Lots of Earthquakes; No Cataclysm

IN California, powerful earthquakes shake both ends of the state like gelatin. Western Europe quivers with its most powerful tremors in 200 years. Several hundred perish in a quake in Turkey.

The earth seems to be shaking in its planetary boots.

But don't think cataclysm has arrived: Scientists say the planet is not experiencing any unusual wave of seismic activity.

It is just the normal upheavals that occur as the earth's massive tectonic plates move. In fact, the world is in a relatively quiet period right now.

Normally, the planet experiences about 18 major earthquakes (defined as magnitude 7.0 to 7.9) each year and one great shaker (8.0 or above).

There have been only 11.5 major quakes on average in each of the past 15 years, and the last great temblor came in 1989, in an island chain south of New Zealand.

"We're not really having an increase in activity," says Waverly Person, a geophysicist at the National Earthquake Information Center in Golden, Colo. It just seems that way, he says, because recent quakes have struck near population centers and the tremors have been more publicized.

The same holds true for California, one of the world's most active seismic areas. While the recent cluster of quakes in the northern and southern part of the state that ranged from 6.0 to 6.9 on the Richter scale may seem out of the ordinary, seismologists say the pattern isn't unusual in geologic time.

The state usually gets at least one magnitude-6 quake somewhere along the infamous San Andreas fault each year. True, the tremors put California over its quota for 1992, but scientists say it is common to have several one year and none the next.

"It always gives us pause when we have earthquakes that occur within a short period of time," says Bill Ellsworth, a geophysicist with the US Geological Survey in Menlo Park, Calif. "In the broad view, what we have been seeing is fairly understandable."

The soothing news that California is quivering on historical schedule is probably of little solace to those whose Victorian homes have turned to pick-up sticks in Ferndale, Petrolia, and other towns along the rugged northern coast.

Residents of Humboldt County are still assessing the damage in the wake of a trio of temblors, led by the 6.9 quake Saturday, that knocked homes askew, triggered fires, and ruined downtown businesses. Damage estimates are approaching $75 million.

Even before the tremors hit, many of the small towns in the redwood-rich area were hurting because of a downturn in logging. Now they face more adversity.

Even so, residents are proving resilient and promising to rebuild. In remote Petrolia, residents quickly set up a makeshift Post Office to replace the one that burned down. In Scotia, Pacific Lumber has said that it will rebuild the town's four-store mall that was destroyed by fire in the aftershocks. Grocers donated food to residents in Ferndale.

In a tour of the area Tuesday, Gov. Pete Wilson lauded the residents for their "tremendous courage." He is seeking to have the region declared a federal disaster area.

The tremors in northern California followed just days after a 6.1 magnitude quake hit in the desert of southern California just north of Palm Springs. Only minor injuries and damage resulted.

The two quakes originated near, but not on, the San Andreas fault, which runs most of the length of the state and then angles out to sea. Scientists say the two events were not related.

The temblors have naturally revived speculation about the "Big One" expected to hit California sometime in the future. Scientists estimate there is a 60 percent chance that a magnitude 7.5 or greater earthquake will hit in the next 30 years along the San Andreas somewhere in southern California, where there hasn't been a great quake since the mid-1800s.

The odds for the San Francisco Bay area, which saw strains along some of its fault systems relieved with the 1989 7.1 earthquake that claimed 67 lives, are lower. Scientists say there is about a 50-50 chance of a size 7 quake there in the next three decades.

Beyond the speculation about the Big One, there is anxiety about smaller quakes occurring along lesser-known faults, of which the temblors of the past week were prime examples.

"The main concern is that we will have earthquakes on faults in heavily urbanized areas," says Mr. Ellsworth.

Around the world, there is a magnitude-6 tremor recorded about once a week. The most devastating in recent months was the 6.8 quake that hit the Erzincan Province of Turkey in mid-March, killing nearly 500 people and leaving more than 100,000 homeless.

Europe realized that its ground isn't as firm as it thought when a 5.6 temblor jolted Germany, France, Belgium, and the Benelux countries April 13. Other countries experiencing strong quakes recently have included China, Taiwan, Japan, and Peru.