It is spring in San Francisco, which by firm local tradition calls for flying kites, falling in love and saluting the apocalypse.

April 18, as any San Francisco school-child learns, is the anniversary of the Great Earthquake and Fire of 1906, when the land under the Pacific Ocean last snapped away from the continent here in its steady trip north. The ground roared open, brick walls plummeted, water mains shattered, the whole city caught fire and 700 people died in assorted horrible ways.

Everybody in San Francisco knows this. Every San Francisco also knows, as surely as he knows his own address, that monster quake is going to come again, and probably in our lifetime. "It's not a matter of if's" U.S. Geological Survey earthquake expert Robert Wallace said recently, in the sort of dismal pronouncement made regularly here, "but when."

When it hits, if it comes during the afternoon rush hour with the intensity of the 1906 quake, 10,000 people will die. That is the estimate of a 1972 National oceanic and Atmospheric Administration report generally cited as the working doomsday document. The deaths will come from collapsed buildings, flying glass, falling parapets, the sudden rending of life-supporting machines, freeway crashes and trampling crowds in stairwells.

Major injuries will strike 40,000; minor injuries 300,000, the predictions says. Thirty-three thousands people will be left homeless. Most of the buildings in Chinatown, which is packed with the brick-and-mortar structures considered most hazardous during an earthquake, will collapse.

Hospitals will lose about 18,000 bed spaces to building destruction, radio and television will be blacked out for up to 24 hours, the airport will close for several weeks because of runways damage and the city will undergo (in the words of one author of the report) "numerous fires but no general conflagration."

Scientists speak with such grim certainty of the impending disaster because they believe the massive plate of land under the Pacific Ocean is moving north in relation to the continental North American plate - about two iches a year. One of the seams between the two, canstantly ripping and mending itself and ripping again, is a network of cracks in the earth 650 miles long called the San Andreas Fault.

There are spots along the fault where the rocks slip bit by bit, releasing the strain in the wrenched earth with occasional little quakes.

There are other places where - for reasons still under debate - the rocks don't snip. They tug, and they strain. The plates keep moving, and sooner or later, when the pull between the plates becomes too much to bear, the rocks give way suddely with the shattering roar of a great earthquake.

That is what is expected to happen under San Francisco. The city has experienced earthquakes since 1906, but nothing of comparable magnitude, and scientists estimate that by now as much as 10 feet of ptential movement has built up along the fault here.

But nobody here wants to think about it.

A 1971 survey on earthquakes by two geophysical researchers faltered because 78 percent of the Bay Area persons contacted to discuss the subject.

A 1977 poll by the San Francisco Examiner fared better but turned up the unsettling finding that of 500 persons polled, only one in five had taken even the slight precaution of setting aside emergency food and water. One-third said they could not disconnect their electricity; nearly half could not turn off the gas.

One San Francisco woman keeps her china in the bottom shelf and says all her friends think she's very odd.

A San Francisco city planner, his desk littered with predictions of ghastly earthquake destruction leans back in his chair and remembers the story they told in high school: A new-comer heard the earthquake tales, moved back to the Midewest in a panic and was promptly killed in a tornado. See? says the planner. And he shrugs.

Earthquake fear hangs over this city like a giant collective trauma, thoroughly repressed most of the time and every so often - usually in April - surfacing as a kind of giddy panic.

This year has been relatively quiet, with a mailing of sober earthquake, safety pamphlets from the Red Cross, some local media coverage and a celebratory run of earthquake movies. ("San Francisco," in which Clark Gable stumbles through the devastation to find Jeanette MacDonald and God, in that order, is the local favorite.)

Other years have been livelier. In 1969, the word - passed with conviction by some, disdain by others and a queasy heh-heh nervousness by most was that April 18 was it.The city of San Francisco, if not the entire state of California, was going to disappear into the Pacific.

So pervasive was the anxiety that then-mayor Joseph Alioto, in a personal gesture of defiance toward the elements, arranged a public earthquake celebration at 5 o'clock in the morning (the hour of the 1906 quake) on the fron steps of the city hall.

To his astonishment, 10,000 people showed up - carrying signs like "Reppent" and "Split the Earth, Not the Muccin" - to watch movies and roar their approval to the speaker who shouted, "I'm crazy. We're all crazy, crazy about a city than can make fun of itself and a good time in the face of impending doom."

It would not be farfetched to extend that philosophy to the general manicipal earthquake policy in San Francisco.

A parapet reinforcement ordinance, adopted after reports showed that faling balconies and gargoyles would cause much injury and death, went unfunded and unenforced for six years;work on removing or shoring up the dangerous parapets bgean only two years ago. A map available to the public lists predesignated earthquake shelter locations; some of the locations are not earthquake-proof.

City Hall may no be earthquake-proof. (A reporter pointed out last year that a six-inch chunk of the ceiling dome fell, on its own, on a serenely calm afternoon.) No one knows what to do about Chinatown.

Engineers say the city's skyscrapers, built either on piles dirven to bedrock or on huge foundation slabs sunk into the bayfill, probably will survive intact but they concede the only way to find out for sure is to watch them in earthquakes.

"It's far overrated, as far as I'm concerned," said Ed Joyce, the city's director of emregency services. Of the quake chaos he is to help abate. "You will have some devastation," Joyce said, "but as far as total devastation, no way." And he added matter of factly, "If you're at the wrong place at the right time, you're gonna get it."

It was probably with that cheerful fatalism that a woman called the San Francisco Chronicle back in 1969, just before the scheduled apocalypse. "What time is the earthquake tomorrow?" she asked. "I cannot give out that information," replied the operator. "You'll have to talk to The Man Upstairs." The caller said, "Okay - put him on."