Quake Expectations Remain High

SAN FRANCISCO —  The geological “window” during which a strong earthquake is expected to strike along a central California segment of the San Andreas Fault peaks next month, but there is no indication from the scores of instruments there that a quake is imminent, scientists reported Tuesday.

But geologists and seismologists who have set up truckloads of instruments around the community of Parkfield are sure the fault will not let them down. Sooner or later, the San Andreas is sure to rupture through that region, just as it has on a regular basis every 21 to 22 years.

Some Local Damage

When it does hit, the quake is expected to be a magnitude of about 5.5 to 6 on the Richter scale, strong enough to cause some local damage but not enough to threaten the nearest communities, including Paso Robles and San Luis Obispo.

However, state emergency planners are preparing to deal with a larger quake should it occur. Mike Guerin of the state Office of Emergency Services said the state is preparing for a possible magnitude of 7, although William Bakun, chief scientist on the project for the U.S. Geological Survey, said that appears to be a long shot.

The consistency of the Parkfield segment of the San Andreas has earned it a special spot in the hearts of seismologists around the world.

Never before have they had such an opportunity to study a quake before, during and after it happens, and they have set up more instruments around Parkfield than anywhere else in the world.

Looking at the Record

Scientists attending a meeting of the American Geophysical Union here said Tuesday that while it doesn’t look like the quake will strike next month, it’s only a matter of time.

The historical record shows that the Parkfield segment, which is in Monterey County, is remarkably consistent, Bakun said.

Scientists have “a 95% level of confidence” that the quake will hit within 5.2 years and are 67% sure it will hit within the next two years, Bakun said. But he said it would not surprise him to see it happen within the next few months, based on the historical record.

Next month, however, appears to be unlikely because none of the events that would be expected to precede the quake are occurring at the present time, he said. Precursor events would include a gradual creep along the fault zone, or fore shocks that would indicate the region is about to let go.

Eye Out for a ‘Slip’

Bakun said historical evidence suggests there was “significant slip” on the fault at least a couple of weeks before the last quake, in 1966. That would be recorded on sophisticated instruments and it would even show up as cracks in the roads.

Nothing like that is happening there now, he said.

The experiments at Parkfield measure everything from land deformation and strain in the fault system to changes in the water level in nearby wells.

Bakun said there have been some false alarms.

“There have been a number of things that have snapped us to attention,” he said. Last September, for example, a swarm of five small quakes hit Parkfield over a seven-minute period, and three different “creep meters” recorded slight movement along the fault.

That was enough to put scientists at the USGS facility in Menlo Park and state emergency officials in Sacramento on alert, but the alert was canceled when nothing else happened.

Cause for Concern

One area of considerable concern to scientists and emergency planners is how to keep the experiments from being buried beneath their own success when they do finally reveal that the quake is near.

Parkfield is a very rural area, surrounded by private ranches, and no one is quite sure what to do about the hordes of scientists, journalists and tourists who would flock to the area as soon as the alert is sounded.

“It’s going to be pretty hard to keep geologists away from an earthquake,” Bakun said. “We’ll have trouble fighting them off with a stick.”

He said he would not be surprised to see “hundreds” of persons descend on the area, possibly jeopardizing the entire project, which depends heavily on cooperation from private land owners.