In the hours before the quake hit provincial China in 1975, small tremors raced this way and that. Well water turned muddy. Snakes stirred from their subterranean slumber and crawled out into the cold. But most quakes, like Loma Prieta, strike without warning. In the 20 years since that Bay Area disaster, scientists have uncovered ways to project roughly where and when the next big one is likely to hit. But they are no closer to the holy grail of earth science: being able to accurately predict an earthquake. State and regional governments, too, have made significant progress at making roads, bridges and buildings more earthquake-safe. But other parts of California, including housing, remain vulnerable.
State and regional governments, too, have made significant progress at making roads, bridges and buildings more earthquake-safe. But other parts of California, including housing, remain vulnerable.
PREDICTING
Scientists long have searched for ways to predict when an earthquake will strike as easily as the Weather Channel forecasts an approaching cold front. But they have consistently come up empty.
“All of the ideas that looked really promising, when you looked at the data more carefully, there really wasn’t anything there,” said Susan Hough, a geophysicist with the U.S. Geological Survey in Southern California.
Hough is one of the nation’s leading experts on short-term earthquake prediction. Earlier this year, after a scientist in Italy claimed to have forecast a quake, she appeared on the National Public Radio program Science Friday to challenge the idea. Her book on the subject, “Predicting the Unpredictable,” is scheduled to be published in December.
The most successful earthquake prediction in California occurred at Parkfield on the central coast, where a team of state and federal scientists forecast that a magnitude 6 or higher quake would hit sometime between 1988 and 1992.
Instead, the quake came in 2004.
“In geological terms, I think we did pretty well,” Hough said. “But in human terms, 12 years is a huge difference. That’s kind of the problem we’re up against.”
Nothing has raised hopes, though, like the Haicheng quake in China in 1975 where government officials alarmed by an unnerving series of small tremors evacuated people hours before the magnitude 7.3 quake struck, saving thousands of lives.
But such tremors, known as foreshocks, don’t always foreshadow quakes. “Sometimes nothing happens,” Hough said. Nor is unusual animal behavior such as the snakes that emerged from underground dens into freezing weather before the China quake a fail-safe indicator, although there could be a connection in some cases.
“Maybe energetic foreshock sequences do cause animals to go nuts,” Hough said. “But as I like to say as a cat owner, animals go berserk for all sorts of reasons.”
PLANNING
It’s not what they know that worries many earthquake scientists. It’s what they don’t know.
“We spend so much effort on the big faults,” said Jack Boatwright, a geophysicist with the U.S. Geological Survey. “And yet a lot of times, you get damaging earthquakes that aren’t on major faults.”
Loma Prieta is a classic example, where a lot of the motion occurred on a relatively unknown fault.
“It’s a reminder that we don’t necessarily know where all the faults are in California,” said Bill Ellsworth, past president of the Seismological Society of America. “And we have to plan for surprises.”
Ellsworth is part of a team that recently sank the first deep well into an active fault the San Andreas midway between San Francisco and Los Angeles. It intersects the fault at a depth of 2 miles.
“This is the first time we’ve been able to obtain the rocks that slide during earthquakes, to hold them in our hands and get them into the laboratory,” Ellsworth said. “And they are going to tell us a lot about how earthquakes work.”
Despite advances, many seismic mysteries remain, including the possibility that one earthquake can trigger another, even thousands of miles away.
That theory gained more credence after the recent magnitude 8 earthquake south of American Samoa was followed a day later by another major temblor on the Indonesian island of Sumatra.
Even under the best of circumstances, unlocking secrets deep beneath the Earth’s surface is an enormous challenge. Ellsworth a geophysicist with the U.S. Geological Survey likened the task to studying weather systems but with a key difference: “Our system is hidden under miles of solid rock that we can’t see through.”
PREPARING
Bridges are sturdier. Highways are safer. But 20 years after Loma Prieta, one part of California remains at high risk to a large earthquake: housing.
“We are worse off on the housing side,” said Mary Comerio, architecture professor at the University of California, Berkeley, and a visiting fellow at the Public Policy Institute of California. “We have a great deal of vulnerability if there is a big earthquake on the Hayward Fault or the San Andre- as Fault of a lot of housing damage.”
An expert in disaster recovery, Comerio said a big problem is California’s state-run, privately funded insurance program the California Earthquake Authority which was put into place by the Legislature two years after the 1994 Northridge quake to cushion the financial blow to private insurance companies.
“It’s a really crummy policy,” Comerio said. “It’s expensive, and it doesn’t provide very good coverage.”
Prior to 1996, Comerio said, most homeowners in quake zones had private insurance. Now they don’t, in part because the authority’s insurance deductible the portion of rebuilding costs that homeowners’ must pay is so high.
“It’s really a policy for catastrophic loss,” she said. “Only like one percent of those who are affected by the earthquake have a catastrophic loss where the whole house collapses. Most people have about $30,000 or $40,000 worth of damage and mostly the deductible is bigger than that. So it doesn’t do you any good.”
The majority of Californians choose not to participate Comerio estimated fewer than 14 percent of Californians have earthquake insurance.
“So, when the next one happens,” she asked, “where is the money going to come from?
PROBABILITY
And now for the long-term forecast: Sometime before 2038, it is almost certain that one or more earthquakes larger than magnitude 6.7 will rattle California. Expect loss of life and property damage in the billions.
Having abandoned the hope of predicting earthquakes in the short-term, scientists are developing methods to determine the probability they will occur over the long haul and where.
Last year, a team of state and federal scientists issued the most detailed long-term earthquake forecast in California history and the results are not encouraging. Over the next three decades, they said, there is a greater than 99 percent chance that California will experience a magnitude 6.7 quake or greater. The odds of a real monster one 7.5 magnitude or greater are nearly 50-50.
They are gazing into the future, in part, by exploring the past, by digging trenches across active faults that reveal evidence of prehistoric earthquakes and offer a chronology of when they occurred.
One of the most worrisome faults is the Hayward, which slices through the highly urbanized East Bay but was not involved in the 1989 Loma Prieta earthquake. Twelve quakes have happened on the southern Hayward over the past 1,900 years, scientists have found. Even more unsettling is their discovery that the last five occurred at intervals of 95 to 160 years for an average of 138 years.
And the last one shook the region in 1868 141 years ago.
“Numerous witnesses reported seeing the ground move in waves,” a U.S. Geological Survey circular says. “Shaking was felt as far away as Nevada and aftershocks rattled the Bay Area for weeks.”
That magnitude 6.8 temblor “still ranks as one of the most destructive earthquakes in California history but this is not the end of the story,” the report adds. “The Hayward fault will rupture violently again, and perhaps very soon.”