System Developed to Warn of Earthquake Aftershocks

Scientists have developed a system that will enable the government to warn California residents of the likelihood of a strong aftershock after an earthquake.

The system is based largely on the history of 62 earthquakes and their aftershocks, but it also grew out of an expanded understanding of the dynamics of major quakes.

The same data used to develop that system also gave the scientists an insight that could relieve some public anxiety after the next large earthquake. They found that quakes of magnitude 5 or above are rarely followed by larger earthquakes.

The scientists, Lucile Jones and Paul Reasenberg of the U.S. Geological Survey, noted that people usually fear right after an earthquake that a more powerful one may be lurking around the corner.

“People want to know if the bigger one is coming tomorrow,” Reasenberg said Tuesday at a meeting of the American Geophysical Union. “The answer is there is only a 5% chance. By the third day, the chance of a bigger one is down to 1%.”

That means there is a 95% probability that California earthquakes greater than magnitude 5 will not be foreshocks to larger quakes.

However, large earthquakes are normally followed by aftershocks, and since the later temblors hit after some structures have been damaged in the main shock, they can be serious. Beginning immediately, the U.S. Geological Survey will use the work by Jones and Reasenberg to issue warnings on the chances of major aftershocks. The warnings will be updated continuously and released to the public after state authorities have been notified, the scientists said.

The warnings will be based on a specific pattern of aftershocks that the scientists discovered while examining the historical record.

“We now have a model to determine the chances of having a damaging aftershock following a quake,” Jones said.

The beauty of the system, Reasenberg added, is that it is based on the dynamics of each specific earthquake, so it should be quite accurate. What the scientists found is that strong aftershocks occur in a direct relationship with the scores of small, insignificant temblors that follow nearly every major quake.

By using a complex mathematical formula to compare the variation in size among the small (magnitude 2 and 3) aftershocks, the scientists expect to be able to predict the likelihood of a magnitude 5 or stronger aftershock for 12-hour periods.

The warning will be continuously updated as more data is fed into the computer.

The Geological Survey hopes the data will discourage people from re-entering damaged structures during high-risk periods, the scientists said.

Meanwhile, another scientist updated evidence of two long-suspected faults in the Los Angeles Basin. Both faults are deep underground and both are in areas known to have other faults.

Egill Hauksson of USC disclosed further research into these so-called thrust faults, first reported by The Times in November, 1987, and blamed for the Oct. 1, 1987, Whittier Narrows earthquake. One of the two faults Hauksson discussed Tuesday was not previously confirmed.

Hauksson analyzed records of hundreds of earthquakes that have struck over the last 14 years in his research.

Both of the faults are thrust faults, created by horizontal compression that is caused by irregular slippage along the San Andreas Fault, and many scientists have warned that such faults probably lie under much of Southern California.

Don Anderson, head of Caltech’s seismology laboratory and president of the American Geophysical Union, made such a warning after the 1972 San Fernando Valley earthquake. The idea has been debated by geophysicists ever since, but that changed with the Whittier Narrows earthquake in October, 1987. That temblor, which killed eight people, struck on the same type of fault system that Hauksson has discovered elsewhere in the basin.

The deep-thrust faults are hard to find because they do not break the surface like the strike-slip faults that have left long scars across so much of California. Instead, they slip below the surface--usually around 10 to 15 miles deep--as one side of the fault slides over the top of the other, sort of like a piece of bread sliding off the top of a sandwich.

However, deep-thrust faults are so poorly understood that it is difficult to determine just how much of a threat the newly discovered faults pose. They may not erupt for hundreds of years, and they may release their energy through many small quakes instead of large temblors, scientists said.

“This is a whole new class of faults,” Anderson said. “It’s got to increase the potential (for earthquakes,) but we just don’t know how much.”

Hauksson indicated he is not sure, either.

“A large earthquake is not imminent,” he said, and then he added: “I hope.”

Hauksson named one of the faults the Torrance-Wilmington Fault because it begins offshore near Newport Beach and passes under the Torrance-Wilmington oil fields. It then extends 40 miles to the north under the Palos Verdes Peninsula and into Santa Monica Bay. The same area is host to the widely studied Palos Verdes Fault.

The second fault, which was discovered shortly after the Whittier earthquake, extends 60 miles from Yorba Linda to Whittier, under downtown Los Angeles and down the Wilshire corridor to Santa Monica Bay. Hauksson calls that the Elysian Park Fault.