earthquake risks, he said.

Ken Hudnut, a U.S. Geological Survey geophysicist in Pasadena, who had no role in the study, said the latest research reaffirms the need to study the mysterious southern San Andreas more closely.

In the fall, Hudnut will head a $240,000 project that would conduct tests on the southern segment to get a better idea of the threat it poses.


--simulation of a quake




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