Researchers have discovered hidden features beneath the Pacific Ocean that help explain why Japan’s devastating 2011 earthquake and tsunami were so destructive. The discovery also provides new clues that could improve predictions of future large earthquakes and tsunamis.
A new study finds that a thin layer of soft, clay-rich sediment beneath the Japan Trench played a key role in the disaster. This unusually weak layer, located just below the ocean floor, allowed the fault to rupture all the way to the ocean trench during the 2011 “megaquake” earthquake. As a result, the ocean floor moved abnormally by 130 to 200 feet, contributing to the generation of a large-scale tsunami.
“This is equivalent to the entire area between Los Angeles and San Francisco moving 130 to 200 feet in just six minutes,” said Christine Regala, an associate professor in Northern Arizona University’s School of Earth and Sustainability and co-author of the study. “In all our years of observing earthquakes, we’ve never seen anything like that. As far as we understand, we never thought it would happen.”
The study, led by Regala and more than a dozen scientists from around the world, science.
Clay layer hidden under the Japan Trench
Most large earthquakes begin deeper than the surface. Regala explained that when tectonic plates move, the ruptures that cause earthquakes usually occur far underground. For example, the rupture that caused the magnitude 6.8 Nisqually earthquake in the Pacific Northwest in 2001 began about 52 miles below the ocean floor.
The 2011 earthquake in Japan was very different. The rift reached only about 15 miles below the ocean floor, and the fault could have broken much closer to the ocean floor. The resulting magnitude 9.1 earthquake caused one of the most devastating natural disasters in Japan’s modern history, killing nearly 20,000 people and causing more than $200 billion in damages.
To understand why this happened, researchers traveled to the Western Pacific aboard a research vessel. Chikyu. They drilled about 26,000 feet into the ocean floor, collected sediment samples, and analyzed the material. Guinness World Records recognized this expedition as the deepest scientific ocean drilling project ever completed.
Samples revealed a layer of pelagic clay 100 feet thick. This is a very soft and slippery deposit that formed over millions of years as microscopic particles slowly settled to the ocean floor. Sandwiched between much stronger rock layers, the clay acted like a natural “tear line” concentrating the cracks along a narrow path.
“At the Japan Trench, the geological formations essentially predetermine where the fault will form,” said study co-author Patrick Fulton, an associate professor in Cornell University’s Department of Earth and Atmospheric Sciences. “It becomes a very concentrated, very weak surface, which makes it easier for cracks to propagate all the way to the ocean floor.”
Why is discovery important?
Because this pelagic clay layer extends for hundreds of miles along the Japan Trench, researchers believe the region may be more vulnerable to shallow-slip earthquakes than previously thought. Regala said understanding where these weak layers exist could improve scientists’ ability to identify areas where the largest earthquakes and tsunamis are likely to occur.
“Earthquakes and tsunamis in Japan not only affect people living locally, but also people in ports and across the ocean,” Regala said. “Think about Hawaii. The most destructive tsunamis come from Japan and Alaska. These are truly global events.”
Improving earthquake and tsunami predictions
The researchers hope their findings will help scientists better understand where powerful earthquakes and tsunamis are most likely to occur. This knowledge could help policymakers strengthen building codes, improve seismic infrastructure, update evacuation plans, and better prepare communities for future disasters.
“Japan is one of the world’s leading countries in earthquake and tsunami preparedness, but even Japan was not prepared for what happened in 2011,” Regala said. “We all need a better understanding of where these events are likely to occur in the future. Only then can we develop emergency plans to keep everyone safe.”
