Sample details from the Nankai Trough Ashizuri transect

Despite the availability of drilling core samples from scientific expeditions, published friction data from the Nankai Trough Ashizuri transect are limited compared to those from the Kumano and Muroto counterparts. This dataset presents laboratory friction results from samples obtained along the Ashizuri transect, elucidating their potential implications for unstable slip behavior. Nine representative sediment samples from two sites (297 and 1177), drilled on the pre-subduction incoming sediments, were investigated. We conducted single direct-shear experiments under an effective normal stress of 10 MPa, employing velocity-stepping procedures with driving velocities ranging from the Nankai Trough plate rate of ~0.002 µm/s (~6 cm/yr) to 30 µm/s on both intact and powder samples. The measured coefficient of friction (μ) showed two distinct groups: μ<0.3 for clayey samples rich in phyllosilicates (55-65%) and μ>0.5 for sandy samples low in phyllosilicates (10-20%). At slow velocities comparable to the plate rate, the velocity-step test revealed predominantly velocity-weakening frictional behavior for Site 297 samples, whereas Site 1177 samples were more velocity-strengthening. Additionally, Stick-slip cycles were observed in sample 1177-46R-01 at driving velocities of 0.1, 0.3, 1, 3, and 10 μm/s. Over this velocity range, the stress drop correlates negatively with the recurrence interval and positively correlates with the average slip velocity. The dataset provides evidence of unstable slip in frictionally weak, clay-rich samples, a condition necessary for earthquake slip. Such materials are expected to slide stably based on previous work.