On the ordinary days we do not think about it. There is no widely disseminated post-earthquake evacuation plan; gargoyles and parapets still hang precipitously over the sidewalk; half of us cannot remember whether we are supposed to stand under a doorway or out in the back yard; almost nobody keeps emergency water or food stores in the basement. No apartment hunters says, "God, this place is wonderful, but when the earthquake comes it's going to fall down the cliff and smash me to smithereens."

Anybody who said that would have to be thinking rationally about the earthquake.

If you are afraid of the earthquake and you think rationally about it, then you have to leave.

If you don't want to leave, you don't think about the earthquake.

This is so simple that Californians don't understand why the rest of the country thinks it's crazy.

Monday night, northeast San Francisco, 10:11 p.m.: Nancy Dunn, writer, in bed reading a novel, glances over a schefflera that has lately been losing its leaves. Gently - so gently that for a wild moment Dunn thinks her plant is going into death throes - the leaves start to shake. Then the house shakes. And Dunn, who has been planning a trip east to visit someone she likes, has one cold taste of impending-disaster lucidity, which she thinks, If this is the big one, I'm not going to be able to see him. Damn.

We don't know what the Big One, the next great heaving of the San Andreas Fault, will feel like. In this city it is not quite like waiting for tornadoes, or hurricanes, the seasonal furies that come at you from a distance and batter you and go away. The Big One is legend; most of us have never felt anything like it at all, and it just waits underneath all the Victorians and condomimiums and landfills we have so cavalierly plopped down on this restless coast.

Two earthquakes in 10 days. One on April 27, 4:44 p.m., 4.4 on the Richter scale, epicenter just off the coast. One on Monday night, 4.8 on the Richter scale, epicenter 10 miles east of San Jose. The seismologists say it probably does not mean much of anything, that the earthquakes were on two different faults, that California has earthquakes all the time.

But each time this happens, each time the walls start to tremble and the floors sways softly and that low rumble comes out of the earth, a little of the raw fear comes back. We have to think about it. We cannot trust the ground. Everything might fall. By Thursday I will have slipped back into my comforting amnesia, but at this moment I see precisely how my balcony will collapse onto Broadway where the nightclubs are (just as I knew at age 14 precisely how the two apartment buildings on either side of my family's home would topple into one another and smash us all), how the hill will avalanche, how we will end up two blocks away, smashing a topless dancer with our living room wall.

At the end of the movie "San Francisco," which of course plays here all the time, Clark Gable and Jeannette Macdonald clutch each other in the ruins of the great earthquake and fire, the sudden love of God and truth dropping down on them both like some billowing parachute, and what do they see? Do they understand the madness of building a gargoyle-studded city on a 650-mile-long wandering crack in the earth? Do they pack up, loving and wiser, and move to more stable ground? No. They rebuild. Damn the elements! Put those gargoyles back where they belong!

There is something mythic about the way San Francisco is supposed to go. The skyscrapers buckling, the streets ripping open, the great city hall dome crashing, the balconied pastel apartment buildings plummeting down the cliffs. One demonic cackle and the earth just heaves up like a horse that has grown tired of the weight on its back. Comptemplate, for a moment, 8.3. That was the magnitude of the 1906 quake. Earthquake strength increases by a factor of 30 for every added number on the Richter scale, which means the intensity of the 1906 quake was somewhere around 30,000 times the shaking we felt Monday night.

A seismologist at the University of California seismographic station in Berkeley, trying to be helpful, described 8.3 as "total destruction . . . somewhere between being in a storm at sea and being in a car wreck."

Every two or three years, the seismologist said, researchers use laser instruments to measure the distance between the Farallon Islands, which lie west of the San Andreas fault, and Mt. Diablo, which lies east of the fault, about 80 miles away. The land mass under the islands is moving north, the land mass under the mountain is moving south, and every year, on the average, Diablo and the Farallons are three centimeters further apart. But the earth has held firm beneath them, and by now so much potential movement has built up along the faul - maybe 20 feet - that the land has become a giant seam, yanked in two directions, waiting to rip.

Seven years ago, the National Oceanic and Atmosphere Administration (NOAA) prepared a kind of catalogue of the apocalypse to determine precisely what would happen to San Francisco when the fault snaps. If it snaps as hard as it in 1906, and chooses the afternoon rush hour to do it, 10,000 people will die, according to the NOAA reports; most of them will be crushed by falling buildings, slashed by flying glass, trampled in crowded stairwells or squashed by dropping parapets. Some hospital patients also will die because their life-support machines will be ripped away. Most of chinatown, which is full of unreinforced brick buildings with too many people inside, will collapse. About 40,000 people will be seriously injured, about 300,000 people will be less seriously injured, and hospitals will lose 18,000 bed spaces.

This, or some lesser variant of it, will happen.

We understand that it will happen.

"Everybody knows it's going to happen," says Helen Ying, legal secretary. "The only way you can deal with it, really, is to not think about it. Otherwise, we'd leave."

"I wouldn't want to live where there's tornadoes," says Lt. Dan Kiely, firefighter. "That'd really bother me. Those people are crazy."

"How about a hurricane" asked Ray Vito, firefighter also, at coffee break with Kiely. "You couldn't get me to live in South Florida."

"How about all the muggers in Central Park?" demands Patrick Buddington, firefighter also. "You couldn't get me to live in New York."

You couldn't get me to live in New York either, because this is home and I like it very much. I salute the New Yorkers and the Floridians and the mad, tenacious people who rebuild their Panhandle homes after the tornadoes smash them down. The sky is quite blue outside my office, the national and state flags are whipping briskly over the firehouse next door, and the ground, for the time being, is not moving at all. CAPTION: Picture, The Great San Francisco Earthquake of 1906.