Explore the megathrust earthquakes in the Japan trench by deep ocean drilling and laboratory friction experiments

The Mw 9.0, 2011 Tohoku-oki earthquake in the Japan Trench caused severe damages to coastal areas in the NE Japan by strong ground motion and huge tsunamis that killed 20,000 people. Seismological observations suggest that the massive disaster was the consequence of huge seismic slip of approximately 50 m on the plate boundary fault (décollement). In 2012, the IODP (Integrated Ocean Drilling Program Expedition / International Ocean Discovery Program) Expedition 343 “JFAST” was carried out to explore why such a huge slip had happened by directly drilling into the décollement, which uncovered that the presence of mechanically weak smectite layer and thermal pressurization along the décollement played critical roles to the fault slip (Chester et al., 2013, Science; Ujiie et al., 2013, Science). 

In 2024, we revisited the drill site during the IODP Expedition 405 “JTRACK” and drilled and collected samples from the seafloor all the way to the subducting Pacific plate basement across the décollement to understand entire geological features, stress accumulation processes, and historical seismic events in the Japan Trench. Especially, several boreholes were drilled within 100 m of spatial distance where we found that the depth of the décollement is spatially heterogeneous by 15 m and that materials above/below the décollement are also heterogeneous (Kirkpatrick et al., 2026, Science). 

Nevertheless, the coseismic slip during the 2011 earthquake overcame such complexities of the décollement. To further elaborate why the massive slip was able to occur, we are now performing laboratory friction experiments on the real fault zone material drilled from the décollement to simulate the coseismic slip and understand its mechanical response. Using a novel apparatus capable of high velocities under high fluid pressure conditions (Okuda et al., 2026, EGU), we confirmed that the smectite-rich décollement material has a less frictional resilience than the overlying prism sediment. We will soon start examining mechano-chemical reactions during the experiment using microstructural, chemical, and magnetic analyses to interpret the fault slip behavior more in detail from the mechano-chemical signals in the actual fault zone in the Japan Trench. Such a combination between laboratories and natural fault zones will be a strong tool to untangle deformation history recorded in geological materials.