'Tardy' earthquake in central California yields treasure trove of seismic data

By MARK SHWARTZ

Sunday, Oct. 17, will mark the 15th anniversary of the Loma Prieta earthquake, a magnitude 6.9 temblor that killed 63 people in Northern California and resulted in more than $6 billion in property loss. Although the quake was centered in Santa Cruz County, about 40 miles south of Stanford, it was powerful enough to damage dozens of structures on campus, including Memorial Church, the Graduate School of Business, the West Wing of Green Library and several buildings on the Quad. By 1999, the university had spent more than $250 million repairing and retrofitting buildings campuswide.

“The takeaway message from Loma Prieta is that it was a very damaging earthquake that wasn’t within the main population center of the Bay Area,” says Mark Zoback, professor of geophysics. “The damage would be much more severe if a similar earthquake occurred on the Peninsula or the Hayward Fault in the East Bay.”

The probability of the Bay Area getting severely shaken by an earthquake in the next 30 years is very high, he warns. What have local governments been doing to prepare for that inevitable event?

“Frankly, there’s a lot of complacency, and some of that complacency is the result of Loma Prieta,” Zoback says. “There is a tendency to believe that, while the damage in parts of San Francisco’s Marina District was pretty serious, by and large the city did fine. Well, the next earthquake could be much closer, much more damaging and it could occur tomorrow.”

Or it could come 30 years from now. No one really knows, because the ability to predict earthquakes has continued to elude geophysicists. Then two weeks ago, researchers got a sudden windfall of seismic data when a long-forecast 6.0 quake struck a remote area near Parkfield, Calif.

Perched along the San Andreas Fault, Parkfield had been rocked by a magnitude 6.0 earthquake every 22 years on average between 1857 and 1966. Based on this historical record, geoscientists forecast in the early 1980s that the next 6.0 quake would occur sometime between 1988 and 1993.

Eager to catch the event, the U.S. Geological Survey (USGS) and the State of California launched the Parkfield Experiment in 1985. Dozens of seismic instruments were installed in and around Parkfield in anticipation of the 6.0 temblor, but 1993 came and went and still no earthquake.

Meanwhile, in 1992, Zoback and USGS scientists Bill Ellsworth and Steve Hickman came up with a proposal to greatly enhance the Parkfield Experiment by building an underground observatory of seismic and other sensors within the San Andreas Fault Zone—a project that would eventually be known as the San Andreas Fault Observatory at Depth (SAFOD).

In 2002, they finally got the go-ahead to drill a 1.3-mile-deep pilot hole close to the fault as a test run for SAFOD. The Parkfield pilot hole was equipped with a vertical seismic array centered about a half-mile below ground. As luck would have it, the instruments were fully operational when the long-awaited 6.0 earthquake finally struck on Sept. 28, 2004, providing scientists with an unprecedented number of seismic recordings.

“The pilot hole seismic array produced some very interesting results,” Zoback says. “But that’s only a small fraction of the data. The scientific beauty of the September earthquake is that the entire Parkfield area is so heavily instrumented. It’s the best documented moderate-size earthquake that has ever occurred.”

Zoback and his colleagues had hoped to have the SAFOD instruments in place before the 6.0 earthquake struck, but the 1.8-mile-deep observatory will not be fully completed until 2008.

“We wanted to be there when it happened,” Zoback says. “That’s a bit of a disappointment, but SAFOD was designed to drill directly into a section of the San Andreas that produces magnitude 2.0 earthquakes that repeat every two years. That way we will get to study the process of energy accumulation and release over and over again for the next 20 years.”

Ultimately, the Sept. 28 earthquake will prove to be a scientific bonanza, Zoback believes: “It may not be immediately visible to the public, but everything we do that improves our understanding of how earthquakes work eventually manifests itself in better estimates of seismic hazards and better strategies for mitigating the hazards.”

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