11:00 AM - 11:15 AM
[SCG62-02] Experimental study on rheological property of pelitic schists in the Nankai Trough seismogenic zone
Keywords:Nankai Trough, Pelitic schist, Pore pressure
Slow earthquake activity has been observed near the upper and lower limits of the plate boundary seismogenic zone of the Nankai Trough. Since the Nankai Trough is a subduction zone composed of an accretionary complex, the fault slip behavior along the plate boundary can be controlled by the rheology of low strength pelitic schists.
In this study, deformation experiments were conducted on pelitic schists to elucidate their rheological properties as they relate to the seismogenic zone of the Nankai Trough. The sample was collected from the Sanbagawa metamorphic belt on Shikoku Island, Japan. XRD measurements indicated that the primary minerals in the pelitic schist are quartz (~55 wt%) and muscovite (~10 wt%).
The shear experiments carried out using Griggs-type apparatus at a temperature of 480 ℃ and confining pressures (PC) of 1250 MPa and 1000 MPa, which correspond to the maximum metamorphic conditions experienced by the pelitic schist. In addition, these temperature and pressure conditions are also roughly consistent with the source region of the episodic tremor and slow slip (ETS) in the Nankai Trough. To investigate the effect of pore fluid pressure (PP) on the rheological behavior of the pelitic schist, experiments were conducted under three conditions: dry (as-is), wet (undrained), and PP-controlled (drained). In the wet condition, experiments were performed under undrained conditions at 1 wt% and 5 wt% water contents, while under the PP-controlled condition, experiments were conducted in a drained condition at pore fluid pressures of 200 MPa, 400 MPa, 600 MPa, 750 MPa, and 800 MPa. All experiments were conducted at a shear rate of 0.23 µm/s, which is close to the slip velocity observed during slow slip events.
The experimental results showed that the strength of the pelitic schist decreased under both the wet and PP-controlled conditions relative to the dry condition, and unstable, slow slip-like behavior was observed. At the PP controlled conditions, shear stress decreased linearly with increasing the pore fluid pressure indicating that the effective pressure law works over a wide range of pore pressure for PP-controlled conditions. The friction coefficient (µ) evaluated from failure envelope was 0.32 before the stress drop and 0.25 after the stress drop. This change in µ likely reflects the formation of an R1 shear surface, with the pre-drop value representing the internal friction of the sample and the post-drop value corresponding to frictional sliding along the R1 surface. These findings suggest that water reduces the strength of pelitic schists and induces unstable behavior. Tokle et al. (2023) demonstrated that in the presence of water, muscovite reacts and deforms preferentially, thereby reducing the strength of quartz–muscovite aggregates. Furthermore, under PP-controlled conditions, a decrease in effective pressure was observed to coincide with a reduction in strength, implying that fluids further weaken the pelitic schist.
In summary, pelitic schists containing muscovite have low strength under dry conditions, but their strength is further reduced and unstable behavior is observed under wet and PP-controlled conditions. Moreover, a decrease in effective pressure (or an increase in PP) corresponds to a reduction in rock strength.
The results of this research show the role of fluids in the Nankai Trough, and provide new insights into the effects of high pore pressure on fault slip behavior.
In this study, deformation experiments were conducted on pelitic schists to elucidate their rheological properties as they relate to the seismogenic zone of the Nankai Trough. The sample was collected from the Sanbagawa metamorphic belt on Shikoku Island, Japan. XRD measurements indicated that the primary minerals in the pelitic schist are quartz (~55 wt%) and muscovite (~10 wt%).
The shear experiments carried out using Griggs-type apparatus at a temperature of 480 ℃ and confining pressures (PC) of 1250 MPa and 1000 MPa, which correspond to the maximum metamorphic conditions experienced by the pelitic schist. In addition, these temperature and pressure conditions are also roughly consistent with the source region of the episodic tremor and slow slip (ETS) in the Nankai Trough. To investigate the effect of pore fluid pressure (PP) on the rheological behavior of the pelitic schist, experiments were conducted under three conditions: dry (as-is), wet (undrained), and PP-controlled (drained). In the wet condition, experiments were performed under undrained conditions at 1 wt% and 5 wt% water contents, while under the PP-controlled condition, experiments were conducted in a drained condition at pore fluid pressures of 200 MPa, 400 MPa, 600 MPa, 750 MPa, and 800 MPa. All experiments were conducted at a shear rate of 0.23 µm/s, which is close to the slip velocity observed during slow slip events.
The experimental results showed that the strength of the pelitic schist decreased under both the wet and PP-controlled conditions relative to the dry condition, and unstable, slow slip-like behavior was observed. At the PP controlled conditions, shear stress decreased linearly with increasing the pore fluid pressure indicating that the effective pressure law works over a wide range of pore pressure for PP-controlled conditions. The friction coefficient (µ) evaluated from failure envelope was 0.32 before the stress drop and 0.25 after the stress drop. This change in µ likely reflects the formation of an R1 shear surface, with the pre-drop value representing the internal friction of the sample and the post-drop value corresponding to frictional sliding along the R1 surface. These findings suggest that water reduces the strength of pelitic schists and induces unstable behavior. Tokle et al. (2023) demonstrated that in the presence of water, muscovite reacts and deforms preferentially, thereby reducing the strength of quartz–muscovite aggregates. Furthermore, under PP-controlled conditions, a decrease in effective pressure was observed to coincide with a reduction in strength, implying that fluids further weaken the pelitic schist.
In summary, pelitic schists containing muscovite have low strength under dry conditions, but their strength is further reduced and unstable behavior is observed under wet and PP-controlled conditions. Moreover, a decrease in effective pressure (or an increase in PP) corresponds to a reduction in rock strength.
The results of this research show the role of fluids in the Nankai Trough, and provide new insights into the effects of high pore pressure on fault slip behavior.