On March 11, 2011, an earthquake with a magnitude of 9.1 on the moment magnitude scale caused a devastating tsunami along Japan’s Pacific coast. Although Japan had prepared for the possibility of a tsunami by erecting protective seawalls along the coast, the scale of the wave exceeded all expectations. As a result, many questions surrounding the Tohoku-oki earthquake remain unanswered and are the focus of extensive international research efforts. Japan is prone to earthquakes because it lies on a so-called subduction zone, where—simply put—the Pacific Plate is pushed beneath Japan. At the boundary between the Pacific Plate and Japan, friction builds up, hindering the movement of the Pacific Plate. Earthquakes occur when this friction is overcome and the rock masses on both sides of the fault suddenly slip past one another. “We know that the largest earthquakes in the world can occur in subduction zones, but that alone does not automatically mean they will also generate tsunamis of this magnitude,” says Jamie Kirkpatrick, lead author of the study from the University of Nevada, Reno.
Drilling in the Japan Trench
To better understand the geological processes and investigate the cause of the tsunami, an international research team participated in Expedition 405 “Tracking Tsunamigenic Slip Across the Japan Trench” (JTRACK)at the end of 2024. The International Ocean Discovery Program (IODP) is the world’s most significant major research program for scientific deep-sea drilling. The expedition was carried out by JAMSTEC, the Japan Agency for Marine-Earth Science and Technology, using the state-of-the-art research vessel Chikyu. “The drill cores revealed how exceptionally complex the structure of the plate boundary is. Even small differences in the composition can determine how easily or abruptly the rocks move past each other during an earthquake,” says Charlotte Pizer. The geologist is part of the Sedimentary Geology Working group at the Department of Geology, led by Prof. Michael Strasser. Unusual for an earthquake in a subduction zone was that the 2011 quake ruptured the seafloor and displaced it by around 50 meters eastward—more than twice as much as the displacement in deeper areas where the earthquake originated. “It was the largest displacement ever observed during an earthquake and contributed significantly to the enormous size of the tsunami due to its location,” says Kirkpatrick.
The drill cores showed that the tectonic plates at the plate boundary are separated by a thin layer of very weak, slippery clay. When the earthquake reduced friction between the plates, this layer provided little resistance, allowing the plates to slide easily past each other. “This weak clay layer may be crucial for enabling such large displacements at this location. It is an important clue to why the Japan Trench may be particularly susceptible to powerful tsunamis,” adds Pizer. The research results suggest that future large earthquakes in this region could potentially generate similarly massive tsunamis. The findings provide important geological insights that can help more accurately assess the potential magnitude of future tsunamis along the Japanese coast.
Publication:
J. D. Kirkpatrick et al.: Extreme plate boundary localization promotes shallow earthquake slip at the Japan Trench. Science (2025) DOI:10.1126/science.ady0234