4:15 PM - 4:30 PM
[SCG45-41] Fault Transition and Short Slip Displacement Induced by Two-Surface Shear Friction Tests
Keywords:Earthquake, Slow Earthquake, Friction experiment
In nature, earthquake faults develop secondary faults that are separate from the main fault on a regional scale, and multiple shear planes (weak planes) develop locally in the fault zone. Therefore, the interaction between the main fault and nearby faults and weak planes cannot be ignored. However, due to physical and technical limitations in the laboratory-scale rock friction experiments, only simplified single-plane shear friction experiments have been carried out to evaluate the earthquake fault behaviors. Therefore, in this study, two parallel faults were prepared to apply the same shear stress simultaneously in order to evaluate the possibility that multiple faults in close proximity affect each other's fault slip, the slip behavior of each fault were investigated.
A rotary shear friction test apparatus was used for the experiments. Three cylindrical titanium simulated rocks (25 mm in diameter) were placed vertically, and a simulated granular fault material (approximately 2 g), was placed between the simulated rocks (Figure 1a). By applying a vertical load from one simulated rock and rotating the other simulated rock, the two fault materials sandwiched between the simulated rocks were sheared simultaneously. Two sets of absolute encoders were placed near the two fault planes to measure the amount of slip displacement (rotation) of each simulated fault. Experiments were conducted with a vertical stress of 4 MPa and either constant slip velocity control (0.18 mm/s) or increased torque control (maximum shear stress of 3.7 MPa). The constant-slip-velocity control is a control in which the velocity is constant with respect to the combined slip of the two faults. Hot spring deposits from the Blood Pond Hell in Oita Prefecture (mainly kaolinite), sandstone (Triassic) from the Kouchigatani Formation in Kochi Prefecture (mainly quartz, feldspar, and illite), beach sand (mainly quartz and feldspar) from the Chirihama Beach in Ishikawa Prefecture, and commercially available quartz sand were used as simulants. Sandstone crushed to less than 1 mm was used in the experiments. Hot spring deposits, sandstone, and beach sand were used for experiments in which different simulated materials were placed on the two fault planes, and quartz sand was used for experiments in which the same materials were placed on the two fault planes.
In the experiment in which the hot spring deposits and sandstone were sheared simultaneously, both faults began to slip as soon as the shear stress was applied. And the hot spring deposits began to slip faster than the sandstone. However, after 2 mm of slip displacement, the sliding speed of the sandstone accelerated, whereas the hot spring deposit slowed down and hardly slid at all (Figure 1b). In friction tests using hot spring deposits and beach sand, the two faults started to slip at almost the same speed. However, the sliding speed of the beach sand increased and the sliding was accelerated, whereas that of the hot spring deposit slowed down and the beach sand hardly slid at all. The two-plane shear experiments on hot spring deposit and sandstone indicate that in fault zones with multiple weak planes (secondary faults) of different faulting materials, the fault transitions to another fault during the slip, and the originally-sliding fault comes to rest with a very short slip displacement.
In the single-plane shear test experiment, the hot spring deposit showed higher peak friction and greater displacement to reach peak friction compared to the sandstone. On the other hand, the increase in shear stress relative to the amount of slip displacement was almost the same for beach sand and hot spring deposit, but the amount of slip displacement until peak friction was reached was smaller for beach sand than for hot spring deposit. Through a series of experiments, we found that the slip-shear stress behavior of the two-plane shear friction experiments can be explained by combining the results of one-plane shear experiments for each fault material.
The experimental results indicate that when multiple fault planes exist in a fault zone and the mineral components of the fault are different, the slip behavior (change in shear strength) at each fault is different, and various slip velocities and displacements appear due to their interaction. In the future, we would like to unravel this interaction experimentally to understand the relationship between slow earthquakes and fast earthquakes.
A rotary shear friction test apparatus was used for the experiments. Three cylindrical titanium simulated rocks (25 mm in diameter) were placed vertically, and a simulated granular fault material (approximately 2 g), was placed between the simulated rocks (Figure 1a). By applying a vertical load from one simulated rock and rotating the other simulated rock, the two fault materials sandwiched between the simulated rocks were sheared simultaneously. Two sets of absolute encoders were placed near the two fault planes to measure the amount of slip displacement (rotation) of each simulated fault. Experiments were conducted with a vertical stress of 4 MPa and either constant slip velocity control (0.18 mm/s) or increased torque control (maximum shear stress of 3.7 MPa). The constant-slip-velocity control is a control in which the velocity is constant with respect to the combined slip of the two faults. Hot spring deposits from the Blood Pond Hell in Oita Prefecture (mainly kaolinite), sandstone (Triassic) from the Kouchigatani Formation in Kochi Prefecture (mainly quartz, feldspar, and illite), beach sand (mainly quartz and feldspar) from the Chirihama Beach in Ishikawa Prefecture, and commercially available quartz sand were used as simulants. Sandstone crushed to less than 1 mm was used in the experiments. Hot spring deposits, sandstone, and beach sand were used for experiments in which different simulated materials were placed on the two fault planes, and quartz sand was used for experiments in which the same materials were placed on the two fault planes.
In the experiment in which the hot spring deposits and sandstone were sheared simultaneously, both faults began to slip as soon as the shear stress was applied. And the hot spring deposits began to slip faster than the sandstone. However, after 2 mm of slip displacement, the sliding speed of the sandstone accelerated, whereas the hot spring deposit slowed down and hardly slid at all (Figure 1b). In friction tests using hot spring deposits and beach sand, the two faults started to slip at almost the same speed. However, the sliding speed of the beach sand increased and the sliding was accelerated, whereas that of the hot spring deposit slowed down and the beach sand hardly slid at all. The two-plane shear experiments on hot spring deposit and sandstone indicate that in fault zones with multiple weak planes (secondary faults) of different faulting materials, the fault transitions to another fault during the slip, and the originally-sliding fault comes to rest with a very short slip displacement.
In the single-plane shear test experiment, the hot spring deposit showed higher peak friction and greater displacement to reach peak friction compared to the sandstone. On the other hand, the increase in shear stress relative to the amount of slip displacement was almost the same for beach sand and hot spring deposit, but the amount of slip displacement until peak friction was reached was smaller for beach sand than for hot spring deposit. Through a series of experiments, we found that the slip-shear stress behavior of the two-plane shear friction experiments can be explained by combining the results of one-plane shear experiments for each fault material.
The experimental results indicate that when multiple fault planes exist in a fault zone and the mineral components of the fault are different, the slip behavior (change in shear strength) at each fault is different, and various slip velocities and displacements appear due to their interaction. In the future, we would like to unravel this interaction experimentally to understand the relationship between slow earthquakes and fast earthquakes.