In 12 million years, barring unforeseen twists, San Francisco and Los Angeles , now about 400 miles apart, will sit more or less side by side.
Picture, then, riding BART (Bay Area Rapid Transit) down Sunset Boulevard from Nob Hill into Beverly Hills, and then perhaps a monorail to Disneyland's Matterhorn. Newspaper columnists like the Bay city's Herb Caen and L.A.'s Jack Smith can carry on their quibble over which city is more livable and compelling across the backyard fence.
Obviously there is plenty of time to work out the details. Meanwhile the two cities move closer at an average speed of about two inches a year. They are each riding the edge of a moving plate of the earth's crust, and it can be a rough ride.
Like cars with sticky clutches, the Pacific plate and the North American plate slide past each other in fits and starts. They catch on rough spots along their uneven edge, the San Andreas fault zone, which cuts from just south of the San Francisco peninsula down into the Gulf of California. Then the sides break loose and spring forward to catch their pace.
And thus the sunny cities of the southern California coast slip and jerk northward - pulling Baja California along with them - while the rest of North America slides south.
This is the general drift of things behind the most lively earthquake zone in the contiguous United States.
It is also, of course, a huge oversimplification. But it's the framework from which explanations hang. Of the dozen or so plates that make up the earth's lithosphere, it is the restless Pacific plate - ringed by the Aleutians, Japan, New Zealand, and California - whose squirming causes the big majority of the earth's temblors.
Lately, southern Californians are being warned they are due any year for a big breaking loose of a locked southern section of the San Andreas. A great earthquake is on the way, many earth scientists say. And they stress that southern Californians are not ready for it. But they can be. Like New Englanders who build their houses to withstand the winter wind, Californians are slowly learning to build their cities so that the earth's clumsy shiftings don't have to mean disaster.
Clarence R. Allen of the California Institute of Technology (Caltech) here - one of the elder statesmen of earthquake research - flatly says it: ''We can solve the earthquake problem. And we are fairly well along the road to doing that.''
The struggle is to inspire a people to action over a risk that is not so tangible, not so immediate, to most of them. It's a puzzling priority: an imminent danger that may wait half a century.
It's an abstract notion, a great earthquake, except to those who have felt big temblors before. And in the meantime, lawns need mowing and cars need oil changes and daily life presses on.
Although southern Californians are more keenly aware of the earthquake threat now than at any time in the past decade, public apathy is still the weakest link in the readiness effort, remarks Richard Andrews, director of the year-old Southern California Earthquake Preparedness Project.
The focus of attention is sharpening. The preparedness project - a government-backed effort to set up earthquake-response plans from the neighborhood level up - is a case in point. New earthquake scenarios by the state geology department give local planners a much-needed common ground to work from. Atlantic Richfield Company recently sponsored a major conference on earthquake preparedness for business leaders in the region.
''Unprecedented,'' comments Dr. Andrews. ''Not too long ago, respectable people didn't talk about this publicly.''
Yet the US is not spending much money - around $100 million a year for earthquake prediction and preparedness nationally, contrasting with Japan's more than $300 million a year over the next five years in just one region. Focusing on the quake-prone Tokai district southwest of Tokyo, the Japanese project ranges from building tsunami, or tidal wave, walls to stockpiling food and water underground. The American effort is lackluster in comparison.
The Earthquake Hazards Reduction Act - the chief US program - is so far scheduled for about the same funding next year as this year, approximately $60 million. There is, however, a requested $3 million extra. This is for a study on how to set up a seismic instrument network in southern California to continuously monitor by computer the most active faults.
Dr. Andrews's basic assessment: ''It is unlikely at the present level of expenditure that we can provide a short-term warning of a major earthquake.''
Technology can't prevent earthquakes, although this is not beyond imagining. Some faults could theoretically be kept slipping along smoothly without big jolts by injecting water. The expense and the legal tangling would be formidable.
Better to be ready for the rumble when it comes, say experts. Hence the urgent question: When is it coming?
There are still some good, live leads in the field, but prediction research so far has been disappointing.
Like a window under the steadily building pressure of a hydraulic ram, explains Thomas L. Henyey, professor of geophysics at the University of Southern California, a locked fault finally cracks at its weakest point, at a flaw. From this first break, the crack spreads along the fault at a couple of miles a second, fading as the tension of the shifting earth is relieved.
"If we knew what the stress distribution in the earth was, we could predict earthquakes pretty well,'' observes Dr. Henyey.
But stress deep in the earth can't be picked up by remote sensors the way starlight can. Stress monitors would have to be on the spot, deep underground along a fault, and this is very expensive.
So seismologists have spent the past decade sleuthing after those more slippery clues that come to the surface. Five years ago scientists held high hopes. That was the year the Earthquake Hazards Reduction Act passed Congress, fueling an almost mission-type study effort.
The search took some exotic twists by earth scientists' standards. Researchers probed all kinds of unexplained changes tied to underground movement - in radon gas burbling from the ground, water level in wells, animal behavior, electrical resistivity in rocks, twisted or distorted land surfaces, and micro-earthquake swarms.
They are still probing these anomalies, but their connections to earthquakes have been more fickle - or at least harder to find - than expected. None of the clues have yet provided reliable or consistent signals of when the ground is beginning to move, and no one is too certain whether they will.
In a sense, scientists are coming to think there are no shortcuts to predicting earthquakes. They can't interpret clues without a better picture of how an earthquake works.
''My own opinion has changed in the last few years,'' admits Dr. Allen. His well-traveled briefcase plopped on a stray chair is a reminder that a geophysicist is an outdoor academician. He is chairman of the National Academy of Science's Earthquake Prediction Evaluation Council. ''We put instruments all over the state without knowing what we wanted to do with them,'' he says of the networks of seismic monitors set up by Caltech and the US Geological Survey, among others.
He now thinks seismologists need to fill in their basic understanding of earthquakes before they can read the earth's next move. Five years ago, he says, ''we were overly optimistic.''
But another line of research has far exceeded the hopes of five years ago. The geological ancient history that Kerry Sieh, a young assistant professor at Caltech, uncovered in a dried-up swamp near Palmdale now supplies the best idea scientists have of when to expect the next great shake.
For 2,000 years this bog left layers of sand and peat on its floor, which lay smack across the San Andreas fault where it passes nearest Los Angeles.
The layers left stripes, from a cutaway view. And the stripes were broken and offset along the fault where earthquakes had shifted them out of alignment. Since peat sediment can be dated using carbon 14, the stripes held the history of 2,000 years of earthquakes.
In that time there have been a dozen great earthquakes (those that score around 8 or higher on the Richter scale). They have come roughly every 150 years , with spans averaging between 125 and 225 years. Since the last one, the Fort Tejon earthquake of 1857, it has been 125 years.
In the meantime, six or seven meters of slip has stored up on this section of the fault. That much slip in an earthquake would register 8 or more on the Richter scale.
The Federal Emergency Management Agency puts the probability of a great quake coming in any given year now at 2 to 5 percent. Dr. Allen thinks this is a little high, based as it is on some creakings and groanings of the earth that no longer bode as ominous as they once did. He estimates a 1-2 percent annual likelihood, or a 20-30 percent probability in the next two decades.
More mysteriously, Dr. Sieh did similar excavations further south on the San Andreas last year, and found that this section has been quiet for 560 years. This ''seismic gap'' now has around 10 meters of slip stored up and no geological clue has surfaced to explain how often it breaks.
Apparently the movement of the earth along the San Andreas is highly uneven from region to region, Dr. Sieh notes, like an ''uncoordinated centipede.''
All appearances to the contrary, the tallest buildings in Los Angeles would be among the safest in a great earthquake.
''You'd get quite a ride,'' smiles George W. Housner, professor emeritus of engineering at Caltech and an eminent researcher in earthquake engineering. But the skyscrapers wouldn't fall down, he says.
Brick buildings, on the other hand, are virtually certain to fall down, Professor Housner says. There are some 8,000 unreinforced brick buildings in Los Angeles County, many of them housing lower-income families.
Much of what Dr. Housner and his colleagues know about how buildings withstand earthquakes they owe to the hard shaking that caught San Fernando, a northern Los Angeles suburb, by surprise in 1971.
Engineering models are one thing, but ''the real test comes,'' says Dr. Housner, ''when you get a good shake. In 1971 we found we were deficient.''
The San Fernando quake was unexpected by geologists. It stemmed from an overlooked complication on the San Andreas. Southern California, to move northward, must turn a corner, squeezing around the deep roots of the Sierra Nevada and San Bernardino Mountains. In negotiating this ''big bend'' in the San Andreas - east and north of Los Angeles - the earth behaves something like a stale brownie crust and crumbles up along lesser faults throughout the region.
The San Fernando quake was an instance of the ''big bend'' pushing the San Gabriel Mountains up and over the Los Angeles basin.
At 6.6 on the Richter scale, scientists now agree that San Fernando gave as vigorous a shaking as an earthquake can muster. Bigger earthquakes don't shake any harder at the epicenter than a 6.5 quake does. The difference is that a bigger quake will shake longer and shake a larger area. So this made San Fernando a fair test of what happens in the eye of an earthquake. Since this chastening temblor, and largely as a result of it, the Los Angeles area has become better braced:
Now straps have been attached to 75 percent of the area's freeway overpass sections, which previously rested unattached on overlapping ledges to allow for expansion and contraction. In 1971, some of them fell down. Two hospitals collapsed. Today hospitals are subject to the same safety code that has battened down the state's schools. California dam owners must now upgrade their dams until they pass earthquake muster, since the San Fernando quake damaged one nearly to the point of bursting over the heavily populated San Fernando Valley.
The city of Los Angeles decreed last year that its several hundred unreinforced brick buildings must be strengthened or condemned. These have been considered vulnerable to earthquakes since the 1933 Long Beach quake, the one that first earned temblors notice in building codes.
The codes have come a long way. Structures of 16 stories or more must now be built on the basis of a computerized, dynamic analysis of their potential behavior in an earthquake, according to the Los Angeles building code. In contrast, the first earthquake codes of the 1930s required a building to be able to bear a lateral shove equal to 10 percent of its weight. This kind of a shove, Dr. Housner points out, is nothing at all like the shock waves of an earthquake.
Hunched over a manual typewriter in the corner of his book-lined office, Dr. Housner - chairman of the National Research Council's Committee on Earthquake Engineering Research - is drawing together a major review of the state of this research. Overall, he is optimistic.
There are key gaps in our knowledge, such as how big pieces of communications , manufacturing, and computer equipment might fare when shaken up. ''But looking at it over the years,'' he says, ''the research has made it into the codes.''
''The real question in design is: What's the worst shaking you can get?''
Nuclear power plants and dams are designed to withstand the strongest of all possible earthquakes. But to design everything to that standard would be far too expensive. Dr. Housner points out that builders now spend $30 billion a year in seismic areas for earthquake preparedness. So tightening of standards spells considerable costs.
California is not the only earthquake zone in the US, but it's the most lively and the easiest to understand. There is at least one temblor rated three or higher on the Richter scale a day somewhere in the southern part of the state on the average. With stronger shakes, the faults break right through to the surface.
East of the Rockies, it's different. Temblors of a given strength carry farther through the ground in the East, says Clarence Allen. In the spongier geology there, faults don't break the surface.
''We have some hints in Missouri,'' he says regarding the geology of earthquakes there. ''But we're still floundering around in Charleston. No clue in Boston.''
Although California is where the quake action is in the US, Japan and China are much more seismically active. Accordingly, earthquakes get much more public attention in the Far East. China suffered one of the worst disasters in earthquake history in the 1976 Tangshan quake. Tangshan was a large city built of unreinforced brick. The Chinese don't have the sophisticated technology of the West, but they have some 10,000 scientists studying earthquakes and 100,000 amateurs keeping seismic data. Japan has technology and manpower.
''If there is a major breakthrough (in seismology), it could as easily happen in Japan,'' says Dr. Allen, chagrined at the prospect of the US slacking in its leading role.
Earthquake concern here was given a boost, Dr. Andrews, of the preparedness project, figures by the debate over the mysterious Palmdale Bulge and the erupting of Mt. Saint Helens.
The Palmdale Bulge, a swelling up of a massive area of southern California between 1959 and 1974, may have been an error in surveying measurements. But some say it could be another decade before we know for sure. There have been several possible earthquake precursors, like the bulge, which California has creaked and groaned with in the past few years. But none has been followed by a significant earthquake.
If experts see a possible quake warning, they are then faced with weighing its importance and how to make it public. These are sensitive matters. A false alarm could be expensive and erode public confidence. Dr. Andrews's project is preparing a report now on the Japanese approach to judging and publicizing predictions.
Dr. Allen, meanwhile, looks down the road 20, 30, or perhaps 50 years to a time when Californians will have learned to live safely on the edges of their slipping and jerking lithospheric plates. Then, he told a recent conference of Los Angeles businessmen, they can write to friends in the East, ''Come on out to California and enjoy one of nature's spectacular phenomena with us.''