Tahoe Fault Raises Red Flag

The biggest fault beneath Lake Tahoe could be due to rupture any time, according to a new evaluation being prepared by researchers who probed Tahoe and nearby Fallen Leaf Lake earlier this year.

The preliminary conclusions, outlined last week at a meeting of the American Geophysical Union in San Francisco, help sharpen a still blurry picture of potentially tsunami-spawning faults that lurk beneath the lake.

Ultimately, the findings will make their way into federal earthquake hazard maps that help determine building codes and set insurance rates.

"We have been looking at Tahoe as one of the biggest changes for California" in new maps due out in late 2007, said Mark Petersen, chief of the national seismic hazard project at the U.S. Geological Survey in Colorado.

What's become clearer with the latest Tahoe expeditions, supported by a UC Davis research vessel and led by scientists from three universities, is both the size and the potential of the West Tahoe Fault.

The fault, which skirts the lake's west shore, runs all the way through Fallen Leaf Lake and beyond to the south, said Graham Kent, a research geophysicist at UC San Diego's Scripps Institution of Oceanography.

While that didn't surprise Kent, it confirms that the fault is Tahoe's "800 pound gorilla," long enough to deliver a big jolt, magnitude 7 or more.

Such a quake could trigger an underwater landslide that quickly displaces huge amounts of water, potentially sending giant waves surging into parks, campgrounds, homes and marinas along the lake's shore, and possibly overtopping a dam that regulates flow into the Truckee River.

In addition, it appears from sediment layers that the last big earthquake on the West Tahoe Fault was 4,000 to 6,000 years ago. That's significant, Kent said, because the fault seems to produce a major quake every 5,000 to 7,000 years, or perhaps a little more often.

Which means another big earthquake could come soon -- although "soon" in geologic terms could be anytime from while you're reading this to a few lifetimes from now.

Both Kent and Robert Karlin, a geology and geophysics professor at the University of Nevada, Reno, stressed that core samples and other data collected this summer and fall in the two lakes are still being analyzed.

More lake expeditions and three or four years of work will be needed to provide "a more definitive answer" about just how serious Tahoe's quake risk is, Karlin said.

Still, the latest assessments join a growing body of evidence about faults and landslides in the Tahoe basin and their potential to spawn waves that could surge up to 30 feet and slosh from shore to shore for hours.

"We're keeping an eye on it," said Michael Reichle, chief seismologist at the California Geological Survey. "There are active faults near the lake, under the lake and to the east in Nevada at the base of the hills. We still don't know very much about all those faults."

Federally funded research overseen by Kent, Karlin, and Gordon Seitz, a research geologist at San Diego State University, is adding crucial pieces to the puzzle of Tahoe's quake history, said Petersen of the USGS.

The three have combined trenching on land with underwater coring and innovative sonar probes in an effort to better understand how often and how forcefully the lake's three faults have ruptured. Much of their work has been done from a research vessel operated by the UC Davis Tahoe Environmental Research Center.

"Without UC Davis having a lab and having a boat (at Tahoe), this research wouldn't be possible," said Kent.

For Tahoe area residents, although wildfires and snow closures are bigger worries, earthquake awareness has been growing since research began revealing potential risks in the late 1990s, local officials said.

In a disaster plan completed last year, Placer County acknowledged a "moderate" threat of Tahoe earthquakes, and projected that a magnitude 6.9 jolt there could cause $125 million in damages. The county's quake scenario included 28 demolished structures and two deaths, along with thousands of structures with moderate damage and dozens of people with lesser injuries.

In South Lake Tahoe, city manager David Jinkens said he wants to look again at tsunami projections, and might ask his council to consider whether shoreside warning or evacuation route signs are needed.

"Probably what has been more in our mind are slides, flooding that's associated with heavy rainfall," Jinkens said, but he and other city officials encourage people to be prepared for earthquakes, too.

That concern was echoed by Kent, a south shore native who has traveled from his San Diego campus frequently to probe the lake using a wide band "chirp" sonar that provides detailed images of sediment layering.

"People in the basin have to make sure they have adequate supplies, that things are bolted down, and that if they're on the beach, they should get off" if they feel a quake, he said.

That's hard advice to keep in perspective, though, said state seismologist Reichle, who pointed out that the region has seen plenty of small earthquakes with no disruption in water levels, and yet a huge shaker could send an almost "instantaneous" wave ashore.

There is one fairly sure bet for the lake's thousands of residents and vacationers, though, said South Lake Tahoe Fire Chief Lorenzo Gigliotti:

"If you see unusual movement in the lake, don't congregate on the shore."