The magnitude 6.0 earthquake struck at 10:15 a.m. Tuesday, about halfway between San Francisco and Los Angeles, seven miles southeast of Parkfield and 21 miles northeast of Paso Robles, according to the U.S. Geological Survey. A major quake in the same area killed two people last year.
The strong earthquake that shook Central California on Wednesday - without causing any significant damage or injuries - could be a boon to researchers who hope intense scrutiny of the state's earthquake capital may help predict future temblors.
Two aftershocks measuring magnitude 5.0 and 4.5 hit Parkfield minutes apart Wednesday morning, among the strongest of the more than 500 aftershocks since the quake. In all, there were six aftershocks of 4.0 or greater.
The initial quake struck seven miles southeast of Parkfield and 21 miles northeast of Paso Robles, according to the U.S. Geological Survey. A major quake in the same area, about halfway between San Francisco and Los Angeles, killed two people last year.
The area of the San Andreas fault where the quake struck is a seismic hot spot that has produced similar temblors every two or three decades and is among the most-monitored quake sites in the world.
"It's going to be a lot of data that we can look at," said Andy Snyder of the U.S. Geological Survey. "It ensures a good payoff for all the work that's been done by the USGS, all the university groups and foreign research institutes that have set up experiments here."
Dozens of sensors - seismometers, strainmeters, creepmeters - dot the remote, sparsely populated region. Drilling is underway there to go 1.4 miles down into the bowels of the 800-mile-long fault that forms the boundary between immense geological plates that grind and produce ground movement.
"This will probably be the most well-recorded earthquake in history," said Mike Blanpied, associate coordinator with the U.S. Geological Survey's earthquake hazards program in Reston, Va. "The jury is still out about whether the earth gives out reliable signals about whether an earthquake can be predicted."
The initial, 10-second quake was felt along a 350-mile stretch, as far north as Sacramento and as far south as Santa Ana, southeast of Los Angeles.
"Things were shaking so bad you couldn't tell where to go next," said Parkfield Vineyard owner Harry Miller, who grows 170 acres of wine grapes. "Trees shaking like brooms, and dust coming from everywhere."
Five or six of Miller's buildings - including his home - were damaged by the quake, which also tipped over about 300 cases of wine.
"I looked at the 10,000-gallon water tank, and there was water shooting 30 feet away," Miller said.
Parkfield, population 37, is subject to small, unfelt shocks all the time. Temblors are so prevalent that the USGS named its long-term earthquake research project the Parkfield Experiment.
Based on the spacing of the previous earthquakes, geophysicists had expected another earthquake of magnitude 6.0 or greater to hit Parkfield 15 years ago. But it didn't happen until Tuesday.
"It was very much the anticipated earthquake - the anticipated magnitude and the anticipated section of the San Andreas fault," Blanpied said.
Scientists put the chance of another similar or larger quake striking in the same area this week at 5 percent to 10 percent, said USGS geophysicist Andrew Michael.
Tuesday's quake, which occurred at a depth of 4.9 miles, was a "strike-slip quake," which means it caused the ground to move horizontally, said Kate Hutton, a seismologist at the California Institute of Technology in Pasadena.
The USGS estimate was strengthened several times, from 5.8, or "moderate," to 6.0, the threshold for a "strong" earthquake. A magnitude 5 quake can cause considerable damage and a magnitude 6 quake severe damage, but damage is usually far less in areas with strong building codes.
"This is earthquake country. It's a larger earthquake than what usually occurs, but it's not unprecedented," USGS spokeswoman Stephanie Hanna said. "We expect big earthquakes in this area, but don't know when they'll occur."
Last December, a magnitude-6.5 earthquake jolted the Central California coast, pitching an 1892 clock tower building onto the street and crushing a row of parked cars. Two people were killed in the state's first fatal quake since the 6.7-magnitude temblor that hit the Northridge area of Los Angeles in 1994.
Since then, area residents have taken precautions to protect their property and valuables from the next inevitable quake.
The very few residents of Parkfield - a half-dozen buildings on either side of a street in a valley surrounded by oak-studded hills - pride themselves on the area's seismic activity. A road sign reads: "Now entering the North American plate."
"The thing about the San Andreas fault is, you have to appreciate it because it's what made California the most spectacular state there is," said John Varian, owner of the Parkfield Cafe, where food spilled out of the cupboards Tuesday. "I'll take my earthquakes over those hurricanes any day."
By Matthew Fordahl