Cascadia's Fault

by Jerry Thompson

  Apparently what everyone needed and wanted was forensic evidence that there had been specific Cascadia earthquakes at specific times in the past. Not just hypothetical scenarios, not just signs that the beaches had been hoisted or the mountains tilted, or even that there had been smaller fractures in the continental crust near the California coast. The only thing that could finally put an end to all the back and forth would be tangible signs of past ruptures along the entire subduction zone. And once again, a clue about where that proof might be found was layered in the story written by Walter Sullivan of the New York Times, just before Mount St. Helens blew.

  The so-called missing trench had to be significant somehow. There were trenches at all the other converging plate boundaries, so why would Cascadia’s subduction zone be different? From what I’d read in the science journals, there was a bit of a trench off the west coast, although it was shallow compared to the others and filled with sediment. Why was the down-going angle of the Juan de Fuca plate almost horizontal while the others were steeper and deeper? And what did it matter that the crack was full of mud? The never-ending dump of sand and silt from the turbulent Columbia River and many others along the Pacific Northwest coast had all but buried the fault, so as with the Juan de Fuca Ridge, nobody knew the trench was there at first.

  Trenches at other plate boundaries in deeper parts of the Pacific are located far enough away from the outflow of big mountain rivers that silt and sediment can’t hide the evidence of subduction. Cascadia’s close proximity to the gushing plumes of mountain run-off had created this accretionary wedge, a blanket of muck two and a half miles thick (4 km) that not only filled the crack between the two converging plates but also made the subduction zone nearly impossible to study. There was something else, though—something buried in the sediment—that would reveal the hidden story of Cascadia’s past. And it too came from that chain of smoky volcanoes.

  Without knowing that Mount St. Helens would soon explode, Walter Sullivan had asked scientists about the eruption of another famous Cascade volcano, just to provide a frame of reference. He wrote about the explosion of Mount Mazama 7,700 years earlier, an apparently world-changing cataclysmic event. Sullivan described it so that readers could visualize what would happen again someday in the Pacific Northwest.

  Roughly a hundred miles (160 km) east of the Pacific coast, Mount Mazama, like Mount St. Helens, had been created by Cascadia’s oceanic plate subducting underneath North America. This ancient stratovolcano was given its name posthumously, because it had exploded and was long gone before geologists arrived on the scene seven millennia later to piece together what happened. The upper part of the mountain had completely disappeared in a spectacular blast that caused the lower walls of the volcano to collapse inward, creating a huge, circular hole in the ground—a caldera—five miles (8 km) wide. This gradually filled with snowmelt and rainwater to form Crater Lake—with a maximum depth of 1,958 feet (597 m), the deepest lake in the United States.

  Magma spilled from cracks along the shattered volcanic rim and surged downhill in avalanches that filled nearby valleys with up to three hundred feet (90 m) of hot rock, pumice, and ash. Somewhere between eleven and fourteen cubic miles (not cubic yards, cubic miles, or 46–58 km3) of magma was ejected. A towering column of ash thirty miles (48 km) high rained down for several days on eastern Oregon, Washington, Idaho, Montana, Nevada, and southwestern Canada. An ash layer half an inch (1 cm) thick was measured in Saskatchewan, 745 miles (1,200 km) from its origin.

  Sullivan quoted Grant Heiken, a volcanologist at the Los Alamos Scientific Laboratory in New Mexico, who suggested that “a safe distance from which to watch such an event might be the Earth’s orbit of a space station.” In more recent times, the U.S. Geological Survey website referred to the Mazama blast as “the largest explosive eruption in the Cascades in the last one million years.” Mazama’s blast was forty-two times more powerful than Mount St. Helens’ in 1980. Only the explosion of Krakatoa (recounted by Simon Winchester in his excellent book of the same name) off the coast of Indonesia in 1883 could compare to Mazama’s magnitude and impact.

  Eventually Mazama’s ash became famous in its own right. Geologists used radiocarbon dating to determine when the eruption had occurred and then, wherever researchers spotted the recognizable Mazama layer, it became an important stratigraphic marker, a distinctly visible line that appeared in bands of sediment all over western North America. Mazama provided geologists and archaeologists with a key reference point for geologic calendars and timelines. If you could find Mazama ash in a drill core sample, you could tell roughly how old the layers above and below it were and in what order major geologic events had occurred.

  And that’s how Mazama became a key factor in the next phase of the investigation of Cascadia’s fault. Within eighteen months of the eruption, much of the ash had been carried away by rain, washed downstream by hundreds of creeks and dozens of rivers, and dumped in great heaps along the offshore continental shelf and in deep-sea canyons that wandered for hundreds of miles along the coast. Thousands of years later, a team of marine geologists from Oregon State University would find the distinctive Mazama line in cores of mud they gouged from the ocean bottom—cores that may have looked insignificant at first glance. On closer examination, it turned out they contained the elusive, muddy fingerprints of Cascadia’s violent past.

  PART 2

  SETBACKS AND BREAKTHROUGHS

  CHAPTER 9

  Mud Cores and Lasers: The Search for Evidence

  At five o’clock in the morning it was still pitch black out on deck and the heavy air promised another sticky day. The boatswain, a muscular young guy in his mid-thirties with a patchy beard and a sunburned face, instinctively stepped back to get clear of the bight of steel cable at his feet.

  He glanced up behind the blue-green glare of the floodlights to make eye contact with the winch operator in a glass booth on the upper deck, then raised his right arm and made a circling motion, fingers pointed upward at the sky. The diesel roared a little louder, the cable drum began to turn, and a heavy metal shaft rose from the shadows.

  A handful of scientists gathered along the starboard rail for a final inspection of their strikingly low-tech research probe—a “Benthos gravity corer.” To the unfamiliar observer, it looked like nothing more than a hollow, vertical steel tube, about ten feet long and four inches in diameter (3 m by 10 cm), fitted at the top with a collar of five lead weights shaped like doughnuts and capped off with a set of angular fins, the kind you used to see on bombs, designed to make the rig fall through the water straight and true. Gravity would soon take it to the briny deeps, where it would stab the sea floor and try to capture nearly ten thousand years of tectonic history.

  The scientists, a mixed team of marine geologists, oceanographers, graduate students, and veteran researchers from the United States, Britain, Spain, Belgium, Germany, Japan, and Indonesia, had crossed the Andaman Sea aboard the Roger Revelle, a research ship operated by the Scripps Institution of Oceanography. They had sailed from Phuket, Thailand, on Monday afternoon, May 7, 2007, slipping between the Nicobar Islands and the northern tip of Sumatra into the Indian Ocean. Just before sunrise on Wednesday they were about to drop their first probe into the ocean mud west of Banda Aceh, that unfortunate beachfront town so memorably wrecked by the earthquake and tsunami of December 26, 2004.

  I had met U.S. chief scientist Chris Goldfinger, a marine geologist from Oregon State University (OSU), a year earlier in 2006 while filming an update to my original 1985 Cascadia documentary. He and his team had been punching core samples out of the sea floor and then adapting a technique used by oilfield geologists to match up the stratified layers of clay, silt, and sand at various points along the rupture zone for clues about the size and timing of past earthquakes. In the spring of 2007, he and an international team of scientists went to sea off Indonesia to find out what the latest event—the magnitude 9.2 Sumatra–Andaman megathrust—might tell them about seismic p
atterns in Cascadia.

  In the ship’s lounge the previous afternoon, he had gathered the graduate students, postdoctoral researchers, and coring technicians for an introductory briefing. Looking more like a veteran California surfer than a labcoated investigator, Goldfinger clicked his mouse and the first image appeared on a roll-up screen: a 3D cutaway view of the sea floor off the Oregon coast. “So here’s what Cascadia looks like in cross-section,” said Goldfinger. “Just a very simplified image of the accretionary wedge and the forearc structure.”

  I had seen this seafloor display at his office in Corvallis several months prior to the voyage and thought of it as a magical mystery tour of a place I’ll never get to visit in person. Cascadia’s undersea topography (bathymetry is the correct term) was a hidden landscape, another whole world, exotic yet oddly familiar. The same rugged terrain you see above ground along the Oregon coast exists in a parallel universe on the ocean floor.

  This deep-sea world had its own substantial hills and valleys, its own cliffs and steep canyons, and what looked like sharply cut river channels running across a wide, flat prairie that vanished in the dim, purple distance at the back of the 3D image. When Goldfinger was at his main computer terminal on campus, he could fly around inside the software, touring the canyons, floating over the folded hills of piled-up sediment (the accretionary wedge) in a trip along the subduction zone that was better even than my childhood memory of Captain Nemo’s fantastic voyage.

  “The main point here,” Goldfinger continued, “is that the rupture of a great earthquake in a subduction zone is almost always completely underwater.” He pointed to a fine line at the base of a row of hills along the sandy bottom roughly sixty-five miles (105 km) offshore, where the Juan de Fuca plate slides beneath the edge of the continent. “For other faults, like the San Andreas, that are exposed at the surface, you can walk right up to it,” again he paused, “get out your rock hammer, and tink, tink, tink. Or get a backhoe and dig a hole across it.” His punchline was that studying Cascadia is a tad more difficult because there is no way to get a backhoe or even a hammer to the bottom of the continental slope beneath thousands of feet of seawater.

  Goldfinger changed slides. “So what we’re gonna do on this cruise,” he explained, “is turbidite paleoseismology—based on the idea that if you have a big enough earthquake, you’re gonna generate a lot of landslides on the submarine margins. And you basically can just go to the bottom of the hill, take piston cores,” he clicked the mouse again, “and you should get a vertical record of landslides.” The new image onscreen showed a cutaway view of a core sample sliced open on a laboratory workbench. A vertical slice through the stratified layers of thousands of years of seafloor mud, sand, and silt.

  One of the younger researchers raised his hand. “Define turbidite.”

  Goldfinger grinned. “A turbidite is a sandy, muddy, high-energy deposit coming from submarine landslides.” He clicked the mouse again to a closer view of the core sample. “It’s an underwater sediment plume, gravity driven.” He waved a laser pointer at the screen. “Because all this material is entrained in the flow, it’s subject to gravity and it’s gonna keep going downhill until it runs out of—downhillness.” He wiggled the laser dot at a series of horizontal lines in the core, each one representing a different landslide.

  “The trick is,” said Goldfinger, “how do you determine if the landslides are earthquakes?” Meaning the sediment might pile up for years at the head of an undersea canyon and then simply collapse under its own, unstable weight. Or it could be knocked loose in the relatively shallow water of the continental shelf by turbulence from a big storm passing overhead. But Goldfinger and the team from Oregon State were pretty sure those dark lines in the ocean cores were the physical remnants of Cascadia’s tectonic past, debris from landslides that had been triggered by big seismic shocks.

  Out on deck, the boatswain signaled to the winch operator, who swung the boom out over the starboard rail where the steel piston hung in a stiff breeze. He then leaned over the side to confirm that the acoustic “pinger” was powered up, a smaller metal tube now clamped to the cable just above the piston probe. It would send signals to the ship’s multibeam sonar system as the coring rig approached the ocean floor. Satisfied that everything was good to go, the boatswain stepped back again, looked over his shoulder at the crane operator and raised his right arm, thumb down. At the working end of the boom, a pulley block began to spin and cable rumbled off the drum. Moments later the piston and its pinger disappeared with a quiet swish into the choppy black water.

  Two hours later, in the main laboratory control room, all eyes focused on flat-panel screens and digital readouts as the piston rig dropped through the dark abyss near the bottom of the Sunda Trench. Numbers clicked over rapidly. More than two and a half miles (4,100 m) of cable had spun through the block when Chris Moser, one of the senior coring technicians from OSU, punched the intercom. “Okay, Eddie, stop the winch.”

  After waiting a few moments for the rig to stabilize, he gave the order to spool it down the rest of the way. The piston hit bottom at 14,400 feet (4,380 m). It punched a five-foot (1.5 m) hole in the mud at the mouth of the seafloor canyon closest to the Aceh rupture zone—where the earth had begun to rip apart in the monster quake of 2004.

  It took another two hours to haul the rig back to the surface, so naturally, it arrived after a hazy, overcast sunrise, just in time for breakfast. One of the coring technicians grabbed an industrial-size pipe wrench to unscrew the nose cone from the shaft before the boatswain and a single deckhand slid out a plastic pipe concealed inside the slightly larger metal piston, like an arm inside a sleeve. The see-through plastic tube containing the core sample looked surprisingly light as the two men hoisted it onto a set of stanchions bolted to the deck. Light, because it was almost empty. After more than four hours from deployment to recovery, the ten-foot (3 m) section of pipe contained maybe eight inches (20 cm) of clay and silt—a disappointment, to say the least. They would have to start over.

  Later the next day five members of the science team lifted another, larger piston core from the sea. The boatswain and the crane operator hoisted the steel pipe casing away, exposing a twenty-foot (6 m) core tube inside (this one looked like ordinary, white PVC drainpipe). This time they’d captured a good sample. In the main laboratory, they mounted the plastic tube in a set of clamps on a long workbench and ripped it in half lengthwise with an electric saw. Everyone in the lab was buzzing with energy, measuring, labeling, and entering data into logbooks and computers.

  Chris Goldfinger watched as Russ Wynn from the National Oceanography Centre in Southampton, England, dragged a putty knife across the surface of the new sample, skimming off the top layer of runny mush to create a smooth, flat finish. Now it was easier to see the four or five horizontal stripes of darker, lumpier material that stood out against the bland, gray goo of deep-sea mud. These brownish smudges presumably were the killer’s fingerprints—the turbidite layers from undersea landslides triggered by the deadly Sumatra temblor of 2004 and perhaps from a subsequent rupture in 2005.

  Chatting later with a few of the grad students in the ship’s lounge, Goldfinger began telling the tale of how this kind of research—this deep-ocean version of earthquake hunting, cross-bred with oilfield exploration techniques—got started at OSU. He described how his thesis advisor, geology professor LaVerne Kulm, “took cores in the late’60s along the Cascadia margin. And remember, in 1968 plate tectonics was only four years old at the time. So people were just kinda getting used to the whole idea.”

  “They were taking cores out here,” said Goldfinger, pointing to a place on the map more than sixty miles (100 km) to sea, southwest of the Columbia River estuary, “and they noticed that there was an ash deposit out there called the Mazama ash.” This gave Kulm and his team a timeline, a starting point from which to gauge the age of the other deposits, those darkish lines of turbidite debris they had found in the vertical column of marine mud.


  Thirteen clearly defined turbidite deposits had been found in core samples taken many miles apart along various riverlike canyons on the ocean bottom. “Here, here, here, and here,” pointed Goldfinger, all over the map. “All of these places had thirteen turbidites. So one afternoon, as Vern tells it over beers, he and his students were puzzling about why—why would all these cores have the same number of turbidites? These come from different river systems, different parts of the margin, different geology. They have absolutely nothing in common except, apparently, they had the same number of turbidites deposited in these cores.”

  Goldfinger was warming to his subject. “And the way Vern tells it, one of the students goes: ‘Hey, maybe it could be earthquakes! And then they would just be triggered all along the whole margin. And you’d get the same answer wherever you went.’ And Vern said, ‘Nah, nobody’d ever believe it.’” Goldfinger looked around the room, paused for effect, and then added, “Well, turns out they were right.”

  At the very least LaVerne Kulm was right to suspect that in 1970, when several scientific papers based on those first Cascadia mud cores were published, nobody would believe thirteen giant earthquakes had caused thirteen landslides on every major offshore river channel along the Oregon coast. Chris Goldfinger didn’t believe it himself. At least not at first.

  Scientists generally don’t trust coincidence, and the absence of a convincing explanation guaranteed that the story told by those mud cores was not over. The final chapter—the way those mysterious layers of gunk from the sea floor eventually changed the history of Cascadia’s fault—started to make more sense to me only after I’d dug up a little background on where all that silt and debris had come from in the first place.