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.