Full-Rip 9.0: The Next Big Earthquake in the Pacific Northwest

by Sandi Doughton

  There’s certainly plenty of precedent for big quakes setting off seismic cascades. The monster subduction zone quake that triggered the 2004 Indian Ocean tsunami lit up the region like a pinball machine. Hundreds of aftershocks rattled the area, some as big as magnitude 8. Aftershocks from Japan’s killer quake in 2011 were still jolting the islands more than a year later. Most were on the subduction zone itself, but aftershocks also radiated on land, breaking shallow faults.

  It’s easy to imagine the same fallout in the Pacific Northwest from the next Cascadia megaquake. Seismologists have an equation that predicts how many major aftershocks a given quake will produce. For a magnitude 9 on Cascadia, the best answer is eleven: one bigger than magnitude 8 and ten between magnitude 7 and 8. Most would hit within days or weeks of the original shock, but smaller rumbles can continue for decades as the crust adjusts. But as far as anyone can tell, the most recent full-rip 9 on the coast did not set the dominos tumbling. Only one of the faults Sherrod and the USGS group trenched showed evidence of a quake in 1700: Devil’s Mountain Fault, which cuts across Northern Puget Sound.

  A fault that appears not to have quaked 1,100 years ago is the one geologists now view as the most dangerous player among the region’s surface faults: the South Whidbey Island Fault, called SWIF for short. It was named for the biggest island in Puget Sound, home to a naval air station and the nation’s premier mussel farm. The fault’s existence was suspected for years, but it wasn’t nailed down until the mid-1990s when the USGS got its hands on the old oil company surveys. When geologists walked the island’s shorelines, they found lots of evidence that the ground shook violently in the past. Waterfront cliffs reveal striking swirls and blob-like patterns that form only in soil that’s been rattled hard enough to liquefy.

  At first, the SWIF seemed like just another fault in the growing pack. The earliest reports estimated its length at about forty miles, cutting diagonally from the old mill town of Port Townsend through Whidbey Island’s southern tip. The terminus appeared to be on the mainland between Seattle and Everett. But the SWIF kept growing. Scientists tracked it northwest to Victoria on Vancouver Island. At the other end, the SWIF struck out for the Cascade foothills, where it appeared to link up with several other faults.

  Sherrod trenched several scarps on the SWIF, including one that ran through the site of a new sewage treatment plant. He found signs of four major quakes in the past sixteen thousand years, with many more smaller quakes likely. The most recent was about 2,700 years ago. It was while excavating a strand near the wineries northeast of Seattle that Sherrod started to puzzle over a possible link with the Seattle Fault.

  The Seattle Fault appeared to terminate in the Cascade foothills. But faults don’t just end abruptly. As they examined the aeromag data and the scarps, Sherrod and Blakely came to a surprising conclusion: the fault that Seattleites most dread is just a branch of the SWIF.

  Eventually, the scientists tracked the SWIF across the Cascade Mountains as far as the town of Richland on the Columbia River—home of Hanford. It’s not a single break in the crust, but a band of fractures up to fifty miles wide. At nearly 200 miles long, the fault zone dwarfs every other shallow fault in the region. Only Cascadia is bigger.

  The longer the fault, the bigger the quake. But nobody expects a full rip on the SWIF. A rupture on one segment could trigger a quake on the adjoining segment—say the Seattle Fault—but the SWIF is unlikely to slip along its entire length at once, Sherrod said. “What we’re dealing with is a system of faults that we think are linked. But if you have a fault system that’s three hundred kilometers long and you rupture half or a third of it, that’s a big earthquake. That’s 7.5.”

  The SWIF forms a bridge that joins Western and Eastern Washington in a common seismic framework. For those who live east of the Cascades, it’s not a welcome alliance. The dry sides of Washington and Oregon have long been considered more like the nation’s heartland than Puget Sound when it comes to earthquakes.

  But red flags were there all along. In 1936 a quake estimated at magnitude 5.8 broke windows, collapsed chimneys, and opened two-hundred-foot-long cracks in the ground near the Washington-Oregon border. The towns of Milton-Freewater and Walla Walla bore the brunt. One of the most powerful quakes in Washington history struck east of the Cascades in 1872, setting off massive landslides along the Columbia River.

  Following the SWIF’s trajectory, Sherrod and his colleagues shifted their assembly line eastward in the mid-2000s. Their focus has been a series of east-west folds in the Earth’s crust that show up on lidar maps like wrinkles in a tablecloth. Viewed up close, some of the long, grassy ridges sprout windmills; others are green with a patchwork of vineyards. They have names like Horse Heaven Hills, Rattlesnake Ridge, and Saddle Mountain. Geologists passed through this country many times over the past century, and most concluded that any faults that underlay the ridges would be too shallow to pose much of an earthquake threat.

  It was a convenient interpretation during an era of nuclear proliferation. Throughout the Cold War, the Hanford reactors that fueled the first atomic bombs continued to churn out plutonium for the nation’s growing stockpile. Then came WPPSS, with plans for three nuclear power plants on the Hanford reservation. Consultants analyzed the seismic risks but tended to err on the sunny side. As one geologist put it, “Nobody wanted to suggest that Hanford was a bad place to build large, sensitive structures.”

  Once Sherrod started trenching the area, it didn’t take him long to find signs of at least seven quakes of roughly magnitude 7. In a region that was never covered with glaciers, the trenches reach back in time fifty thousand years. That suggests Central and Eastern Washington enjoy thousands of years of quiet between big quakes. But there’s a rule of thumb for active faults, Sherrod explained. For every magnitude 7 or bigger quake that splits the ground and leaves behind evidence in a trench, smaller quakes of magnitude 6 to 6.5 or so strike much more frequently.

  Data from recent oil and gas surveys confirmed that the faults under Central Washington’s ridges aren’t shallow. They originate more than twelve miles below ground and cut through massive layers of basalt. In other words, the faults that formed the ridges are much more dangerous than anyone realized. “It’s a fundamental rethinking of the seismic risk over there,” Sherrod said.

  What it will mean for the Hanford Nuclear Reservation and the vast accumulation of radioactive and toxic waste stored in underground tanks isn’t clear yet. In 2012 the Department of Energy, which is responsible for cleaning up the mess, ordered new studies of earthquake risk. Hanford is also home to the Columbia Generating Station, WPPSS’s only atomic success story and the Pacific Northwest’s sole nuclear power plant. The reactor generates about 6 percent of Washington’s electricity.

  After the Fukushima meltdowns spawned by Japan’s 2011 tsunami, the NRC ordered several safety upgrades at the plant but decided there was no need to bolster its seismic safety. The agency also extended the plant’s license another twenty years, allowing it to operate through 2043.

  The 1970s-era reactor wasn’t designed for a specific earthquake, but rather for a specific level of ground shaking. The shaking is expressed in g’s, a measure of gravity that also describes the g-forces pilots experience. At 1g, earthquake shaking is fierce enough to overcome gravity and objects and people fly into the air. Based on what they knew at the time, engineers designed the reactor to stand up to .25 g.

  So it was disconcerting in 2009 when a swarm of more than a thousand quakes shook the eastern edge of the Hanford site. None of the quakes was bigger than magnitude 3. But because they occurred so close to the surface, the peak motion force was .15 g, which isn’t far below the nuclear plant’s design level. Blakely and Sherrod traced the swarm back to one of the ridges they’ve been studying—and the fault that lies beneath it.

  Geology can feel like an avalanche of numbers—every fault a recurrence interval, every quake a magnitude, every seismic wave an amplitude and frequency. But at it
s core, earth science is about telling stories. What happened to create the landscape we see today? What is the future likely to bring? There’s no better storytelling tool than a picture, which is why Ray Wells travels with a three-foot-wide briefcase like artists use to carry their portfolios.

  Wells’s masterpiece is a laminated map of the Pacific Northwest with movable sections. Northern California is purple. Western Oregon is pink, Western Washington green. The map represents the culmination of more than two decades of research by dozens of earth scientists—and the key to calculating an earthquake budget for the region.

  Wells assembled the map to show why it’s hard to turn a corner in Washington without running into a fault. By sliding the pieces, he can animate the tectonic forces squeezing the region. “Looking at it as an integrated system helps explain what we see,” he said. Though it took a long time to tease apart the details, the story turned out to be fairly simple. It’s a train wreck on a geologic scale.

  On a visit to Seattle in 2012, Wells held the map in his lap like a ventriloquist’s dummy. The main driver behind the train wreck is the giant Pacific Plate, he explained. Moving northward at the geologically brisk pace of two inches a year, the Pacific Plate pulls California in its wake. Wells slid the purple California piece north, setting the wreck in motion.

  California rammed into Oregon. But Oregon is also being shoved from the side by the Juan de Fuca Plate, which is subducting under North America. The pink Western Oregon block on Wells’s map accordingly slid north and pivoted clockwise.

  Then came the crunch.

  Washington is caught between Oregon pushing from the south and the unyielding bedrock of inland British Columbia to the north, Wells explained. The Evergreen State crumples like a line of box cars slamming into a mountain. “That’s why you have the Seattle Fault, you have the Tacoma Fault, you have the Whidbey Island Fault. They’re all driven by this north-south compression.” Ditto for the rumpled ridges and faults in Central and Eastern Washington.

  Much of the groundwork for Wells’s map was laid in the 1970s by geologists who laboriously drilled samples from ancient lava flows across the Northwest. Iron-bearing minerals in the rocks act like tiny compass needles to reveal how the rocks have shifted since they formed. Today, scientists track ground motions in real-time thanks to a network of more than five hundred GPS stations.

  The measurements show the Puget lowlands are being compressed by about a quarter of an inch a year. That adds up to more than twenty feet of crunch since the last time the Seattle Fault fired off. Central and Eastern Washington are being squeezed at a slightly lower rate. Inexorably, the pressure is accumulating, loading the Seattle Fault and its associates like springs.

  “The plates are moving, but the faults are stuck,” Wells said. “You build up strain on the fault, and then the fault slips.” Around the world, the rate of tectonic-driven strain buildup is a rough yardstick of seismic activity.

  The Juan de Fuca Plate is bearing down on North America at about 1.5 inches a year, which makes the Cascadia Subduction Zone one of the continent’s most active faults. Strain buildup on the San Andreas and its offshoots adds up to about 2 inches a year, which explains why California gets more frequent earthquakes than Washington. Japan, where noticeable earthquakes pop off several times a day, is being squeezed more than 3 inches a year.

  Wells used strain rates to calculate a rough earthquake budget for the Pacific Northwest. The squeeze on the Puget Sound region is enough to produce a magnitude 7 quake every five hundred years. That’s about twice as often as Sherrod’s trenches indicated, but trenches are spotty recorders.

  It’s possible the last quake was so big it left a budget deficit that still hasn’t been filled, Wells said. But it’s also possible that the region’s earthquake account has accumulated a dangerous surplus that will be expended in more powerful quakes in the future.

  Back at Madin’s fault on Mount Hood in 2011, a week had passed since the trench was opened. The volcano sported a fresh coating of snow, and huckleberry bushes were tinged red by frost. Madin and his assistant were rushing to finish their work before the backhoe returned in a few days to fill in the hole.

  Multicolored pins and bits of flagging dotted the walls of the trench, outlining the different types of rock and dirt—geologists call them units—exposed in its walls. Wearing two jackets, rubber boots, and fishermen’s gloves, Madin was plucking bits of charcoal from a crevice and tucking them into Ziploc bags.

  “Ohhoho, that’s a nice one,” he said, displaying a fat morsel ripe for carbon dating.

  Oregonians have plenty to worry about seismically, with the possibility of megaquakes off their coast every 250 years or so. But the state isn’t rattled nearly as often as Washington is by other types of quakes, which has been a bit of a puzzle. Madin spent much of his career studying the Portland Hills Fault, which runs through the center of the city and across the Willamette River. But he still hasn’t found any signs of recent earthquakes.

  The train wreck model helps explain Oregon’s relative quiet—though Madin prefers to visualize a car. “If you’ve seen those slow-motion crash-test-dummy movies, you know that the damage is always worst at the bumper,” he said. “Washington is the bumper and Oregon is the backseat of the car that’s going along for the ride.”

  Oregon isn’t free of damaging quakes. In 1993 the small town of Klamath Falls, east of Ashland, was hit by back-to-back magnitude 6 quakes that damaged more than a thousand homes and businesses. One man was killed when a boulder dislodged by the quake smashed his truck. But so far, lidar surveys have revealed only a handful of scarps in Oregon. Of those, the few that have been studied don’t seem to crank out quakes very often.

  In the midst of his excavation, Madin theorized that the fault on Mount Hood might be a more significant player in the region’s geology. “It’s probably no coincidence that it appears to run directly under the volcano,” he pointed out. “It’s quite possible that the mountain is here because the fault is here.” Fractured rock can create zones of weakness, making it easier for magma to percolate up from the depths.

  But when the fieldwork was done and the results from the carbon-dating were in six months later, the trench yielded evidence for only a single quake in the past twelve thousand years. “That’s kind of sleepy,” Madin conceded.

  He presented the findings in March 2012 at a Seattle workshop to reevaluate seismic hazards in the Northwest. At the same time, he also reported that the most recent lidar surveys found no hint of surface fractures from the Portland Hills Fault. Maybe, Madin suggested, it was time to downgrade the hazard rating there.

  “Lidar gives,” he said, “and lidar takes away.”

  CHAPTER 7:

  THE EARTHQUAKE THAT WOULDN’T STAY PUT

  LIKE MUST OUTLAWS John McBride knew how to wiggle out of jams. When a handyman fingered him for selling liquor to Indians, McBride and his partner slipped $150 worth of gold dust in the pocket of the prosecutor, a man with a taste for whiskey and a sizable saloon bill. Not only were the charges dropped, but the court branded the handyman a liar and ordered him out of town.

  McBride wasn’t able to buy off the jury the next time a deputy served him with a warrant for the same offense. But he spent only a couple of weeks of his six-month sentence behind bars. On a June night in 1873, McBride pried loose a piece of sheet iron and shimmied through a hole in the ceiling of his cell in Walla Walla. “The pursuit has been made quite lively,” reported the local newspaper. The writer speculated that a popular purveyor of spirits like McBride would no doubt find friends to assist his flight. “He is now probably half way to the British line, and will soon shake the dust of Washington Territory from his feet.” Indeed, McBride was reported to be selling horses in British Columbia by the time the snow fell.

  A rancher with a spread near Wenatchee described McBride and his sidekick, Jack Ingram, as “scoundrels who, for pure cussedness, could not be excelled anywhere on the border.” So it’s a toss
-up as to who would laugh louder—those who knew McBride or the rascal himself—to learn that more than a century after he skedaddled to Canada his credibility would lay at the heart of a high-stakes duel over nuclear energy and one of the most powerful earthquakes in the Pacific Northwest.

  It struck in 1872, the year Ulysses S. Grant won his second term as president. Margaret Hopper, a USGS geophysicist, has studied dozens of historic earthquakes but none so memorable. Newspaper clippings, tattered maps, and reports about what transpired in 1872 occupy two full drawers in the cabinets lining her office in Golden, Colorado. “It was easily the most colorful quake I ever looked into,” she said. “There were some characters up there, for sure.”

  The shaking started on the evening of December 14. A steamboat captain in Olympia named James S. Lawson used his chronometer to time the initial jolt at forty and one-half minutes past nine. The quake was felt in such far-flung places as Jasper, Alberta; Eugene, Oregon; and Victoria, British Columbia, where windows shattered and glassware tumbled to the floor. “Not since Washington has been known to white man has there been so great an earthquake within its confines,” one newspaper declared.

  Printing presses hadn’t arrived yet in the Central Washington foothills where McBride and Ingram plied their trade. According to an early head count, the pair comprised two-fifths of the white population in the Wenatchee area. Most of the initial newspaper reports on the quake came from the bustling towns west of the Cascades. In Olympia chimneys cracked, doors swung on their hinges, and chickens were rattled off their roosts. A group of gamblers fled the saloon “in such haste they forgot to carry off the stakes.” Trees toppled along the Puyallup River valley. Dishes broke and water slopped from tanks in Seattle. Fissures split the marshy ground south of town.