Scientists have drilled into the deepest ocean sediment ever recovered, from the Japan Trench, seven kilometres below sea level. The expedition, part of the International Ocean Discovery Program (IODP) Expedition 405, also known as JTRACK, took place from September to December 2024 aboard the drilling ship Chikyu.
The team of 60 scientists, including sedimentologists, geochemists and micropaleontologists, drilled more than 800 metres beneath the seafloor into the fault zone responsible for the devastating 2011 Tōhoku mega-earthquake. This magnitude 9.1 quake triggered a catastrophic tsunami that killed over 18,000 people and caused an estimated US$235 billion in damages.
The researchers successfully sampled the décollement, or basal detachment, of the fault that ruptured in 2011. They recovered cores containing smectite, a slippery clay mineral that reduces friction, allowing tectonic plates to slip easily at shallow depths—conditions that can generate tsunami-producing earthquakes.
One of the most significant findings was reaching layers of chert, a hard glassy rock marking the transition from deep-sea sediments to oceanic crust, at the plate boundary. The cores also revealed beautifully banded clays in shades of chocolate, vanilla and caramel, formed by geological processes deep within the Earth.
The team installed a long-term observatory to monitor temperature and fluid pressure at the earthquake's source. The cores will help scientists better understand how such powerful tsunamigenic earthquakes are triggered, providing crucial insights for future hazard assessment.
