On March 11, 2011, a magnitude 9.0 earthquake shook Japan for nearly six minutes, triggering a tsunami and a nuclear disaster that collectively killed nearly 20,000 people. But beneath the surface, the tectonic plates off the eastern coast of Japan had quietly started shifting long before the shaking began. In February 2011, two quieter earthquakes began slowly creeping along the Japan Trench toward the point where the massive, megathrust quake would erupt a month later.
These strange, quiet quakes are called slow slip events, or slow earthquakes—umbrella terms for the spectrum of subdued moving and shaking that happens at the boundary between tectonic plates. Discovered only in the last 20 years, slow earthquakes are still a seismic puzzle. They can shift tectonic plates as much or more than a magnitude 7 quake. But whereas a regular earthquake suddenly releases seismic waves that can topple buildings, a slow earthquake lasts days, months, sometimes even years—and people nearby never feel a thing.
These imperceptible rumblings are thought to have preceded massive quakes that ripped through Japan, Mexico and Chile—but we don’t know if slow earthquakes triggered the massive temblors or even how they relate to their faster, more dangerous counterparts. Decoding when, where and why slow earthquakes strike could help us understand the most dangerous fault zones on our planet—and, possibly even help us forecast devastating quakes and tsunamis before they take their toll.
“It’s a true mystery,” says Heidi Houston, a geophysicist at the University of Washington in Seattle. “We studied regular earthquakes for decades and we understand some things about them—and then this process comes along and it’s the same in some aspects, and so very different in some other aspects.”
Before the late 1990s, geoscientists thought they had a grasp on how the jigsaw puzzle of tectonic plates covering the Earth’s surface move and fit together. They assumed that as one slab of the Earth’s crust slides past another, the plates either steadily creep past each other or become stuck, accumulating stress until they explosively slip free in an earthshaking quake that ripples from the fault zone.
But starting right around the new millennium, a flurry of scientific publications described a new class of recurring and widespread slow earthquakes observed on opposite edges of the Pacific Rim.
The first report of a clearly defined slow slip event came from the Cascadia Subduction Zone, which is formed by the Juan de Fuca plate pushing beneath the North America plate from northern California to Vancouver Island. There, the regions some 20 miles beneath the surface are softened by the depths and high temperatures and slide smoothly past each other. But shallower, brittle portions of the sliding tectonic plates can become stuck together until the stuck region ruptures in a giant megathrust. Cascadia hasn’t unleashed a giant quake since the 1700s—but rumblings in the seismic community suggest the next big one is coming.
In 1999, geophysicist Herb Dragert with the Geological Survey of Canada noticed that some continuous GPS monitoring stations on southern Vancouver Island and the Olympic Peninsula were behaving oddly. Seven of them jumped about a quarter of an inch over several weeks in the opposite direction of the plate’s normal movement. This kind of backwards jump is what you would expect to see in an earthquake—but there had been no detectable shaking.
“Herb was very worried at first—he thought something was wrong with the data,” says Kelin Wang, a scientist at the Geological Survey of Canada who worked with Dragert and geoscientist Thomas James to decode this puzzle. “He tried everything to prove himself wrong, and everything failed.”
That’s because there was nothing wrong with the data. The team soon realized they were seeing the North America plate and the Juan de Fuca plate gently slipping as the patches where they were stuck together unzipped. At 18 to 24 miles beneath the surface, these stuck patches were above the high-temperature, high-pressure region where the plates slide smoothly, but below the locked, earthquake-generating portions of the subduction zone. And it turns out that the sticky, intermediate zone slips on a schedule, about every 14 months.