From LA Times:

A quake lull, but how long?
Study says the L.A. area has had 1,000 years of lighter seismic activity.
By Sharon Bernstein
Los Angeles Times Staff Writer

August 24, 2007

Earthquakes that struck Southern California over the last century killed more than 200 people and caused billions of dollars in damage. But new research to be released today says the Los Angeles area may actually be in the midst of a prolonged seismic lull.

Geologists examined the size and frequency of quakes going back 12,000 years, finding patterns of heavy and lighter seismic activity every 1,000 to 1,500 years.

In a paper to be published today in the journal Geology, the scientists from the Southern California Earthquake Center argue that when the lull ends, metropolitan Los Angeles will experience significantly bigger and more frequent temblors -- up to 15 times larger than the destructive Northridge earthquake of 1994.

That could be soon -- or 500 years from now.

"We've been having fewer earthquakes than our long-term average," said James Dolan, a USC geologist and the lead author on the paper, which was co-written with geologists David Bowman of Cal State Fullerton and Charles Sammis of USC.

Even more dramatic is the geologists' explanation -- welcomed by some scientists and questioned by others -- of why the lull is occurring.

The three theorize that two of the region's most active fault zones are essentially taking turns producing earthquakes, with faults in the Mojave Desert producing bigger and more frequent quakes, while faults under Los Angeles take a break, and vice versa.

If future studies bear out the theories of Dolan and his colleagues, the work could go a long way in helping scientists understand the fault zones in Southern California, and the apparently contradictory information that scientists have gathered about the size, frequency and causes of earthquakes in the area.

The paper is expected to generate much debate and new research, as scientists try to parse whether its conclusions are fully warranted by the evidence.

Susan Hough, scientist-in-charge for the U.S. Geological Survey in Pasadena, said she is inclined to agree with Dolan.

"I think the observation is right that there are these systems that kind of trade off," Hough said.

She likened the trade-off between the two fault systems to the process of a person putting on a pair of pants -- first you have to pull on one side, then the other, until you finally get the pants up.

Ross Stein, a geophysicist with the U.S. Geological Survey in Menlo Park is more skeptical. The idea is intriguing, he said, but doesn't take into account the massive and seismically active San Andreas fault.

It may look like the two regional fault zones are trading seismic energy back and forth, but it's more likely that quakes in the region are controlled by the San Andreas, which is capable of producing larger temblors than either of the two systems, Stein said.

"To conclude that these more remote systems are interacting without being able to determine the impact of the big San Andreas earthquakes seems to me to be very speculative," he said.

The researchers found that the current earthquake lull period began about 1,000 years ago.

Dolan and his colleagues reached their conclusions after examining years of data drawn from fieldwork in which scientists, students and others dug under city streets, ditches and other places where there are earthquake faults, comparing the formation of the underlying earth and stone.

They first looked at the Los Angeles Region Fault Network, which lies under much of metropolitan L.A., and saw that there were periods in which earthquake activity seemed to increase and periods in which it would decrease.

Then they compared the information with data that had been gathered in the Mojave region, where the Eastern California Shear Zone is located. They found similar periods of activity there.

By comparing the two fault systems, Dolan said, the scientists saw that they seemed to alternate, with one becoming more active as the other became more quiet.

The fault zones are located along the boundary of two of the massive plates that make up the surface of the Earth, and the three authors theorized that each was absorbing in turn energy from the collision of the Pacific and North American plates.

First one region absorbs most of the shock, and creates more quakes, and then the other one does, Dolan said.

The San Andreas, he said, should be considered separately from the two fault systems.

Dolan said the paper would help scientists better understand how the apparent build-up of seismic energy along the plate boundary actually plays out in terms of earthquakes.

For example, he said, high-tech monitoring devices show that the region's earthquake faults are building up high amounts of energy, yet the historical record shows that, as an average over time, seismic activity has been much lower.

The geologists' research, Dolan said, could help explain that apparent contradiction.

There are indeed periods of high activity, but they alternate with periods of low activity, bringing down the average over time, he said.

The research also has policy implications, Dolan said, because once the lull ends, the quakes experienced in the region could be significantly larger than the ones we have experienced during the last 1,000 to 1,500 years.

These quakes will not only be bigger, Dolan said, but they will likely produce large, slow seismic waves, which can be very damaging to tall buildings and large structures like dams and bridges.

Such a quake "is going to pump enormous amount of energy into the L.A. Basin, causing it to resonate," Dolan said. "We're going to have a metropolitan area-wide disaster on our hands."

But Hough of the U.S. Geological Survey stressed that even a seismic lull period has its risks.

"Even if we believe Southern California is in a lull, we still had Long Beach and Northridge and Sylmar," she said. "If it's quiet, it's not dead."

sharon.bernstein@latimes.com