16:00 〜 16:15
[SCG44-09] Laboratory friction experiments for understanding fault slip behavior in the shallow Nankai subduction zone
★Invited Papers
キーワード：Friction experiment、Fault slip、Subduction zone
Subduction zones have hosted huge megathrust earthquakes that have caused severe damage to regions nearby. Seismological observations have revealed a wide spectrum of fault slip behavior along the plate boundary fault from “slow” to “fast” earthquakes controlled by various factors at and around the fault, e.g., slip velocity, lithology, fluid, temperature, etc. Especially in the shallow depths, slow earthquakes have been detected and the coseismic slip to the trench is expected to induce devastating tsunamis. Therefore, understanding the frictional properties of sediments at shallow depths in subduction zones is indispensable for evaluating future fault slip activities. For this purpose, we have performed two series of laboratory friction experiments on natural and simulated samples that compose the shallow Nankai subduction zone.
(1) Spatial heterogeneity of frictional properties induces complex slip behavior, and therefore knowing such information in natural conditions is important to anticipate future fault slip activities. We performed experiments at 0.01-100 μm/s on intact sediment core samples from different locations along the Kumano transect in the Nankai subduction zone system collected by IODP NanTroSEIZE projects. Combined with seafloor topography, we found that friction coefficients within the Nankai accretionary prism and along the plate boundary fault were spatially varied, which may be related to lithology and diagenetic processes of sediments. In addition, the samples representing the plate boundary fault exhibited the velocity weakening behavior requisite for nucleating an earthquake, and their critical nucleation lengths increased landward, possibly corresponding to the spatial variation of slow earthquakes in the shallow Nankai Trough. [Okuda, Ikari, et al., 2021, JGR Solid Earth]
(2) Volcanic glass is one of the main components of sediment incoming the Nankai Trough. Although volcanic glass easily alters into mechanically weak smectite at shallow depths and thus weakens the bulk strength of the shallow faults, frictional properties of volcanic glass and mixtures with smectite are largely unknown. We performed low-to-high velocity experiments from 10 μm/s to 1 m/s on simulated gouges of a mixture of volcanic glass and smectite at different weight fractions under water-pressurized conditions. At the intermediate velocities of 1-3 mm/s, samples with a small amount of smectite exhibited a conventional response to the velocity step (i.e., direct and evolutionary effects) followed by a large weakening behavior toward a velocity-dependent steady-state friction coefficient. Local thermal pressurization caused by flash heating at grain contacts may induce this weakening behavior, which can be applicable for other types of clay-mixed gouge. This result implies that the strength of fluid-saturated shallow fault systems can effectively weaken when the slip is slowly accelerated, whereas it remains strong when the acceleration is large enough not to initiate the weakening. [Okuda, Hirose, Yamaguchi, 2021, JpGU]
(1) Spatial heterogeneity of frictional properties induces complex slip behavior, and therefore knowing such information in natural conditions is important to anticipate future fault slip activities. We performed experiments at 0.01-100 μm/s on intact sediment core samples from different locations along the Kumano transect in the Nankai subduction zone system collected by IODP NanTroSEIZE projects. Combined with seafloor topography, we found that friction coefficients within the Nankai accretionary prism and along the plate boundary fault were spatially varied, which may be related to lithology and diagenetic processes of sediments. In addition, the samples representing the plate boundary fault exhibited the velocity weakening behavior requisite for nucleating an earthquake, and their critical nucleation lengths increased landward, possibly corresponding to the spatial variation of slow earthquakes in the shallow Nankai Trough. [Okuda, Ikari, et al., 2021, JGR Solid Earth]
(2) Volcanic glass is one of the main components of sediment incoming the Nankai Trough. Although volcanic glass easily alters into mechanically weak smectite at shallow depths and thus weakens the bulk strength of the shallow faults, frictional properties of volcanic glass and mixtures with smectite are largely unknown. We performed low-to-high velocity experiments from 10 μm/s to 1 m/s on simulated gouges of a mixture of volcanic glass and smectite at different weight fractions under water-pressurized conditions. At the intermediate velocities of 1-3 mm/s, samples with a small amount of smectite exhibited a conventional response to the velocity step (i.e., direct and evolutionary effects) followed by a large weakening behavior toward a velocity-dependent steady-state friction coefficient. Local thermal pressurization caused by flash heating at grain contacts may induce this weakening behavior, which can be applicable for other types of clay-mixed gouge. This result implies that the strength of fluid-saturated shallow fault systems can effectively weaken when the slip is slowly accelerated, whereas it remains strong when the acceleration is large enough not to initiate the weakening. [Okuda, Hirose, Yamaguchi, 2021, JpGU]