🌊 A record drilling reveals very bad news for Japan

Why did the 2011 earthquake in Japan cause such a massive tsunami?

This disaster, one of the deadliest in modern history, had surprised scientists by the way the fault ruptured much closer to the surface than expected. Faced with this observation, researchers therefore examined the particular geological conditions of the Japan Trench.

To solve this mystery, an international team conducted an exceptional ocean expedition. Aboard the ship Chikyu, scientists drilled to a record depth in the Pacific seafloor, collecting samples. This coring campaign, recognized by the Guinness Book of Records, made it possible to directly analyze the geological layers at the very spot where the oceanic plate dives beneath Japan.


The analyses revealed the presence of a band of pelagic clay, about a hundred meters (approximately 328 feet) thick, trapped just below the ocean floor. This layer, formed of very fine particles accumulated over millions of years, has a soft and slippery consistency. Unlike the more resistant rocks that frame it, it offers a surface of low resistance, like a plane of weakness naturally integrated into the geological structure.

This discovery explains how the seismic rupture was able to propagate so close to the surface. During the 2011 earthquake, tectonic forces concentrated along this clay layer, guiding the slip. The movement thus reached the seafloor, lifting it by forty to sixty meters (131 to 197 feet) over a vast area. Such a vertical displacement in shallow water constitutes a direct mechanism for generating a large-amplitude tsunami wave.

This behavior differs from many other large earthquakes, which typically rupture at depths of tens of kilometers (several miles). For example, the Nisqually earthquake that occurred in 2001 in the Pacific Northwest began more than fifty kilometers (about 31 miles) below the surface.

Understanding this phenomenon will also allow for a reassessment of risks in other subduction zones around the world. By identifying the presence of similar layers of pelagic clay, geologists could better anticipate locations where shallow ruptures are possible.

The team's work is published in the journal Science.