A 12-kilometer cable of hydrophones towed by the Marcus G. Langseth is capturing waves reflected off Cascadia’s fault zone.
At the Cascadia subduction zone, which has generated some of North America’s greatest earthquakes, the silence is deafening. Lying off the Pacific Northwest, where a plate of ocean crust dives beneath North America and into the mantle, Cascadia is best known for a mammoth magnitude 9 earthquake in 1700 that sent a tsunami all the way to Japan. But in modern times, it has been ominously quiet, with almost none of the small, daily earthquakes that are common at other subduction zones. Stress building up at the fault seemingly has had no release. “It’s just way, way, way too quiet,” says Chris Goldfinger, a marine geologist at Oregon State University, Corvallis.
Last month, however, that silence was shattered with the arrival of the Marcus G. Langseth, a research vessel that is generating miniearthquakes of its own in a 2-month campaign. On the ship, owned by Columbia University and funded by the U.S. National Science Foundation, scientists use an airgun to blast sound through the water, sending waves into the crust below. A long chain of hydrophones trailing the ship catches echoes from the innards of the 1300-kilometer-long Cascadia fault (see map, below). Other receivers, dropped on the ocean floor and scattered across coastal farmland and woods, listen for reflections from the deeper parts of the fault, which slopes east, down under the coast.
The resulting pictures of the fault, sharper than any collected before, could show whether its silence is cause for alarm. “We’ve been waiting for this moment for quite a few years,” says Kelin Wang, a geophysicist at the Geological Survey of Canada.
Cascadia’s quiet has long been taken to mean the fault is entirely locked, with the edge of the North American continental plate stuck to the subducting Juan de Fuca oceanic plate as it plunges by about 4 centimeters a year. As the continental plate flexes and builds up stress, Cascadia could be headed toward a megaquake like the one in 1700, when the fault ruptured along its entire length, from north of Vancouver, Canada, to south of Portland, Oregon. Such a quake would inundate the coast with a wave up to 30 meters high, liquefy soil under cities, and likely claim thousands of lives.
Large quakes have struck Cascadia every 500 years or so, and building codes in the Pacific Northwest are based on a worst case scenario. But without a modern example of the 1700 strike, no one knows for certain whether the next Cascadia quake will rupture the entire fault again, says Lydia Staisch, a geologist at the U.S. Geological Survey. “It really fuels mystery.”
Movement captured by GPS stations in recent years offers some reassurance, suggesting the part of the fault off central Oregon is creeping, releasing some stress without earthquakes. Paleoseismologists have also found evidence that many of the large quakes in the past 10,000 years did not rupture the whole fault. Rough patches along the fault may split it into segments, acting as “gates” that can stop a rupture in its tracks. Fuzzy images from previous imaging campaigns hinted at potential gates: eroded undersea mountains on the oceanic plate or faults in the continental plate. Decisively identifying these structures would firm up the idea of Cascadia’s segmentation—and lower the odds of large, catastrophic earthquakes, says Suzanne Carbotte, a Columbia marine geophysicist who is leading the cruise.
Perilous slope
A research ship, zigzagging up the coast of the Pacific Northwest, is building a picture of the eerily quiet Cascadia subduction zone by firing seismic shots into the water and capturing reflections from under the sea floor. More than 800 receivers on land will help image deeper parts of the fault, which last ruptured more than 300 years ago in a magnitude 9 earthquake.
(GRAPHIC) K. FRANKLIN/SCIENCE; (DATA) KATHLEEN CANTNER/AMERICAN GEOSCIENCES INSTITUTE
The imaging might also gauge another dimension of a future rupture: how close to the surface it might extend. Scientists thought subduction zone ruptures could not slip all the way up to the ocean floor because the water-logged clays of the upper crust are too weak to build up strain. But the 2011 Tohoku undersea earthquake in Japan punched through to the sea floor, causing the devastating tsunami that triggered the Fukushima nuclear disaster. Images of sediments trapped in Cascadia’s trench, where the plates meet, could reveal layering that would indicate how often past ruptures have reached the surface.
Those kilometers-thick piles of sediments could hold clues to another potential catastrophe. The researchers will be looking for pockets of gas trapped in the sediments, which might make them liable to collapse during an earthquake in tsunami-forming submarine landslides. And measuring how quickly the airgun waves travel through sediments across the region will improve early warning systems that predict how quickly earthquake waves will arrive at a given place and how damaging they will be, says Harold Tobin, director of the Pacific Northwest Seismic Network, who helped expand an alert system called ShakeAlert to the Pacific Northwest this spring.
Fundamental insights into how subduction zones work could emerge, too. About 15 years ago, seismometers in the Cascadia region began to pick out a small shudder—as subtle as the rumble of a passing train—lasting for several weeks. The noise turned out to come from the depths of the fault, where the rock is so hot the plates transition from locked to smooth sliding.
Some scientists believe the rumble, which has been detected every year or so, stems from “slow slip”—an earthquake in slow motion. Similar events preceded Tohoku, as well as a 2014 magnitude 8.2 earthquake in Chile, suggesting slow slip events can add stresses to locked parts of the fault and trigger major quakes. But other scientists tie the tremors to noise generated as rising water is expelled from hot minerals in the slab. The 800 seismic receivers deployed on land should add 3D detail to this deepest part of the fault, helping resolve this debate, says Emilie Hooft, a marine geophysicist at the University of Oregon. That is, if the teams can recover the receivers, with their stored data, from across 130,000 square kilometers after a summer’s worth of plant growth, she says. “Our challenge now will be to find them.”
It’s not the only hurdle the seismic campaign has faced, Carbotte says, speaking to Science from the Langseth as huge swells buffeted it. During the airgun shots small chase boats scurry to drive off whales and dolphins, in an effort to protect them from potential hearing damage. And early in the cruise, fishing gear got tangled in the 15-kilometer-long hydrophone cable. Then, she says, “we actually lost it” when the cable broke in rough weather. They found the tail buoy quickly, but lost several days bringing the line back on board and switching to a 12-kilometer cable.
Later this month, calm will once again return to Cascadia. It will take researchers years to stitch together all the data collected by the cruise. But once they do, if they get lucky, something in those images may shout out why this subduction zone, compared with all others, stays so hushed. “That’s hanging out there as a great big question mark,” Goldfinger says. “People are still tiptoeing about it.”
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