EXTRA : Web of Fault Lines Makes Predictions Difficult : Caltech: 1 in 20 Chance of Another

There is a one in 20 chance that a second large earthquake will occur in the Los Angeles area within five days, a scientist said today.

Kate Hutton, a geophysicist at Caltech, said a quake of the magnitude of the one that struck here today increases the likelihood of a second earthquake.

Hutton and her colleagues were still trying to analyze the earthquake, which sent seismographic needles wildly off their scales at Caltech’s observing station because of its proximity to the quake’s epicenter.

She said the quake, which was later estimated at 6.0 on the Richter scale, was followed within minutes by two aftershocks reaching magnitudes of 4.4 on the Richter scale.

By about 45 minutes after the earthquake, 12 aftershocks measuring at least 3 on the Richter scale had been recorded, Hutton said.

A previously developed emergency plan that would have admitted reporters to Caltech’s seismology laboratory for up-to-the-minute reports on the earthquake was foiled when a fluorine tank in the basement of the building that houses the laboratory sprang a leak, forcing evacuation of the entire building and two neighboring buildings, said Rober Finn, a Caltech spokesman.

Field workers from Caltech were roaming over the Whittier fault, where they believed the earthquake’s epicenter was located. The most recent large earthquake on that fault occurred in 1929, Hutton said.

Scientists have said there is a 50% chance that a great quake--measuring 8 or more on the Richter scale--will occur along the San Andreas fault in Southern California during the next 30 to 50 years.

The fault runs approximately north and south, passing about 60 miles east of Los Angeles.

California is riddled with a complex web of fault lines, however, complicating efforts to predict future quakes.