1:45 PM - 2:00 PM
[SCG40-11] Fluidization of a fault in a shallow portion of an accretionary prism during faulting
Keywords:fault, pore pressure, fluidization, ring-shear experiment
An increasing number of the studies of slow earthquakes repeatedly suggested an important role of high-pore-fluid pressure conditions as the underlying possible mechanism of slow earthquakes. However, experimental studies performed aiming to understand the mechanical properties of subduction-zone materials in the presence of pore fluids are limited. In order to understand the faulting process in the shallow part of the subduction zone, we have performed large-displacement shear experiments on shallow-subduction-zone material under pore-fluid pressure conditions.
Experimental samples were collected from the hanging wall side of the imbricate thrust fault developed in the upper Miocene Misaki formation on the Miura Peninsula, central Japan. Due to the burial depth of only ~1 km depth, shallow deformation structures during subduction have been preserved in the complex. Fault gouge of the thrust faults in the Miura-Boso accretionary complex shows a possibly fluidized structure, which is characterized by injection of the gouge into the hanging wall side of the fault (Yamamoto et al., 2005).
Experiments were performed on hemipelagic siltstone at drained and undrained conditions, using the large ring shear testing machine at the Disaster Prevention Research Institute of Kyoto University (Sassa et al., 2004). In the experiments, the shear stress was increased gradually at a rate of 0.1 kPa/s from an initial stress at 500 kPa until deformation reached the failure (slip) condition. Macroscopic fault slip was observed to initiate when deformation reached the failure line. In the experiment conducted under the drained condition, no increase in pore-fluid pressure was observed. Under the undrained condition, pore-fluid pressure increased with shear loading even before deformation reached the failure line. After the initiation of the macroscopic slip at a shear stress of ~200 kPa, the effective normal stress gradually decreased with the continuing increase in pore-fluid pressure and the shear stress decreased to a steady state value of ~50 kPa. The slip weakening behavior of the drained condition required a displacement of ~450 mm for the shear stress to decrease to a steady state, whereas the corresponding weakening displacement for the undrained condition was ~100 mm. The increase in pore-fluid pressure due to fault deformation before the initiation of fault slip is of the same magnitude as the increase in pore-fluid pressure during slip. This demonstrates the important role of the shear-induced compaction of the fault-zone material for the pore pressure increasing process. Observations of the internal structure of faults showed that injection vein which consisted of fine-grained gouge material was developed during shear when tested in undrained condition. This may be a candidate deformation structure which suggests fluidization of the fault-zone material during fault shearing.
Experimental samples were collected from the hanging wall side of the imbricate thrust fault developed in the upper Miocene Misaki formation on the Miura Peninsula, central Japan. Due to the burial depth of only ~1 km depth, shallow deformation structures during subduction have been preserved in the complex. Fault gouge of the thrust faults in the Miura-Boso accretionary complex shows a possibly fluidized structure, which is characterized by injection of the gouge into the hanging wall side of the fault (Yamamoto et al., 2005).
Experiments were performed on hemipelagic siltstone at drained and undrained conditions, using the large ring shear testing machine at the Disaster Prevention Research Institute of Kyoto University (Sassa et al., 2004). In the experiments, the shear stress was increased gradually at a rate of 0.1 kPa/s from an initial stress at 500 kPa until deformation reached the failure (slip) condition. Macroscopic fault slip was observed to initiate when deformation reached the failure line. In the experiment conducted under the drained condition, no increase in pore-fluid pressure was observed. Under the undrained condition, pore-fluid pressure increased with shear loading even before deformation reached the failure line. After the initiation of the macroscopic slip at a shear stress of ~200 kPa, the effective normal stress gradually decreased with the continuing increase in pore-fluid pressure and the shear stress decreased to a steady state value of ~50 kPa. The slip weakening behavior of the drained condition required a displacement of ~450 mm for the shear stress to decrease to a steady state, whereas the corresponding weakening displacement for the undrained condition was ~100 mm. The increase in pore-fluid pressure due to fault deformation before the initiation of fault slip is of the same magnitude as the increase in pore-fluid pressure during slip. This demonstrates the important role of the shear-induced compaction of the fault-zone material for the pore pressure increasing process. Observations of the internal structure of faults showed that injection vein which consisted of fine-grained gouge material was developed during shear when tested in undrained condition. This may be a candidate deformation structure which suggests fluidization of the fault-zone material during fault shearing.