New drill may reveal Tahoe quake history
David Perlman - San Francisco Chronicle
Published 9:27 p.m., Sunday, September 2, 2012
A web of dangerous seismic faults deep beneath the placid waters of Lake Tahoe has triggered earthquakes in the past and will do so again.
Over the past 60 years, small quakes have rattled communities around the lake while larger ones have been felt in towns like Truckee, farther inland from the shore. But scientists know little about when and how often big quakes struck the region along one of the lake's three major faults.
In their quest for fresh clues, they have begun a search for answers in the rocky rubble of an ancient giant landslide beneath Tahoe's western shore, using a powerful new tool designed to explore a far distant lake hidden under a half-mile of Antarctic ice.
For the first time, California geologists have hammered a drill into thick sediments atop the landslide's rocks to recover samples of ancient life.
The dated layers of sediments could reveal the recurrence rate of past quakes along what scientists call the West Tahoe fault, which runs for more than 20 miles beneath the lake, roughly between Emerald Bay and Tahoe City, and continues north and south on land.
Knowing when and how often that fault ruptured could lead to upgraded seismic hazard maps for the entire Tahoe region, said Gordon Seitz, an engineering geologist with the California Geological Survey who has long studied seismicity at the lake.
Seitz and his science team completed their first exploratory drilling effort Thursday, and will return in a month to probe the bottom again for more samples.
Ancient avalanche
The enormous landslide at McKinney Bay occurred at least 50,000 years ago, Seitz said, and sent an avalanche of rocks that buried more than 5 miles of the lake bottom, with some boulders ending up on what is now the Nevada shore, Seitz said.
Successive layers of sediment samples, holding the remains of long-buried living organisms, will be clearly dated by specialists at the Lawrence Livermore National Laboratory, he said. Rock layers that have been deformed by quakes can be read like a book, he said.
There's also a story of luck and teamwork behind the California geologist's venture, said Ross Powell, a geologist at Northern Illinois University and a leader of a far different $10 million exploration venture in Antarctica called Wissard, the Whillans Ice Stream Subglacial Research Drilling project.
Powell has been testing his group's underwater instruments at Lake Tahoe this week for his Antarctic project. His venture calls for a unique robot submarine, a cigar-shaped vehicle only 22 inches wide and designed to be lowered through the West Antarctic Ice Sheet, to explore a lake beneath the ice called Lake Whillans. The lake, which has never been seen before, may hold samples of unknown life forms millions of years old.
Tahoe water like Antarctica
The sub, named SIR for Sub Ice Robot, was built by a specialized engineering firm in Alameda called Doer Marine. The firm's president, Liz Taylor, knows members of the California Seismic Safety Commission, Powell said, and she suggested that the new robot and its ice-penetrating drill could also bring up samples from Tahoe for geologists.
The commission and the Gordon and Betty Moore Foundation put up $50,000 for Seitz to piggyback on Powell's $10 million venture - financed by the National Science Foundation - and use the drill and its instruments to recover crucial seismic samples from the lake bottom.
"Tahoe's fresh water was glaciated and it was pretty much like Antarctica during the last ice age," Powell said. "So it's an ideal place to test our robot's instruments for exploring the fresh-water lake there."
The drill, called a percussion corer, does not screw into the ground, but carries a 2,000-pound weight above its drill bit, which "basically hammers the drill down into rocks or ice again and again," Powell said.
In the Tahoe trials it could penetrate the McKinney Bay landslide's surface for more than 16 feet, he said.
The three major seismic faults at Tahoe are called Normal faults, Seitz said. Unlike the San Andreas Fault, where two massive blocks of the Earth's crust slide past each other, one block of crust abruptly drops down below the other.
"And a big earthquake beneath the lake could cause a towering rush of water like a 30-foot tsunami, followed by a succession of more waves, one after the other, with each one called a seiche," Seitz explained.
"On land when you feel a strong quake, the rule is 'duck, cover and hold,' " he said. "Up along the Tahoe shore, when you feel a strong quake, the rule is 'duck, cover, hold on, and then move quickly to higher ground.' "
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