Synthesis of the laboratory frictional properties of a major shallow subduction zone: the Nankai Trough, offshore SW Japan

The Nankai Trough subduction zone, located off the southern coast of southwestern Japan, has a well-documented history of large Mw > 8 earthquakes and significant tsunamis (e.g., Ando, 1975; Garrett et al., 2016). This region has been the focus of extensive research, including numerous scientific ocean drilling expeditions conducted through the Deep Sea Drilling Project (DSDP), the Ocean Drilling Program (ODP), the Integrated Ocean Drilling Program (IODP), and the International Ocean Discovery Program (IODP).

In this study, we compile all available friction data and shipboard routine X-ray diffraction (XRD) analyses from across the Nankai Trough. Our findings reveal that while individual friction studies show systematic variations related to mineralogy (e.g., Ikari et al., 2018), temperature (e.g., den Hartog et al., 2012), and pore-fluid pressure (e.g., Bedford et al., 2021), only the correlation between friction and clay mineral content is consistently observed across the entire dataset. Specifically, the friction coefficient measured over velocity scales from nanometers per second to millimeters per second generally remains below 0.6, which is lower than the typical ‘Byerlee’ friction value of 0.85 under normal stress conditions below 200 MPa (Byerlee, 1978), and exhibits an inverse correlation with clay mineral content. The rate-and-state friction parameter (a-b) varies significantly between -0.01 and 0.02, showing no clear relationship with clay mineral content. This lack of correlation is likely due to the diverse experimental conditions across different studies. However, it is notable that velocity-weakening behavior becomes less frequent at the higher end of this velocity scale (>10 μm/s), which may help explain the widespread occurrence of slow slip events in the Nankai Trough. In contrast, samples tested at higher velocity scales (centimeters per second to meters per second) display pronounced frictional weakening, with the friction coefficient dropping to very low values (~0.1) once slip velocities exceed 0.1 m/s.

The wide variation in experimental friction data reflects the complex and heterogeneous frictional properties of the Nankai Trough and aligns with the diverse seismic behaviors observed in the region. The dataset compiled in this study serves as a robust basis for constraining the frictional characteristics of the shallow portion of the Nankai Trough subduction zone.