The key to the earthquake and tsunami that occurred in northeast Japan in 2011 may not only be its magnitude but also “where” and “how” the rupture occurred.
A scientific campaign in the Japan Trench has obtained core samples and records at the greatest depth ever from the plate boundary (7,906 metres). The findings show that at the center of the earthquake’s extreme behavior was an extremely weak, thin clay layer that facilitated the rupture reaching the shallowest part of the fault and pushing the seafloor with devastating effectiveness.
The study, published in the journal Science, was carried out on the drilling ship Chikyu in 2024, within the scope of the 405th Expedition of the International Ocean Drilling Program (JAMSTEC).
According to the research, megafaults “prefer” to develop at the top or bottom of a pelagic clay layer, that is, in the region where the physical properties between sediments and rocks change suddenly. This positioning creates a narrow, low-friction fracture surface; It creates a “rupture plane” that can intensify slip in the shallowest part of the contact between the plates.
The result of this is known, but it was difficult to explain for many years. The Tōhoku Earthquake (Mw 9.1) was one of the largest earthquakes measured instrumentally. However, what surprised many models was the amount of displacement observed near the pit; There was an estimated peak slip of between 50 and 70 meters in the shallowest part of the fault. It was this “push” effect that raised the seafloor within seconds, multiplying the tsunami-generating potential of the event. This mechanism led to giant waves, a major humanitarian crisis, and the Fukushima Daiichi nuclear power plant accident.
7 THOUSAND 906 METERS DEPTH
The research also has an important technical dimension in terms of geosciences. On September 21, 2024, the project broke the world record for the “deepest scientific ocean drilling” by reaching a pipe length of 7,906 meters in drilling and recording operations and was registered by Guinness. This achievement allows for more direct sampling and measurement of the region that determines whether a major rupture will stay deep or “rise” all the way to the ocean edge, as it did in 2011.
The researchers describe the clay layer in question as an “extraordinarily weak” material. This layer consists of extremely fine sediments accumulated over millions of years in the Pacific and then dragged under Japan as the oceanic plate advanced.
The clay is “sandwiched” between the stronger layers, increasing mechanical contrast. According to ANU’s Ron Hackney, when the tension built up over centuries was released, this structure allowed the rupture to propagate into the shallowest part of the fault with little resistance; Thus, the deformation of the sea floor and the size of the tsunami increased.
The results support a growing view in subduction zone seismology: Not all tectonic margins are the same. The presence or absence of particularly weak materials in areas close to the trench can make the difference between a “conventional” large earthquake and one that can produce extreme seafloor rise.
The study addresses this within the framework of a comparative diagnosis. Coastal hazard assessments could be made more precise if similar sediments were detected in other subduction zones, such as around Sumatra. However, the authors emphasize that new drillings are needed to confirm this.
The research is accompanied by a documentary about the work on the Chikyu ship and the difficulties of operating in one of the deepest pits on the planet. The background question is strategic for a country living with seismic risk: Japan cannot prevent earthquakes, but it can better determine which faults have the capacity to “rupture to the edge” and produce disproportionate tsunamis. This drilling reveals that sometimes the decisive difference may be hidden in just a few meters of clay, a structure that acts like the geological hinge of a catastrophe.
