[S-SS02] Frontier studies on subduction zone megathrust earthquakes and tsunamis

[SSS02-17] High-velocity frictional strength across the Tohoku-Oki megathrust determined from surface drilling torque

*氏家 恒太郎^1,2、井上 朝哉³、石渡 隼也³ (1.筑波大学生命環境系、2.海洋研究開発機構海洋掘削科学研究開発センター、3.海洋研究開発機構地球深部探査センター)

キーワード：frictional strength, drilling torque, Japan Trench Fast Drilling Project

High-velocity frictional strength is one of the primary factors controlling earthquake faulting. The Japan Trench Fast Drilling Project drilled through the shallow plate-boundary fault, where displacement was ~50 m during the 2011 Tohoku-Oki earthquake. To determine downhole frictional strength, we analyzed the surface drilling torque data acquired at rotation rates equivalent to seismic slip rates (0.8–1.3 m/s). The results show a clear contrast in high-velocity frictional strength across the plate-boundary fault: the apparent friction coefficient of frontal prism sediments (hemipelagic mudstones) in the hanging wall is 0.1–0.3, while that of the underthrust sediments (mudstone, laminar pelagic claystone, and chert) in the footwall increases to 0.2–0.4. The apparent friction coefficient of the smectite-rich pelagic clay in the plate-boundary fault is 0.08–0.19, which is consistent with that determined from high-velocity (1.1–1.3 m/s) friction experiments. This suggests that surface drilling torque is useful in obtaining downhole frictional strength.