Quake Forecasters Watch and Wait : Seismology: A magnitude 6 temblor is expected by Thursday in an area along the San Andreas Fault. If it happens, it will be the first time an earthquake has been accurately predicted.

With a magnitude 4.7 earthquake and seven tiny aftershocks this week, the hamlet of Parkfield has caught the attention of seismologists who hope this could be the year of the first predicted earthquake.

Scientists are waiting eagerly to see if a Central California section of the San Andreas Fault will behave as expected and produce a larger temblor by the end of the year--perhaps before the end of the week.

The U.S. Geological Survey forecast Tuesday a 40% chance that a magnitude 6 quake would strike by 10:28 p.m. Thursday, 72 hours after the first. Geologists said the odds were diminishing as time passed, but they remained on edge and excited.

The governor’s Office of Emergency Services issued a warning Tuesday to seven counties--Fresno, Kern, Kings, Monterey, San Benito, Santa Barbara and San Luis Obispo--to take precautions, although no major injuries or damage would be expected.

“If it comes about, this would really be an event of historic proportions,” said Jim Gould, a state emergency operations planner who drove from Sacramento to Parkfield for the alert period.

“The first earthquake prediction by the state and the federal government,” he said. “If we can call it, especially in three days, that’s a major leap forward.” Andy Michael, a USGS geophysicist in Menlo Park, said: “Anything can happen. We don’t necessarily know what this means. But we have hope right now.”

For seven years, the sparsely populated valley around Parkfield has been larded with seismic instruments, measuring everything from the water table to changes in the planet’s crust to radio emissions. The region, about halfway between San Francisco and Los Angeles, straddles a 16- to 25-mile segment of the San Andreas Fault that has become the largest earthquake laboratory in the world.

This part of the fault acts as a bridge between a northern section that continually releases stress and a long southern portion that is locked. Consequently, said Richard Liechti, a USGS technician who oversees the monitors, the Parkfield segment “makes up the deficit by having this characteristic (magnitude 6) earthquake” on average every 22 years. Like the one in 1857. And those in 1881, 1901, 1922. Then there was an earthquake “11 years early,” Liechti said, in 1934. The last came June 28, 1966.

In 1985, the USGS concluded that a quake of magnitude 6 or higher would jolt the Parkfield region by January, 1993. So far, the agency has spent about $19 million installing and maintaining equipment that could reveal what happens before, during and after a quake. Scientists hope such information can eventually help them predict earthquakes, a goal that has proved elusive.

The Parkfield segment responded to the barrage of interest by slumbering. Researchers were beginning to despair, wondering if they would run out of money for their experiments before they got to record anything of note. But the new flurry of activity has dispelled their gloom.

And if the past is any indication, they have reason to be hopeful. A magnitude 4.7 quake preceded each of the last two magnitude 6 shocks, Michael said, and both, like Monday’s, began under Middle Mountain just northwest of Parkfield.

Michael was visiting with friends when his beeper went off, indicating that computers were picking up a Parkfield quake about five miles below the Earth’s surface. He headed into his Menlo Park office to check the data.

Liechti was at home about 20 miles from Parkfield. The quake “got my attention direct,” he said. “There was that sound like a freight train approaching, then a rolling motion for six seconds or so.”

All the aftershocks were recorded within the next two hours, the last at 11:44 p.m. Monday.

If the Big One--the Big One in the Parkfield sense--occurs, it will not be the prototypical Parkfield quake, Liechti said. In the usual sequence, he said, the 4.7 temblor is followed by the main shock “in 17 minutes and almost exactly 20 seconds.”

Still, Monday’s movement was enough to prompt about a dozen state and federal officials to drive to the site, swelling the usual population of 34.

Liechti and another USGS scientist planned to set up a laser experiment Tuesday night to measure changes in the valley floor.

And everyone prepared for disappointment, just in case.

If the weekend arrives, and the earthquake hasn’t, “we just go back to base-level status,” Michael said. “The chance of it coming by the end of the year will be the same as it was on Monday.”

Temblor Alert Zone

Parkfield, a ranching town where a quake of about magnitude 6 is expected, has more seismic instruments than any other spot on the planet .