Strength characteristics of Japan Trench borehole samples in the high-slip region of the 2011 Tohoku-Oki earthquake

• We perform shearing experiments on borehole samples from the Tohoku earthquake zone.
• The fault zone is weak and exhibits partially velocity weakening slip behavior.
• Large contrast in smectite content and strength between the fault zone and wall rock.
• The fault is cohesionless, reducing fracture energy and favoring earthquake rupture.

The 2011 Tohoku-Oki earthquake demonstrated that the shallowest reaches of plate boundary subduction megathrusts can host substantial coseismic slip that generates large and destructive tsunamis, contrary to the common assumption that the frictional properties of unconsolidated clay-rich sediments at depths less than ∼5km should inhibit rupture. We report on laboratory shearing experiments at low sliding velocities (<1mm/s) using borehole samples recovered during IODP Expedition 343 (JFAST), spanning the plate-boundary décollement within the region of large coseismic slip during the Tohoku earthquake. We show that at sub-seismic slip rates the fault is weak (sliding friction μs=0.2–0.26μs=0.2–0.26), in contrast to the much stronger wall rocks (μs>∼0.5μs>∼0.5). The fault is weak due to elevated smectite clay content and is frictionally similar to a pelagic clay layer of similar composition. The higher cohesion of intact wall rock samples coupled with their higher amorphous silica content suggests that the wall rock is stronger due to diagenetic cementation and low clay content. Our measurements also show that the strongly developed in-situ fabric in the fault zone does not contribute to its frictional weakness, but does lead to a near-cohesionless fault zone, which may facilitate rupture propagation by reducing shear strength and surface energy at the tip of the rupture front. We suggest that the shallow rupture and large coseismic slip during the 2011 Tohoku earthquake was facilitated by a weak and cohesionless fault combined with strong wall rocks that drive localized deformation within a narrow zone.