Synthesis of the Laboratory Frictional Properties of a Major Shallow Subduction Zone: The Nankai Trough, Offshore SW Japan

Abstract The Nankai Trough has a long history of ≥ M w 8 earthquakes and associated tsunamis. It has been the focus of decades of research, including drilling, providing the opportunity to measure the laboratory frictional properties of shallow accretionary prism samples. We collate all available friction data from the Nankai Trough and correlate it with clay abundance determined from consistent, routine IODP analyses. We find low‐velocity frictional resistance is generally lower than typical “Byerlee” friction, and that this strength is inversely correlated to clay abundance. Frictional stability ( a–b ) varies from velocity strengthening to velocity weakening and shows no clear relationship with clay abundance. There is also no relation of ( a–b ) with smectite abundance. Samples tested at high slip velocity (>0.1 m/s) show strong weakening to low values of shear resistance ( μ ∼ 0.1). While the compilation shows inherent variability, individual studies of friction show a number of key systematic variations with mineralogy, temperature, pore‐fluid pressure. The experimental friction data are consistent with the range of observed behavior in the Nankai Trough. The data demonstrate heterogeneity in frictional properties within a natural setting and constrain the frictional character of the shallow part of this major subduction zone.