A Thin Clay Layer May Explain How This Earthquake Turned So Catastrophic

In March 2011, a magnitude 9.1 earthquake struck off the coast of Japan, triggering a tsunami that killed nearly 20,000 people and caused more than $200 billion in damage. Scientists have long understood the scale of the catastrophe. What puzzled researchers was how the rupture propagated so near to the ocean floor, amplifying the tsunami’s destructive power.

Now, after drilling 26,000 feet into the Pacific seafloor at the Japan Trench, researchers say they have identified the geological feature that made this extreme shallow slip possible. According to the study published December 18, 2025 in Science, the megathrust fault preferentially developed within a thin pelagic clay layer marked by sharp contrasts in physical properties.

A Fault Guided by an Ultra-weak Clay Layer

The drilling expedition, conducted aboard the research vessel Chikyu, set a Guinness World Record for the deepest scientific ocean drilling ever achieved. Core samples revealed a roughly 100-foot-thick layer of pelagic clay beneath the seafloor.

According to the Science paper, drill cores and geophysical logs from International Ocean Discovery Program Expedition 405 demonstrate that the megathrust developed at the top or base of this clay layer, where it meets silica-rich mud or chert. The contrast in mechanical properties created a narrow, weak fault zone.

At the Japan Trench, this layering effectively predetermined the rupture path. Study co-author Patrick Fulton explained, “It becomes an extremely focused, extremely weak surface, which makes it easier for ruptures to propagate all the way to the seafloor.” Because the clay layer is sandwiched between stronger rock units, it acted like a structural guide. The fault zone itself was just about a centimeter thick, yet it enabled extraordinary movement.

Why the 2011 Rupture Defied Expectations

Most large earthquakes originate deep below the surface, where tectonic plates fracture at significant depths. For comparison, the 6.8-magnitude Nisqually earthquake in 2001 began roughly 32 miles beneath the seafloor.

The 2011 Tōhoku event was different. According to reporting on the expedition, the rupture occurred at a depth of about 15 miles, much closer to the seabed. The largest slip reached between 50 and 70 meters, or roughly 164 to 230 feet, along the shallowest portion of the plate boundary.

Associate Professor Ron Hackney noted, “The rupture plane was just a centimetre [0.4 inches] or so thick, yet it allowed between 50 and 70 meters [164 and 230 feet] of movement on the fault and caused the seafloor off Japan to rise abruptly by several metres during the earthquake.”

Christine Regalla emphasized the scale of displacement in stark terms: “That’s equivalent to the entire area between Los Angeles and San Francisco moving 130 to 200 feet in just six minutes. 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.”

According to the editor’s summary in Science, the 2011 earthquake was exceptional among great earthquakes for having peak slip of approximately 50 to 70 meters on the shallowest portion of the megathrust.

Global Implications for Other Subduction Zones

The pelagic clay identified beneath the Japan Trench formed from microscopic particles that settled on the seafloor over millions of years. Hackney described the process: “This clay-rich ancient mud formed from microscopic particles that slowly settled on the seafloor beneath the Pacific Ocean over time – a process that took place over 130 million years – as the Pacific tectonic plate slowly moved west to ultimately be forced under Japan.”

Because this weak clay layer extends for hundreds of miles along the trench, researchers suggest the region may be more susceptible to shallow-slip earthquakes than previously recognized. According to the Science abstract, the presence of pronounced contrasts in physical properties enhances the tendency for shallow seismic slip and may make the Japan Trench more prone to ruptures with large shallow slip than margins without weak clays.

Hackney also pointed to possible parallels elsewhere, noting there are indications that sediments being drawn toward and under Sumatra may contain a similar weak clay layer, though confirmation would require direct sampling.

For Regalla, the stakes extend well beyond Japan. “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,” she said. “Think about Hawaii: Their most devastating tsunamis come from Japan and Alaska. These are truly global events.”