Low-speed shear experiment on solid-liquid two-phase flow system: Hysteresis of shear stress

In recent years, slow slip events (SSEs) have occured in subduction zones across the world, and become a very interesting subject of research as a universal physical phenomenon. The spatio-temporal relationships between SSEs and triggering of regular earthquakes, such as dynamic and static stress change, have been elucidated clearly in an observational study, which provided insight into the contribution of fluids to the process of SSE generation (e.g., Hirose et al., 2021). To understand the fault mechanics of subduction zones accurately, it is necessary to consider the rheological properties caused by the collective behavior of granular materials, such as fault gouges. It is generally known well that the shear stress in a granular layer is not uniform from a microscopic point of view and is accompanied by a local torque. In contrast, very few rheological studies have been conducted from a macroscopic point of view, especially those that investigate how the physical properties averaged over the entire granular layer behave in response to the shear rate. In addition, few examples consider the effect of water in the granular layer. To better understand the fault mechanics within a fluid-rich granular layer in a subduction zone, it is essential to understand the rheological properties of the solid-liquid two-phase flow system. Here we investigate the rheological behavior of the solid-liquid two-phase flow system under low-speed shear, in which we use the rheometer AR2000 (e.g., Taylor and Brodsky, 2019). Quartz grains and water are mainly used as solid and liquid samples, respectively. Shear stress measurements from these two-phase samples showed that they exhibit pseudoplastic behavior, with increasing shear rate at low velocities. We also observed a strong hysteresis in the measured shear stress, as changing shear rate. Additionally, the friction coefficients measured simultaneously showed that there was a characteristic hysteresis under both dry and water-saturated conditions. In this presentation, we will also report the results of visualization experiments using stress emitters and describe the relationship between the stress inhomogeneity observed in the plane and the hysteresis mentioned above.