Development of Large-scale Rock Friction Apparatuses at NIED, Japan

Development of large-scale rock friction apparatus started around 2010 at National Research Institute for Earth Science and Disaster Resilience (NIED) in Japan (Fukuyama et al., 2014, NIED Rep.). Until now, three generations of the apparatus have already been developed, whose sliding area ranges from 1.5 m to 6.0 m in length. The purpose of this project was to investigate the rock frictional properties and earthquake rupture process at various sliding scales. There are two important characteristics in these apparatuses. One is that the nucleation zone size can be generated within the sliding fault area. The other is that the nucleation process can be spatially monitored by local strain measurement array installed close to the sliding surfaces. When nucleation zone is confined to the experimental fault surfaces, the whole rupture process from initiation to termination could be able to be observed in the experiments (Yamashita et al., 2026, EGU). Using these apparatuses, many kinds of large-scale rock friction experiments have been conducted. Through such experimental research, the following results have been reported. 1) The spatial distribution of strength on the fault surface is heterogeneous, which had not been properly considered by small-scale experiments (Yamashita et al., 2015, Nature). 2) Such spatial heterogeneity of strength on the fault surface could contribute to the fault-size dependence of macroscopic rock friction as well as the rich spectrum of rupture behaviors, which are quite important for the modeling of earthquake generation process (Xu et al., 2023, Nat. Geosci.). Especially, due to high-speed loading (~1 mm/s) and long-distance sliding (~40 cm), ten-decimeter-scale heterogeneity on the fault surface could be generated, which is found to play an important role in large-scale friction experiments (Yamashita et al., 2018, Tectonophys.). In addition to these spatially heterogeneous fault friction experiments, rupture propagation has been investigated in detail to investigate the fracture energy (Okubo et al., 2026, EGU; Matsumoto et al., 2026, EGU). In the near future, we expect that such large-scale rock friction experiments would contribute significantly to seismology, especially, physics of earthquake generation process, by establishing new constitutive law(s) of the rock friction through the usage of dense arrays of near-fault observations.