Seafloor study reveals one reason why Japan's 2011 earthquake turned deadly

A new study from the bottom of the Pacific Ocean has partly revealed why the 2011 Japan earthquake and tsunami were so devastating—and how scientists may be able to better understand intense disasters in the future.

The study, co-authored by NAU associate professor Christine Regalla of the School of Earth and Sustainability, found that at the Japan Trench—a deep ocean boundary where one tectonic plate dives beneath another—the fault zone narrows into a thin, clay-rich layer hidden just beneath the seafloor. That weak layer enabled the 2011 “megathrust” earthquake to rupture all the way to the trench, producing 130-200 feet of shallow slip and displacing large portions of the seafloor.

“That’s equivalent to the entire area between Los Angeles and San Francisco moving 130 to 200 feet in just six minutes,” Regalla said. “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 study, authored by Regalla and more than a dozen other scientists from across the globe, was published in December in Science.

The geologist explained that when tectonic plates move, causing earthquakes, parts of the plates typically rupture very deep underground. For example, the plate rupture that started the 6.8-magnitude Nisqually earthquake in the Pacific Northwest in 2001 was about 32 miles under the seafloor. But the 2011 earthquake was different: Tthe Japan Trench rupture was just 15 miles deep, much closer to the seafloor. The resulting 9.1-magnitude earthquake and major tsunami was one of the deadliest natural disasters in modern Japanese history, killing nearly 20,000 people and causing more than $200 billion in damage.

Regalla and her colleagues discovered the weak layer of sediment at the Japan Trench by taking the research vessel Chikyu to the western Pacific, drilling 26,000 feet deep into the ocean floor and analyzing the extracted material. The effort was recognized by Guinness World Records as the deepest scientific ocean drilling ever conducted.

The sediment samples recovered from the drilling operation revealed a 100-foot-thick layer of pelagic clay, a very soft and slippery material formed from microscopic particles that slowly settled to the seafloor over millions of years. With stronger layers surrounding it, the clay acted like a natural “tear line” that concentrated the rupture along that surface.

“At the Japan Trench, the geologic layering basically predetermines where the fault will form,” said study co-author Patrick Fulton, an associate professor in the Department of Earth and Atmospheric Sciences at Cornell University. “It becomes an extremely focused, extremely weak surface, which makes it easier for ruptures to propagate all the way to the seafloor.”

Because the pelagic clay layer extends for hundreds of miles along the Japan Trench, the region may be more prone to shallow-slip earthquakes than previously recognized. That will be important for geologists to dig further into, Regalla said, because major natural disasters can have ripple effects beyond their countries of origin. 

“An earthquake and tsunami in Japan doesn’t just impact people who live locally—it also impacts people at the ports and people who live across the ocean,” Regalla said. “Think about Hawaii: Their most devastating tsunamis come from Japan and Alaska. These are truly global events.”

Regalla said she and her co-authors hope their work will help scientists better understand and better forecast large-magnitude earthquakes and tsunamis by identifying the locations that are most susceptible. Policymakers could use those scientific findings to alter building codes, build earthquake-proof infrastructure, revise their evacuation plans and more, helping communities become more resilient.

“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.”