Living on the fault line.

Last October, every single citizen, of Parkfield, California, was put on warning. The town's only fault: the San Andreas-- and a history of shaky quakes.

The people who live in Parkfield, California, are used to earthquakes. Ask them about the one that hit this past October and they sound kind of "cool."

It was nothing special, hints 16-year-old Yolanda Garcia. "My books and a bunch of roses fell off my TV."

But scientists working for the United States Geological Survey (USGS) thought the medium-sized Parkfield quake, measuring 4.7 on the Richter scale (see caption, left), might be a sign of something bigger. They even thought they knew when it might come. For the first time in history, they issued a "Level A alert"--a warning that a quake measuring 5.5 to 6.0 on the Richter scale might hit the tiny ranching town within the next 72 hours.

EYES ON PARKFIELD

"I'm not thrilled to think about the ground splitting open," admits Yolanda. Neither is anyone else. That's why the USGS set up a hightech earthquake monitoring project in Parkfield. Since 1985, geophysicist John Langbein and his team have been using delicate monitoring devices to search for clues about how quakes happen.

The answers could come from almost anywhere. The instruments measure all kinds of variables such as ground movements and the levels of certain gases in the air. The hope is that one or more of these variables will show a correlation with earthquake occurrence--a regular change that happens before quakes that can be used to predict them. Last October, the scientists thought Parkfield's 4.7 quake--and a few smaller rumbles before it--might be such a telltale sign.

But why focus on tiny Parkfield instead of the big cities that stand to suffer more damage if hit by a quake? Well, for one thing, the town has a history of quakes. One has struck there just about every 22 years since 1857. If scientists find something that helps them explain the Parkfield cycle--and predict a quake there--the data might help them predict other earthquakes. Accurate predictions for cities like Los Angeles and San Francisco might prevent catastrophes. The millions of people who live there might be more prepared.

The reason for Parkfield's shaky history? The town sits right atop the San Andreas fault, the 1,127-km crack in Earth's rocky crust that is the breeding ground for many of California's quakes (see map, below). The fault marks the place where two pieces of crust meet--in this case the North American and Pacific plates.

Like all of Earth's tectonic plates, these solid slabs of rock "float" about on Earth's semisolid, sometimes-molten mantle. They creep past each other in opposite directions like huge chunks of ice drifting very slowly on the surface of a thawing lake.

But don't let the relaxed pace fool you. When the rocks from two plates "catch" on one another, tremendous pressure is created: It builds...and builds...and builds. ..until--CRACK!--the plates suddenly lurch past each other and an earthquake occurs.

You can feel a similar buildup of pressure and sudden release of energy when you press your fingers together to snap them. Like your snapping fingers, the sudden shifting of Earth's plates sends out vibrations called shock waves. In the case of your fingers, the vibrations reach your ear as a sound--SNAP! In a quake, the energy of the vibrations moves mountains and buildings for miles around. You may remember that the quake that jolted San Francisco in 1989 was centered 75 miles from the city. Nonetheless, 47 San Franciscans lost their lives when the upper deck of a major highway collapsed. The quake also caused fires and levelled houses and stores, leaving many people homeless and jobless.

THE WATCH

Luckily, Parkfield's tallest building is only two stories high. So residents there were fairly calm even after the USGS issued its ominous warning.

"The truth is, in a quake," says 15-year-old Jeremy Howard, "you'd be better off in Parkfield" than in a big city. "After a quake, Los Angeles could have 15 feet of glass on the ground."

Parkfield students, in particular, are well prepared for quakes. From earthquake drills and presentations at school, they know to stockpile food and keep fresh batteries in their flashlights. "Remember," advises 12-year-old Lilla Thomason, "your worst enemy is panicking."

This time, there was no panic--the predicted quake never came.

For geophysicist Langbein, that doesn't spell s-u-c-c-e-s-s. Instead, he says, it means "there are still a lot of unknowns" in earthquake prediction.