An international research expedition, involving scientists from Cornell University, has revealed significant findings regarding the unusual behavior of the 2011 earthquake off the northeastern coast of Japan. This earthquake, which triggered a devastating tsunami and the Fukushima Daiichi nuclear disaster, was influenced by a hidden layer of clay beneath the seafloor. The study, published on December 18, 2023, in the journal Science, highlights how this clay-rich layer contributed to the earthquake’s characteristics, particularly its ability to rupture up to the Japan Trench.
The Japan Trench, a deep ocean boundary where one tectonic plate subducts beneath another, contains a fault zone that narrows into a thin, weak layer of clay. This condition allowed for a significant amount of shallow slip—between 50 to 70 meters—displacing large portions of the seafloor. According to Patrick Fulton, an associate professor of earth and atmospheric sciences at Cornell Engineering, this discovery provides crucial insights into the behavior of subduction zone earthquakes.
“This work helps explain why the 2011 earthquake behaved so differently from what many of our models predicted,” Fulton stated. He emphasized that understanding the fault zone’s structure can lead to better predictions of where seismic slip is likely to occur and the potential tsunami risks associated with various subduction zones.
Typically, in subduction zone earthquakes, rupture begins deep within the fault, with the slip decreasing as it approaches the surface. However, the 2011 earthquake demonstrated a surprising increase in slip as it neared the surface. This unexpected behavior has puzzled geoscientists for over a decade. Fulton served as the co-chief scientist for the International Ocean Discovery Program Expedition 405, known as JTRACK, which conducted drilling operations in 2024 to collect data directly from the fault and sediment on the Pacific Plate.
The JTRACK expedition achieved a remarkable total drill-pipe length of 7,906 meters beneath the sea surface, setting a Guinness World Record for the deepest scientific ocean drilling ever conducted. This accomplishment underscores the advanced capabilities of the drilling vessel, Chikyu, and the collaborative efforts of the Japan Agency for Marine-Earth Science and Technology, industry partners, and the international scientific team. Fulton spent nearly two months aboard the ship, actively participating in the research.
The sediment samples retrieved during this expedition revealed a 30-meter-thick layer of pelagic clay, a soft and slippery material composed of microscopic particles that have settled over millions of years. Surrounded by stronger geological layers, this clay acted as a natural “tear line,” concentrating the rupture along its surface. “At the Japan Trench, the geologic layering basically predetermines where the fault will form,” Fulton explained. “It becomes an extremely focused, extremely weak surface, which makes it easier for ruptures to propagate all the way to the sea floor.”
Fulton noted that the extensive presence of this pelagic clay layer along the Japan Trench could mean that the region is more susceptible to shallow-slip earthquakes than previously thought. “Ultimately, our goal is to translate this kind of detailed fault zone knowledge into better assessments of earthquake and tsunami hazards for coastal communities around the world,” he added.
Accompanying the publication of the study is a 30-minute documentary that chronicles the JTRACK expedition. The film captures the experiences of Fulton and dozens of scientists over 105 days at sea as they planned, drilled, and installed long-term observatories that delve into the fault zone. More data from this expedition will become publicly accessible through the International Ocean Discovery Program, further enhancing the scientific community’s understanding of seismic hazards.
The findings from this research not only enhance our comprehension of the 2011 earthquake but also serve as a critical step towards improving safety measures for vulnerable coastal areas worldwide.
