The 2011 Tohoku-oki earthquake and tsunami killed nearly 20,000 people and caused more than $200 billion in damage. The earthquake was a magnitude 9.1, far larger than Japanese seismic hazard models had predicted for the Japan Trench, and the tsunami it generated was devastating because the seafloor rupture propagated all the way to the surface.
For 14 years, seismologists have asked why. A new study, published in Science and based on the deepest scientific ocean drilling expedition ever attempted, provides the answer: a 100-foot-thick layer of soft, slippery pelagic clay that acted as a natural tear line, concentrating the rupture along a narrow path and allowing it to reach the seafloor.
The Expedition
In 2024, an international team of more than 60 scientists aboard the drilling vessel D/V Chikyu, operated by JAMSTEC (the Japan Agency for Marine-Earth Science and Technology), set out to drill into the Japan Trench fault zone at a depth that had never been attempted before. The expedition, IODP Expedition 405 (JTRACK), deployed a total drill-pipe length of 7,906 meters beneath the sea surface, a record recognized by Guinness World Records as the deepest scientific ocean drilling project in history.
The target was the shallow portion of the megathrust fault, the plate boundary where the Pacific Plate dives beneath the Okhotsk Plate. This is the region that slipped catastrophically on March 11, 2011, generating the tsunami.
The Clay Layer
What the drilling revealed was unexpected. Sandwiched between much stronger rock layers was a deposit of pelagic clay approximately 30 meters (100 feet) thick, formed over millions of years from microscopic particles settling through the water column. The clay is extremely soft and slippery, mechanically, it has almost no strength.
When the earthquake struck, this clay layer provided an exceptionally weak surface for the fault rupture to travel along. Rather than stopping deeper in the crust (most subduction zone earthquakes rupture at depths of around 50 kilometers, or about 32 miles), the Tohoku rupture followed the clay layer upward along a focused path, reaching the seafloor at a depth of only about 15 miles.
“The fault rupture plane was just a centimeter or so thick within this clay layer,” said Christine Regalla, associate professor at Northern Arizona University and a co-chief scientist on the expedition, in comments to the university’s press office. The clay “is an extremely focused, extremely weak surface that predetermines where the fault will form.”
Six Minutes of Displacement
The result was seafloor displacement of 40 to 60 meters (130 to 200 feet) in just six minutes. For comparison, the 2001 Nisqually earthquake in Washington displaced its fault by only a few meters. The Tohoku earthquake shifted the entire seafloor over an area the size of the Los Angeles to San Francisco corridor by that distance in the time it takes to boil an egg.
“That slip of 50 to 70 meters on the shallowest portion of the megathrust is what generated the devastating tsunami,” the authors write in the Science paper, whose lead author is J. D. Kirkpatrick (University of Illinois Urbana-Champaign).
The implication is sobering. Pelagic clay layers are common in subduction zones around the Pacific Rim. Wherever they are present, they may create the potential for shallow, focused rupture propagation, and the tsunamis that follow.
Forward Warning
The discovery does not mean every subduction zone with pelagic clay will produce a magnitude 9 earthquake. The stress conditions, plate convergence rate, and thermal structure all contribute. But it does provide a geological marker that can help assess tsunami risk: if a thick pelagic clay layer is present in the shallow portion of a subduction zone, the potential for a shallow, tsunamigenic rupture is higher than previously assumed.
Regalla and her colleagues are now advocating for expanded drilling campaigns at other subduction zones, including the Cascadia subduction zone off the coast of the Pacific Northwest, where a similar clay layer may exist. The 2011 Tohoku earthquake was a surprise because the models did not include the clay. The next one should not be.
Disclosure: Based on a peer-reviewed paper in Science, December 2025 (online), January 2026 (print). DOI: 10.1126/science.ady0234. Title: “Extreme plate boundary localization promotes shallow earthquake slip at the Japan Trench.” Lead author J. D. Kirkpatrick. First author Christine Regalla (NAU) is the NAU-affiliated co-chief scientist.