A record-breaking deep-sea drilling expedition has uncovered the hidden mechanism that made Japan's 2011 earthquake and tsunami so catastrophically deadly — a thin, slippery layer of ancient clay lurking in a fault zone 26,000 feet beneath the ocean surface.
Published in the journal Science, the study — led by Northern Arizona University and involving more than a dozen international scientists — reveals that a 100-foot-thick layer of pelagic clay beneath the Japan Trench acted as a natural "tear line," allowing the fault rupture to race all the way to the seafloor. The result: the ocean floor shifted by an extraordinary 130 to 200 feet in just six minutes, generating a tsunami that killed nearly 20,000 people and caused over $200 billion in damage.
"That's equivalent to the entire area between Los Angeles and San Francisco moving 130 to 200 feet in just six minutes," said Christine Regalla, associate professor at Northern Arizona University and a co-author of the study. "We've never seen anything like that in the time we've been observing earthquakes. Based on what we understood, we didn't think that could happen."
The expedition, which Guinness World Records recognized as the deepest scientific ocean drilling project ever completed, took place aboard the research vessel Chikyu. The team drilled about 26,000 feet into the ocean floor, recovered sediment samples, and analyzed the material to understand what had happened.
The samples revealed a 100-foot thick layer of pelagic clay — an extremely soft, slippery sediment formed over millions of years as microscopic particles slowly settled to the seafloor. Sandwiched between much stronger rock layers, the clay concentrated the rupture along a narrow path, allowing the earthquake to propagate much closer to the surface than most megathrust earthquakes.
Most large earthquakes begin much deeper underground. For comparison, the rupture that caused the 6.8 magnitude Nisqually earthquake in the Pacific Northwest in 2001 started about 32 miles beneath the seafloor. The 2011 Japan earthquake rupture reached only about 15 miles deep.
Because this pelagic clay layer stretches for hundreds of miles along the Japan Trench, researchers believe the region may be more vulnerable to shallow-slip earthquakes than previously thought. The findings could help scientists identify other subduction zones around the world that harbor similar weak layers, potentially improving forecasts of where the next megaquake and tsunami might strike.
"Japan is one of the world leaders in earthquake and tsunami preparation, but even they weren't prepared for what happened in 2011," Regalla said. "We all need to gain a better understanding of where these events might happen in the future. Only then can we make emergency plans that will keep everyone safe."
