Friction experiments under in-situ stress reveal unexpected velocity-weakening in Nankai accretionary prism samples

Spatial Patterns in Frictional Behavior of Sediments Along the Kumano Transect in the Nankai Trough

“…Hence, differences in velocity dependence may influence the spatial variation in fault slip activities beneath the outer prism and inner prism. Although previous studies have often reported velocity-strengthening behavior for clay-rich gouges from the Nankai Trough , the velocity-weakening behavior in this study is consistent with the results of Roesner et al (2020) who reported that intact samples tested under in situ effective normal stresses showed the velocity weakening, but their equivalent powdered samples under larger effective normal stresses showed velocity strengthening. Because we used intact samples, the observed velocity-weakening behavior could be related to the lithification state of the samples.…”

“…Our results on input sediments do not show such a transition (Figure 4a), suggesting that such a mechanism for limiting slip velocity is absent for the décollement beneath the outer prism, which is therefore more unstable and may generate faster events. We speculate that the difference in a‐b values between the two locations might possibly originate from grain interactions during shear deformation (den Hartog & Spiers, 2013; Roesner et al., 2020) although we cannot verify this hypothesis without microstructural information for the sample C0002T‐2K‐1. Because we conducted our tests at room temperature, depth‐dependent temperature effects could induce some uncertainty in our calculations as well.…”

“…To reveal the fault mechanics and explain fault slip activities at the Nankai Trough subduction zone system, the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) project of the Integrated Ocean Drilling Program/International Ocean Discovery Program (IODP) drilled over 10 sites to measure physical properties and to collect sediment samples below seafloor along the Kumano transect offshore Kii Peninsula, Japan (Figure 1). Because friction directly controls the slip behavior of a fault (Marone, 1998;Scholz, 2019), the frictional properties of recovered prism and input sediments have been studied through laboratory experiments (Ikari et al, 2009(Ikari et al, , 2013(Ikari et al, , 2018Ikari & Hüpers, 2019;Roesner et al, 2020;Takahashi et al, 2013Takahashi et al, , 2014Tsutsumi et al, 2011;Ujiie & Tsutsumi, 2010). Frictional properties are mostly controlled by lithological variations of sediments (Ikari et al, 2013(Ikari et al, , 2018; Tobin et al, 2019), showing the three drilling sites from which our samples in this study were retrieved.…”

The stabilizing effect of high pore-fluid pressure along subduction megathrust faults: Evidence from friction experiments on accretionary sediments from the Nankai Trough

“…Although the gouge material tested in this study exhibited exclusively rate-strengthening behaviour, previous investigations have shown that rate-weakening material can also be found within the Nankai Trough (e.g. Roesner et al, 2020), suggesting there is a heterogeneous distribution of material properties within the subduction zone.…”

“…Based on the frictional stability, the material in the study would be expected to experience stable aseismic creep, whereas the material tested by Roesner et al, (2020) could experience unstable stickslip behaviour, depending on the elastic stiffness of the surrounding materials (Leeman et al, 2016).…”