Shakin' on the fault line.

Parkfield, California -- a quaint town with a one-room schoolhouse, a county library, and a single main street. Except that a large sign on the water tower by the cafe says it all: "Earthquake Capital of the World."

Over the past 150 years, sizable earthquakes (about magnitude 6.0 on the Richter scale) have shaken the town an average of once every 22 years. That's because Parkfield sits right atop the San Andreas Fault, the 1,290-km (800-mi) crack in Earth's rocky crust (outer layer) that is the source of many of the Golden State's quakes.

Based on the town's somewhat regular cycle, researchers from the United States Geological Survey (USGS) predicted in 1985 that a shaker would rock the town between 1988 and 1993. (The Earth doesn't work like a clock -- researchers didn't pick a single year for the quake.) This was the first time U.S. scientists dared to forecast a future quake. But guess what? Five years later everybody is still waiting for Parkfield to shake, rattle, and roll.

Every year around the globe about 120 strong earthquakes (magnitude 6.0 to 6.9), 18 major quakes (magnitude 7.0 to 7.9), and one great quake (magnitude 8.0 and higher) strike.

Geologists use the Richter scale to measure earthquakes' intensity. (A magnitude of 7.0 is 10 times stronger than 6.0.) But as of now, no one can predict when and where quakes will happen.

That's why USGS scientists scrambled to hard-wire Parkfield with an array of quake-detecting instruments. They sought to gather data to help them predict upcoming earthquakes, and to find out how the earth changes before, during, and after a shake-up.

SHAKE 'N' QUAKE

Parkfield residents are so used to earthquakes that the USGS's prediction didn't even bother them. "But when a couple of little shakers come one after the other, I wonder what's going on down under there," admits Lilla Thomason, 17, a Parkfield resident who feels her bedroom tremble now, and then.

What is going on "down under there" anyway? The Earth's outer layer actually resembles a cracked eggshell. Cracks like the San Andreas Fault divide the crust into several large slabs of solid rock, called tectonic plates, that "float" on Earth's partially molten mantle (the layer of earth below the crust). The plates slowly slide past each other in different directions, at about the same rate that fingernails grow.

To the north of Parkfield, the North American and Pacific plates creep past each other smoothly. But south of town the plates are locked -- "stuck" together. As the plates strain against each other, tremendous pressure builds up, until CRACK!

Rocks suddenly give way, releasing colossal amounts of energy. The energy spreads in the form of seismic waves -- vibrations that "squeeze" solid rock and move anything in their wake for miles around -- from mountains to cities. Earthquake!

The last medium-size quake shook Parkfield in 1966. According to the 22-year earthquake cycle, another should have struck before now. So why no earthquake? The theory is wrong, admits USGS geophysicist John Langbein. Just because quakes shook Parkfield regularly for the past century does not mean the pattern will hold.

WIRED UP!

Langbein would love to gaze into a scientific "crystal ball" and forecast quakes accurately. The instruments at Parkfield monitor a range of variables -- from the Earth's magnetic field to Parkfield's water level -- that may signal a coming shaker.

Langbein even enlisted the help of townspeople like school teacher Duane Hamann to operate some of the instruments. At night, Hamann controls the two-color geodimeter, an instrument that aims laser beams at reflectors across the fault to measure plate movement. Computers measure how fast laser beams take to reach the reflector and return. Changes in return time mean the plates have moved as little as a fraction of a millimeter.

Just as sensitive are creepmeters, a series of wires stretched across the fault. (Geologists use the term creep to describe barely detectable movements of the Earth's crust along the fault line.) "Anytime the fault moves, the tension on the wire changes," Hamann explains.

One of the more sensitive instruments is the strainmeter, which scientists lower into a hole 152 meters (500 ft) deep, where earthquakes may start. A strainmeter detects how pressure from the earth warps rocks under normal conditions and in a quake.

In 1993, strainmeters in Parkfield picked up signs that fault movement in the San Andreas had speeded up slightly. But the movement was so minuscule that scientists didn't recognize the trend until 1996. "It's a very subtle change and took us a long time to realize it was really happening," Langbein says.

Does the data mean the long-awaited quake is finally coming? Langbein won't say -- he doesn't want to be wrong again. In the meantime, life goes on as usual in Parkfield, including the regular earthquake drills. "People in Kansas don't worry too much about tornadoes," Lilla says. "They just go on with their lives."

RELATED ARTICLE: Quakes Near You?

If there's news of a major temblor in the U.S., you can almost bet it's on the West Coast. Or can you? Quakes rock that area often, thanks to the San Andreas Fault running almost the entire length of California. Faults are the birthplace of many quakes.

But earthquakes have also struck other parts of the U.S., where there are no known faults. Three of the biggest quakes in U.S. history hit New Madrid, Mo., along the Mississippi River in 1811. A major quake also rocked Charleston, S.C., in 1886.

What caused these quakes? Scientists aren't sure. They suspect hidden cracks in the North American plate are to blame. In fact, geologists from the University of California at Berkeley have uncovered faults buried deep under the Mississippi River. Who knows where other faults may be lurking? Researchers say cities on the East Coast, like Boston, New York, and Washington, D.C., are due for a shaker. So be prepared. A quake near you may be coming soon.