2:45 PM - 3:00 PM
[SCG44-11] Frictional properties of clay minerals in shallow subduction zones
★Invited Papers
Keywords:Clay minerals, Friction, Subduction zone
Clay minerals such as smectite and illite are abundant in sediments deposited on the ocean floor. These clay minerals have low frictional strengths, and mineral species are varied species during subduction. Therefore, frictional properties of clay minerals are important for the process of earthquake generation in subduction zones. To understand the slip behavior of plate boundary faults in subduction zones, I have conducted friction experiments on clay mineral-rich materials that simulate subducting sediments. In this presentation, I present the results of friction experiments on smectite, which is abundant in marine sediments, and illite, which is formed as a result of diagenetic processes.
Among the various clay minerals, smectite is one of the mechanically weakest. Therefore, the influence of smectite on the slip behavior of faults has been extensively studied under both low velocity conditions (~1 μm/s) or high velocity conditions (~1 m/s). However, the frictional behavior under intermediate velocity conditions (~1 mm/s) has remained unclear, even though it characterizes how spontaneous rupture propagation evolves during earthquake nucleation. Therefore, I performed friction experiments on smectite-glass mixtures under velocity conditions ranging from 10 μm/s to 1 m/s. I found that when a small amount of smectite was included, anomalous slip weakening behavior occurred under the velocity conditions of 1-3 mm/s. Considering the instability of fault slip due to slip weakening, unrealistic fault sizes are estimated to allow spontaneous acceleration of slip on faults. Therefore, fault gouge containing clay minerals may self-arrest fault slip and rupture propagation at intermediate velocities during earthquake nucleation, resulting in slow seismic-like behavior. [1]
Smectite transforms to illite by depths where temperature is about 150°C. This transformation of smectite into illite has been thought to determine the updip limit of the seismogenic zone in subduction zones, but it is known that both smectite and illite exhibit little or no unstable slip under low velocity conditions. However, the frictional properties under hydrothermal conditions, the actual environment of the subduction zone, have not been fully understood. Therefore, we modeled the transformation from smectite to illite along the plate boundary of the Nankai Trough and conducted friction experiments under conditions that reproduced in-situ temperature and pressure conditions to explore changes in the frictional properties due to smectite-illite transformation and their contribution to seismogenesis. As a result, no transition from stable to unstable slip was observed under conditions up to 150°C, where smectite illitization almost completed. This indicates that the updip limit of the seismogenic zone is not controlled by smectite-illite transition. On the other hand, unstable slip was observed at deeper depths with higher temperatures. This is caused by the frictional property of illite at high temperatures. Compared with the distribution of seismic activities in the Nankai Trough, the depth of the initiation of unstable slip of illite is consistent with the location of the updip limit of the seismogenic zone, suggesting that the frictional properties of illite are important for the updip limit of the seismogenic zone. Shallow slow earthquakes occur in regions which sediments exhibit stable slip at. In general, stable slip cannot spontaneously accelerate fault slip, so the mechanism of shallow slow earthquakes may not be simply unstable slip, but a combination of mechanisms such as slip weakening. [2]
[1] H. Okuda, T. Hirose, A. Yamaguchi (2023). Potential role of volcanic glass-smectite mixtures in slow earthquakes in shallow subduction zones: Insights from low- to high-velocity friction experiments. JGR Solid Earth. 128, e2022JB026156.
[2] H. Okuda, M. Kitamura, M. Takahashi, A. Yamaguchi (2023). Frictional properties of the décollement in the shallow Nankai Trough: constraints from friction experiments simulating in-situ conditions and implications for the seismogenic zone. EPSL. 621, 118357.
Among the various clay minerals, smectite is one of the mechanically weakest. Therefore, the influence of smectite on the slip behavior of faults has been extensively studied under both low velocity conditions (~1 μm/s) or high velocity conditions (~1 m/s). However, the frictional behavior under intermediate velocity conditions (~1 mm/s) has remained unclear, even though it characterizes how spontaneous rupture propagation evolves during earthquake nucleation. Therefore, I performed friction experiments on smectite-glass mixtures under velocity conditions ranging from 10 μm/s to 1 m/s. I found that when a small amount of smectite was included, anomalous slip weakening behavior occurred under the velocity conditions of 1-3 mm/s. Considering the instability of fault slip due to slip weakening, unrealistic fault sizes are estimated to allow spontaneous acceleration of slip on faults. Therefore, fault gouge containing clay minerals may self-arrest fault slip and rupture propagation at intermediate velocities during earthquake nucleation, resulting in slow seismic-like behavior. [1]
Smectite transforms to illite by depths where temperature is about 150°C. This transformation of smectite into illite has been thought to determine the updip limit of the seismogenic zone in subduction zones, but it is known that both smectite and illite exhibit little or no unstable slip under low velocity conditions. However, the frictional properties under hydrothermal conditions, the actual environment of the subduction zone, have not been fully understood. Therefore, we modeled the transformation from smectite to illite along the plate boundary of the Nankai Trough and conducted friction experiments under conditions that reproduced in-situ temperature and pressure conditions to explore changes in the frictional properties due to smectite-illite transformation and their contribution to seismogenesis. As a result, no transition from stable to unstable slip was observed under conditions up to 150°C, where smectite illitization almost completed. This indicates that the updip limit of the seismogenic zone is not controlled by smectite-illite transition. On the other hand, unstable slip was observed at deeper depths with higher temperatures. This is caused by the frictional property of illite at high temperatures. Compared with the distribution of seismic activities in the Nankai Trough, the depth of the initiation of unstable slip of illite is consistent with the location of the updip limit of the seismogenic zone, suggesting that the frictional properties of illite are important for the updip limit of the seismogenic zone. Shallow slow earthquakes occur in regions which sediments exhibit stable slip at. In general, stable slip cannot spontaneously accelerate fault slip, so the mechanism of shallow slow earthquakes may not be simply unstable slip, but a combination of mechanisms such as slip weakening. [2]
[1] H. Okuda, T. Hirose, A. Yamaguchi (2023). Potential role of volcanic glass-smectite mixtures in slow earthquakes in shallow subduction zones: Insights from low- to high-velocity friction experiments. JGR Solid Earth. 128, e2022JB026156.
[2] H. Okuda, M. Kitamura, M. Takahashi, A. Yamaguchi (2023). Frictional properties of the décollement in the shallow Nankai Trough: constraints from friction experiments simulating in-situ conditions and implications for the seismogenic zone. EPSL. 621, 118357.