Experts Say Small Quakes May Warn Seconds Before Big One

The discovery that moderate temblors in the 4 and 5 magnitude range occurred seconds before both the Loma Prieta and Landers earthquakes gives scientists hope they may one day be able to send out flash warnings of imminent big quakes to urban centers, it was reported Tuesday.

Such warnings, conveyed electronically, might be able to stop elevators or cut off gas in time to significantly mitigate damage, even if they were to arrive only two or three seconds before major shaking began.

However, it is clear that scientists are not yet at the point where they can distinguish precursors to big quakes from other moderate temblors, according to presentations at the annual meeting of the American Geophysical Union.

Rachel E. Abercrombie, a USC post-doctoral associate, told of seismographic recordings at Morongo Valley the morning of the June 28, 1992, Landers earthquake that showed two quakes--of magnitude 4.4 and 5.6--in the three seconds before the 7.6 quake began.

Unfortunately, Abercrombie added, there were no clearly identifiable characteristics that would have shown that a larger quake was imminent.

Although she said she is pessimistic that distinguishing characteristics will be found, her colleague, James Mori, head of the Pasadena field office of the U.S. Geological Survey, said he believes that further, more intricate instrumentation in quake-prone regions may yet show that such anomalies do exist.

Another Geological Survey scientist, William L. Ellsworth of Menlo Park, Calif., suggested that the instruments available near both Landers and Loma Prieta were too elementary to display all the intricacies of the shocks.

Ellsworth is one of the scientists responsible for monitoring seismic events at Parkfield in Central California, where experts have long expected an earthquake on the San Andreas Fault of at least magnitude 6.

The most sophisticated instruments available have been deployed around Parkfield in an attempt to record precursory phenomena when that quake finally does occur.

The prospect that scientists could team up with governmental authorities to fashion a warning system that could mitigate damage in urban areas is one of the main reasons given for investing research money in studying the seismic phenomena that immediately precede quakes.

But several scientists appearing at the half-day symposium on the subject here cautioned that it is complex and there may be many disappointments before success is achieved.

Terry E. Tullus of Brown University, for instance, displayed a model for Parkfield that postulated possible precursory events deep underground as long as eight years before the eventual big earthquake.

But Tullus freely acknowledged that “the extent to which this model is realistic is yet to be determined,” and he said that even if the model is accurate, it may remain hard to tell what events will lead to a magnitude 6 quake as compared to a magnitude 4. “It is frustrating,” he said.