"You're going to see some really cool geology," Brian Sherrod says, running his finger across the screen of his laptop in the cab of his pickup.
Sherrod, a U.S. Geological Survey paleoseismologist with a salt-and-pepper mustache, tents his hands and interlocks his fingers, illustrating how seismic forces created the craggy hillsides and deep fissures of Washington's Yakima Fold and Thrust Belt. In the satellite map on his computer, the region stretching east from the Cascade Range's volcanoes to the Columbia River resembles an ocean of frozen waves.
Sherrod will spend this spring day exploring the Wenas Valley, about 100 miles southeast of Seattle. He believes there's an active fault here, part of a not yet fully mapped system that he and six colleagues describe for the first time in a paper published in July.
Until now, the extensive seismic hazards of western Washington were treated separately from those east of the Cascades, which were thought to be smaller and farther from population centers. But the fault Sherrod is seeking appears to be part of an interconnected system underlying the Cascades, from Puget Sound to Umtanum Ridge and Rattlesnake Mountain, which loom above Hanford Nuclear Reservation and the Northwest's only commercial nuclear plant, the Columbia Generating Station. Should it span the mountains, it could mean emergency planners have significantly underestimated eastern Washington's earthquake risks. Longer fault systems -- and this would be a long one -- generally produce larger quakes, though they don't always rupture along their entire length.
Sherrod closes his laptop and heads down a rutted dirt road toward Umtanum Ridge's southern flank. He's excited and confident, like a treasure hunter sensing buried loot. "I'm looking for stuff that happened thousands, even tens of thousands of years ago," he says. "I go out and look in the rocks and dig in the dirt."
But he starts from the air, using tools that measure minute changes in elevation and sensors that reveal differences in rocks' magnetic properties. Such differences could result from earthquakes nudging distinct rock layers closer together. He'll visit the most likely faults, ruling out landslides or anything besides earthquakes that might explain the data. Then he'll dig trenches and look beneath the surface for telling details -- fossils from different climatic periods that appear in adjacent soils, for example. Such clues suggest ground movements that broke contiguous layers of earth. Geologists can pinpoint the breaks in time by comparing the age of the fossils.
Seismic monitoring has been done in the Northwest since only 1969 -- a blip in geologic time. Monitors have mostly tracked activity west of the Cascades, where a subduction zone quake like the one that devastated Japan earlier this year will likely strike someday. Unexpectedly strong shaking from a magnitude 6.8 quake near Olympia in 2001 prompted an effort to map the entire state's hazards. Still, USGS geologists concentrated first on surveying the more populated west side, largely ignoring the territory Sherrod is now exploring. "We were amazed by how little actual information there was dealing with the activity of these faults," Sherrod says. In the past three years, he's found at least three new faults east of the mountains -- and they are just the tip of the iceberg, he thinks.
"You think about the Cascades as being a big façade -- a big edifice that looks like it blocks (seismic activity)," Sherrod says. But he and his comrades have detected similar magnetic alignments on either side of the Cascades, some at depths indicating that the faults formed long before the range's volcanoes did. That suggests, Sherrod says, that they extend all the way through the mountain range -- a major shift in the geologic understanding of the region.
Further trenching and analysis of suspected faults is needed to determine just how significant the risk is, Sherrod says. In the meantime, emergency officials for cities and major infrastructure aren't rushing to update disaster mitigation plans, hazards maps and building codes. If Hanford and the Columbia Generating Station are at greater risk than previously thought, they may be forced to re-evaluate the adequacy of their plans. But John Schelling, earthquake program manager for the Washington State Emergency Management Division, says there needs to be greater scientific consensus before the state adjusts its planning and public outreach.
Sherrod is still trying to develop a more complete seismic survey of the region. Recounting run-ins with rattlesnakes and other perils encountered on his quest, he scrambles up a hill toward one likely fault. "Even when you know where they're at, sometimes they're really hard to find," he says.