In 2025, the International Ocean Discovery Program's Expedition 405 drilled beneath the Japan Trench, focusing on understanding the fault zone involved in the 2011 Tōhoku earthquake to improve earthquake and tsunami hazard assessments.

The drilling efforts uncovered that the largest slip during the earthquake occurred just beneath the seafloor, challenging previous assumptions that slip along subduction zones was slow and gradual, emphasizing the significance of shallow fault zones in tsunami generation.

The Japan Trench Fast Drilling Project (JFAST), which drilled 850 meters below the seafloor after the 2011 earthquake, found that the fault was extremely thin and filled with slippery clay, enabling rapid rupture and amplifying tsunami effects.

The expedition, conducted from September to December 2024 aboard the drilling ship Chikyu, collected over 800 meters of deep-sea sediment cores from the fault responsible for the 2011 quake, marking only the second deep-drilling mission in this area.

Analysis of these cores revealed layers of sediments and rocks, including slippery clay like smectite, which can reduce friction and facilitate fault slipping, and chert layers indicating the transition to oceanic crust, providing vital insights into fault structure.

In 2025, the JTRACK expedition drilled over 800 meters beneath the trench to analyze sediments recording past earthquakes and tsunamis, studying fault slip conditions and water flow that influence earthquake behavior.

These scientific efforts aim to understand the conditions that lead to mega-earthquakes and tsunamis, with the goal of improving hazard assessments and developing early warning systems to better protect coastal regions.

Discoveries from these studies challenge previous models by showing that weak zones in the Earth can produce large, shallow ruptures, significantly increasing tsunami risk.

Researchers also examine geological signatures of past seismic events, such as turbidite sequences, to estimate the frequency of large earthquakes and tsunamis over millions of years.

The insights gained from this research are intended to enhance tsunami hazard assessments and deepen understanding of megathrust fault behavior, contributing to improved earthquake prediction and risk mitigation worldwide.

Since the 2011 quake, the Chikyu has monitored fault evolution through an advanced borehole observatory, observing real-time changes in temperature and fluid flow to better understand fault dynamics and simulate earthquake conditions.

The findings from the Japan Trench studies have global implications, highlighting the need to update earthquake preparedness and hazard models, especially for densely populated coastal areas vulnerable to shallow fault slips.