In the 1975 movie thriller Earthquake, a young scientist comes up with data that show a major quake is on its way. But, despite the prediction, authorities refuse to issue a public warning. Recently Caltech geophysicist James H. Whitcomb found himself playing a somewhat similar role when he predicted an earthquake would occur in the Los Angeles area within the next year. Whitcomb and the California Institute of Technology, where he is a senior research fellow in the seismological laboratory, were immediately threatened with suit by a city official, who charged that such reports hurt property values. Political tremors of this kind are new to Whitcomb, 35. A country boy from the Colorado plains, he majored in geophysics at Colorado School of Mines, earned a master's in oceanography at Oregon State and helped develop instruments for studying rocks on the moon. After a Fulbright scholarship in Sweden, he came back for his Ph.D. in geophysics at Caltech. Whitcomb spoke recently about earthquake prediction with S.J. Diamond of PEOPLE.

When and where in California can we expect an earthquake?

My theory predicts an earthquake of a magnitude of 5.5 to 6.5 on the Richter scale—that is, a geologically moderate earthquake—between now and next April, within an area about 80 miles in diameter, including parts of Los Angeles and the San Fernando Valley. We cannot set a more precise location; it could be along any one of about 20 active faults.

How certain are you?

Well, this is a test of the hypothesis that I submitted to the California Earthquake Prediction Evaluation Council along with my data. Earthquake prediction is still in its infancy, testing unproven methods. But we're working in a goldfish bowl of public interest and it's impossible to keep quiet. The trouble in the past was that predictions as to specific time, place and magnitude were rarely published before the earthquake, so the field had trouble documenting the track record of different methods.

What made you decide to go public with your predictions?

Because the ice must be broken. The only way to compile such track records is to write up our tests and document the results. There are going to be false predictions along with correct ones, and we need this data as well. When we can say we've tested a method of prediction 20 times and it worked 18 of them, it obviously will have some statistical credibility. Pretty soon we may be able to set odds, like Jimmy the Greek, for government agencies responsible for warning the public.

How did you arrive at your prediction?

On the velocity "bay" theory formulated over 10 years ago by Russian scientists. They noted that before an earthquake, the speed with which sound waves pass through the earth's crust decreases, and then returns to normal just before the earthquake. The word "bay" describes the dip on a graph when the sound velocities are low. After-the-fact evidence from a number of earthquakes, including the one in the San Fernando Valley in 1971, indicates that the longer the low period, the larger the earthquake. In the present case we noted velocities K decreasing in early 1974 and returning S to normal this past December, a two-year "bay."

What causes the "bay"?

If you squeeze rock, small cracks open up under the stress, swelling the rock. This increase in the rock's volume slows down sound waves that pass through it. Then something fills the cracks—water, perhaps—or closes them, and sound wave velocity returns to normal, though the rock is weakened.

Have you tested this prediction method before?

Once. Our first written test of the hypothesis—by which I mean we published our prediction—was in January '74. We predicted an earthquake of magnitude 5.5 in the Riverside area. Two weeks later there was one; we had the right area and the right time, but it was a magnitude 4.

Didn't the Chinese correctly predict an earthquake in February 1975?

Apparently they did, evacuating an area just before the quake. No one outside China has actually looked at their data yet, but they have said they used every method known—the velocity "bay" theory, ground distortion, change in tilt, changes in level and sediment in well water and chemical content in ground water, and unusual animal behavior. But the Chinese also had at least one false alarm, during which inhabitants were sent into the fields in freezing weather, and nothing happened.

What, precisely, is a fault?

An earthquake is a slippage along a surface of rocks in the earth due to stress. According to current theory, the earth's surface is composed of plates of rock moving around, and as they jerk past one another, earthquakes occur at their boundaries or at other "fault" lines of rupture in the rock, and they send out waves like those caused by a pebble tossed in a pond.

Is the San Andreas Fault, which runs the length of California, the area's major fault?

Yes, but that's somewhat of a misconception. If you consider the four greatest California earthquakes in the past 200 years, the San Andreas Fault accounts for only two of them—at Tejon Pass in 1857 and in San Francisco in 1906. People say, "As long as we don't live on the San Andreas Fault, we're okay." But California is a map of many faults and there's no way you can get away from them here. When people choose to live anywhere in California, they choose to live with earthquake hazard, as other states have floods or tornadoes.

Is California the only area of geological concern?

No, most of the Western states have high earthquake hazard, but there are areas even in the East—Charleston, S.C., near Boston, the Mississippi Valley. In these areas, the frequency of large earthquakes is not as great as in California, but the hazard is considered by some to be just as great. While California has had earthquake-resistant building codes since the 1930s, there are few in the East.

How do you measure the size of an earthquake?

Before Charles Richter developed his scale, they were measured by estimates of the shaking strength, like whether dishes fell off shelves or chimneys toppled. The Richter scale is a logarithmic scale based on vibrations registered on seismographs even great distances away, and each number represents a magnitude of tremor 10 times more severe than the one before, and releasing about 30 times the vibrational energy.

What constitutes a major earthquake?

Well, geologically speaking, a magnitude of 8 is major. The largest known were 8.9—in Ecuador in 1906 and in Japan in 1933. San Francisco's 1906 quake was about 8.3, and Anchorage in 1964 was 8.4. But there's really no upper limit.

What's the greatest danger in an earthquake?

It's well recognized in L.A. that the greatest hazard to property and life comes from old buildings constructed before the first earthquake building code in 1933. There are many of these, and the problem is the $3 billion investment it would take for total reconstruction. There was a lot of really misleading information in the movie Earthquake, which showed the total destruction of everything, including single-family dwellings. In L.A. such dwellings are mostly wood frame houses, which in fact stand up well in an earthquake.

What precautions do you advise in case of earthquake?

If you're inside, for example, don't run out, because the most hazardous places are just outside doorways and near the sides of buildings. If outside, stay in the open, away from buildings. If you leave your house during or after, shut off things as if you were going on vacation, and above all don't make unnecessary calls or go sightseeing. In general, I think people who live in California should go through their houses for hazards—like heavy objects on open shelves—once a year.

What preparations have you made in your own home?

I took my stereo speakers off the wall and set them on the floor. I'd meant to for years. Besides, to be honest, we had a painter coming.

Are you surprised by the reactions to your prediction?

No, not really. I'm not naive enough to think it wouldn't cause some disruption, but now that it's received so much attention, I hope to establish in the public mind that this is a test prediction. There's no question that there'll be some false alarms, and I don't want to be a hero if it happens—or a villain if it doesn't.