The researchers focused on the plate boundary where the Pacific Plate dives beneath the overriding plate along the Japan Trench in the western Pacific Ocean. There they found that the main fault localizes into a thin, mechanically weak layer just beneath the seafloor, rather than remaining broad and deep within the crust as is more typical in many subduction zones. This configuration helped the 2011 rupture reach the shallowest part of the plate boundary and enabled the unusually large seafloor motion that powered the tsunami.
Regalla, an associate professor in NAU's School of Earth and Sustainability, explained that the amount of shallow slip during the Tohoku event was unprecedented in the modern instrumental record. At the trench, the fault slipped by an estimated 130 to 200 feet in only about six minutes, displacing huge portions of the seafloor. She compared that motion to moving the entire area between Los Angeles and San Francisco by the same distance in that short time interval and noted that existing models had not predicted that such behavior was possible.
In most large earthquakes, the rupturing parts of the plate interface lie much deeper below Earth's surface. As an example, the rupture that initiated the magnitude 6.8 Nisqually earthquake in the U.S. Pacific Northwest in 2001 began roughly 32 miles beneath the seafloor. In contrast, the Tohoku earthquake nucleated and propagated along a plate boundary that extended to only about 15 miles depth, which placed the most energetic slip close to the ocean bottom and set up conditions for an exceptionally powerful tsunami that killed nearly 20,000 people and caused more than 200 billion dollars in damage across Japan.
To probe the structure of the plate boundary, the team used the drilling vessel Chikyu to core into the ocean floor above the Japan Trench. The expedition drilled to a record-setting depth of about 26,000 feet below the seafloor, retrieving long sections of sediment and rock from the subduction plate boundary zone. That effort has been recognized by Guinness World Records as the deepest scientific ocean drilling achieved to date and provided the first direct physical samples of the shallow fault system involved in the 2011 rupture.
Analysis of the recovered material showed a roughly 100-foot-thick layer of pelagic clay at the plate boundary. This soft, slippery clay formed over millions of years from fine particles that slowly settled through the water column and accumulated on the seafloor before being carried into the trench. Mechanical contrasts between this weak clay and the stronger surrounding rocks appear to have created a natural tear line where strain could concentrate and rupture could propagate efficiently along a narrow surface.
Study co-author Patrick Fulton, an associate professor in the Department of Earth and Atmospheric Sciences at Cornell University, said the geological layering at the Japan Trench effectively predetermines where the fault forms. He described the pelagic clay horizon as an extremely focused and extremely weak surface that makes it easier for ruptures to travel all the way updip to the seafloor. That behavior explains how the Tohoku earthquake generated such extreme shallow slip and helps distinguish this subduction segment from others that may be less prone to similar events.
Because the pelagic clay layer runs for hundreds of miles along the Japan Trench, the researchers conclude that this margin may be inherently more susceptible to shallow-slip earthquakes than many previous assessments suggested. Regalla noted that this kind of structural predisposition has implications far beyond Japan's immediate coastline. Tsunamis generated along the trench can cross the Pacific, affecting distant communities and ports and contributing to the global risk posed by major subduction earthquakes.
The work underscores how earthquakes in one region can drive destructive waves across ocean basins, threatening locations such as Hawaii whose largest historical tsunamis often originate from events in Japan and Alaska. Regalla emphasized that such disasters are truly global in scope because they can disrupt trade, infrastructure and coastal populations far from the source region. Improved understanding of which plate boundaries are configured to foster shallow slip can therefore inform risk assessments for countries around the Pacific Rim and beyond.
The researchers hope that their findings will help scientists better anticipate where large-magnitude earthquakes and tsunami-generating ruptures are most likely to occur. By mapping weak layers and fault-localizing horizons in other subduction zones, geologists may be able to refine scenarios for extreme events and provide more realistic inputs to tsunami models and hazard maps. Regalla stressed that the goal is to move toward forecasting frameworks that identify not only whether a region is seismically active but also which parts of the plate interface are capable of producing the most damaging shallow ruptures.
Policy makers and planners could apply such scientific insights to strengthen building codes, design more earthquake- and tsunami-resilient infrastructure and update evacuation strategies in vulnerable coastal zones. Even in a country like Japan, widely regarded as a world leader in preparing for earthquakes and tsunamis, the scale of the 2011 catastrophe exceeded prior expectations. Regalla said that societies everywhere need to deepen their understanding of where similar events might occur so that emergency plans keep pace with the full range of plausible geologic behavior.
The study, which Regalla co-authored with more than a dozen collaborators from institutions around the world, appeared in the journal Science in December. The paper, titled "Extreme plate boundary localization promotes shallow earthquake slip at the Japan Trench," details how the localized clay-rich fault zone structure helped drive the shallow rupture and documents the mechanical properties of the materials that hosted slip. The authors present their work as a framework for evaluating shallow slip potential in other subduction settings.
Research Report:Extreme plate boundary localization promotes shallow earthquake slip at the Japan Trench
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