When will ‘the big one’ hit? Researchers dig for San Andreas Fault secrets in SLO County

Most people prefer to be dead before they are buried. I’m one of them.

But if things shake out differently, we won’t be that lucky.

In 2011, I stood with three researchers in a trench 10 feet deep, a strip of blue sky overhead. That was unremarkable, except for the fact that we were inside the San Andreas Fault.

At that point near Wallace Creek on the Carrizo Plain, the fault averages a major violent rupture about every 150 years.

A major quake was four years past the average when we were inside the fault. It’s a dozen years overdue now.

Since 1985, scientists have been working in Parkfield — roughly 70 miles north west up the fault — trying to divine the secrets of how to predict earthquakes.

The Parkfield portion of the fault shakes with 6.0-magnitude quakes regularly, roughly every 22 years.

Further south, the San Andreas Fault has been locked. The last time it broke bad was 1857, when it ripped from Parkfield to Cajon Pass near San Bernardino — a distance of 255 miles.

Earthquakes are most frequently found where the earth’s massive tectonic plates bump into or scrape past each other.

But the San Andreas Fault only accounts for about half of the geologic collision between the Pacific Plate moving north and the North American Plate moving south. There are thousands of faults in the west that can or will wreak havoc.

The media obsesses over “the big one,” but countless quakes happen every year in California.

Only a few are large enough to be noticed and named. Temblors in Coalinga, San Simeon, Northridge, Whittier Narrows, Loma Prieta and now Ridgecrest are all remembered for loss of life and millions of dollars in damage.

None of those quakes were “the big one.”

Nathan Welton wrote this story, published in The Tribune on Oct. 5, 2004.

Quake forecasts a shaky science

It’s like Old Faithful.

The San Andreas fault in Parkfield, like the legendary geyser, grumbles regularly — but predicting its geologic burps with clocklike accuracy has always been a shaky science.

Still, the area might yet be a fount of reliable earthquake activity. Because scientists have predicted a relatively powerful magnitude-6.0 rattler in that area for years, last week’s tremor seems to support mathematical models used for long-term quake warnings.

Besides showing that a temblor’s strength, location and approximate time — at least within a multiyear window — can be reasonably foreseen, scientists say last week’s quake also gives credence to other formulas that warn of shakers statewide.

One prediction says a 6.7-magnitude earthquake will hit San Francisco within 30 years, for example. The science behind that warning is similar to the science that forecasted last week’s event in Parkfield.

Experts hope to further polish their crystal ball with the help of a 10,000-foot-deep hole they’re now boring into the ground in Parkfield, into which they’ll place an array of sensors to measure the earth’s movements.

While long-term quake prediction appears sound, it’s the short-term predictions — such as warnings in the early 1990s that a quake could hit within days — that need to be re-evaluated.

“There was nothing there (in Parkfield) that met the rules we set up in the ‘80s,” explained Andy Michael, a U.S. Geological Survey seismologist. “We’re now looking at the data to see what might have been there that would have been new and interesting or could help us next time” in short-term predictions.

Long-term predictions

In the late 1970s, scientists began tracking seismic activity around Parkfield and developing a statistical warning system for earthquakes.

That system was and still is based largely on the fact that the area has experienced a magnitude-6.0 quake every few decades for the past 150 years.

David Oppenheimer, project chief for USGS Northern California seismic network, said long-term prediction “is not rocket science — it’s just a matter of doing your homework.”

The last two Parkfield temblors for which scientists have decent data — in 1966 and 1934 — occurred because of the same geologic phenomenon, he said.

North of Parkfield, the San Andreas fault slides along, with one side rubbing in a direction opposite the other. This so-called strike-slip fault — different from the up-and-down thrust that jolted San Simeon last December — typically moves smoothly because of relatively frictionless rock beneath the surface.

“There are little pieces inside the fault zone of different kinds of rock that are higher friction,” Oppenheimer said. “They get stuck and pop off quakes of lower magnitudes like one and two.”

On the south side, however, there lies a bend in the fault line that stalls the continual creeping. Eventually that stall builds up enough force and a stronger quake, such as the one on Sept. 28, occurs.

Oppenheimer said that the geology of the area guarantees the fault will let loose a quake in that area every so often.

Although the numbers say last week’s shaker should have happened around 1988, according to Michael, it wasn’t statistically late. And while forecasters didn’t pinpoint the exact time, they still nailed the location and the magnitude.

In some ways, earthquake prediction is akin to the “best before” date on a milk container: Scientists can predict what’s going to happen (a curdle or a quake) and where it’s going to happen (in the carton or in Parkfield) but they can’t say precisely when.

But they say their mostly accurate predictions validate the methods used to forecast future large earthquakes in metropolitan areas. And in the meantime, scientists are probing the San Andreas fault for clues to improve their accuracy.

“It’s still called the Parkfield Prediction Experiment,” Oppenheimer said, emphasizing the last word. “We saw some quasi-periodic behavior in the earthquake cycle when we first started in 1979, so the idea was let’s get instrumentation in the ground and get funding and record data before a significant earthquake.”

In the short term

What could be improved most about earthquake prediction in Parkfield is the short-term warning system, Oppenheimer said, which for years has been in place to caution people of imminent, strong quakes.

For more than two decades, and particularly in the early 1990s, experts continually warned of the possibility of an impending temblor, using a scheme similar to the federal government’s current terror alert system.

The warnings have an A through D severity rating and say a quake could theoretically occur within several days.

The system has cried wolf with every alert issued.

The reports are based on two main measurements along the fault in Parkfield: the speed and distance one side moves when compared with the other, and the strength of minor quakes in the area.

Those minor quakes are relatively common — about 30 shake the state daily.

A key to the prediction system was historical: The last few magnitude-6 Parkfield shakers followed a strong, magnitude-5 foreshock less than 20 minutes before.

So when a magnitude-4.7 tremor occurred in October 1992, scientists for three days warned the public of a 37 percent chance a magnitude-6.0 quake would strike.

It never did.

In 1996, after realizing their cautions came too frequently, scientists redefined the area where a foreshock would have to occur, making it more specific, and increased by 1.5 the magnitude necessary for a foreshock to trigger an alarm.

The future of predictions

Scientists will soon have a new tool to better their understanding of subterranean geology and maybe even improve their ability to accurately predict quakes.

In June, a 182-foot-tall drill began boring a 10,000-foot hole into the Earth on a ranch in Parkfield with the goal of placing hundreds of sensors that will measure temperature, pressure and the tiniest of the Earth’s movements.

The goal of the $300 million project, called the San Andreas Fault Observatory at Depth, or SAFOD, is to allow scientists to study how faults work and how earthquakes happen. The fault zone is poorly understood at that depth.

Scientists hope that the information gained during future quakes will help accurately predict earthquakes.

Since drilling began in June on the Bear Valley Ranch, 25 miles northeast of Paso Robles, the rig has already penetrated more than 7,000 feet. Scientists will conclude their drilling in 2007 and then invest 15 years monitoring the fault.