Scientists Searching for ‘Missing’ Quakes : Geology: Data shows too few temblors to account for fault shifts, researchers tell House panel. Some say this could mean a series of Northridge-level jolts in coming decades, or a single much bigger one.

Scientists examining movement along faults in Southern California said Wednesday that the region may be in for a series of earthquakes comparable to the Northridge temblor--or a single far greater quake--in upcoming decades.

Lucile M. Jones, a geophysicist with the U.S. Geological Survey in Pasadena, told a House committee that the Southern California Earthquake Center has looked at 150 years of quake data and concluded that there have not been enough quakes to explain all the shifting along faults.

Testifying at a hearing of the Science, Space and Technology Committee, Jones said the region has experienced “only one-third the number of large and major earthquakes” expected, given the rate of movement along faults.

“Where are the missing earthquakes?” Jones said. Her answer was that stresses accumulated inside the earth may be unleashed in the coming decades.

In Los Angeles, the executive director of the Southern California Earthquake Center said he was “a little bit aghast” that the conclusions were announced Wednesday, but he added that the findings are endorsed by scientists in the group. There is some disagreement, however, about the probability of quakes.

“We’ve reached a point now where our models are predicting more Northridge-type earthquakes or bigger earthquakes in the future than we’ve had in the last 150 years,” said Tom Henyey, the center’s executive director. The Southern California Earthquake Center was established by the National Science Foundation to improve the methodology for forecasting quake movement in Southern California.

Highlighting the lack of certainty in earthquake analysis, Henyey added that it is also possible that seismic stress has not increased. Scientists are still evaluating data for a detailed report to be released this summer, he said.

Nonetheless, the center’s overall finding is consistent with concerns expressed by other scientists that the Jan. 17 Northridge quake may have been a forerunner, if not a foreshock, to a more intense temblor or a series of quakes.

Allan Lindh, chief of seismology for the Geological Survey at Menlo Park, said Wednesday there were one or two serious earthquakes in the San Francisco Bay Area each decade for 70 years before the catastrophic quake in 1906. He said that such a pattern--which has also been recorded in Japan and Russia--appears to be repeating itself in Southern California in recent decades.

“Is this evidence that the stress level in the ground in Southern California is building back to the point where there will be another great earthquake?” Lindh said in an interview. “The answer is probably yes. Then the question is: How soon? We probably don’t know.”

Jones, who is also a visiting research associate at Caltech, offered three possible explanations for what she called “the L.A. dilemma.”

The first is that the region is in for “at least five more earthquakes as large as the Northridge earthquake,” a magnitude 6.8, she said. Under this scenario, the high rate of damaging quakes in the Los Angeles area in the last six years may be the long-term norm.

The second option, Jones said, is that “we are building up the stress to a much larger earthquake” of magnitude 7.5 to 8. She said that “only one magnitude 8 would be needed to release the energy of 30 magnitude 7 events.”

The third possibility is that movement on the faults is occurring without precipitating earthquakes. Jones said such slippage might explain some of the “missing” quakes but not all. Slippage of faults without corresponding quakes has been “shown to be an important part of the picture elsewhere in the world but not so far in Southern California,” she said.

Determining which of the explanations, or combination of them, is right would require “analysis of five to 10 years of data from improved geodetic and seismic networks,” Jones said. She said that funding is inadequate for research on earthquake prediction and analysis.

Henyey said that the earthquake center’s study was based on historic seismic activity, examination of the sidewalls of major faults dating back as far as 3,000 years and a relatively new process of using satellites to measure movements of Earth’s crust as small as a millimeter.

He said that the center has devised an analytic model that indicates “the earth is apparently straining at a much more rapid rate than would be suggested by past earthquake history.”