1:45 PM - 3:15 PM
[SSS06-P04] Influence of smectite-illite transition on fault frictional behavior in the shallow Nankai Trough
Keywords:Shallow slow earthquakes, Seismogenic zone, Nankai Trough, Friction experiment, Smectite-illite transition, Sediment
Smectite-illite (S-I) transition is one of the major diagenetic processes in subduction zones and has been one candidate for the control of the updip limit of the seismogenic zone (e.g., Oleskevich et al., 1999). However, previous friction experiments on the S-I mixtures did not report the transition in frictional instability (i.e., the transition from positive to negative velocity dependence of friction coefficient; (a−b)) with a progress of illitization (Saffer & Marone, 2003; Saffer et al., 2012; Tembe et al., 2010). Therefore, the role of the S-I transition in seismogenesis in subduction zones remains uncertain, especially under elevated temperature conditions despite significant effects of temperature on frictional behavior.
Here, we studied the transitions in frictional properties associated with the S-I transition along the Kumano transect in the Nankai Trough. Spatial variation of illite content in the S-I mixture (I% in S-I) can be modeled (Hüpers et al., 2019) via the kinetic expression of the S-I transition (Pytte & Reynolds, 1989), evolution of the accretionary prism (Screaton et al., 1990), and thermal structure (Sugihara et al., 2014). Based on the modeled S-I content, we selected five locations along the plate boundary (the bottom of the accretionary prism) whose I% in S-I were 30-50-70-90-95%. Simulated fault gouge materials having in-situ I% in S-I for given locations were prepared by mixing quartz (20%), albite (10%), orthoclase (10%), and the S-I mixture (60%). The gouge material was sandwiched by two porous alumina spacers with 30-degree saw-cut. Shear deformation was applied to the gouge with axial loading rates of 0.01-100 μm/s under the in-situ hydrostatic pressure and in-situ temperature conditions for the selected location. Friction experiments were performed with the gas-medium high-temperature-high-pressure triaxial deformation apparatus in GSJ, AIST.
Friction coefficient of the gouge showed an increase trend in accordance with the increase in illite content in the S-I mixture. The Reuss average of friction coefficients of mineral components well-explained the observed increase in the friction coefficient. The (a−b) values were always positive up to the temperature conditions of 150℃ (I% in S-I = 90%) whereas those at 0.1-10 μm/s and 171℃ (I% in S-I = 95%) were negative.
The shallow earthquakes (VLFEs and SSEs) in the Nankai Trough locate from 10 to 30 km landward from the trench where the temperature conditions are less than 150℃ (I% in S-I is less than 90%). As the (a−b) values were positive at all velocity conditions in this temperature range, and as the illitization is almost completed at 30 km landward from the trench, the experimental results indicate that the S-I transition does not have a significant influence on the (a−b) values. On the other hand, the observed transition in (a−b) from positive to negative values at 171℃ (~40 km landward from the trench) may be associated with the temperature dependence of frictional behavior of illite (den Hartog et al., 2012). As the hypocenter of the 2016 Off-Mie earthquake (Mw 5.9) located at ~40-50 km landward from the trench, this transition in (a−b) due to the frictional properties of illite rather than the S-I transition could correspond to the updip limit of the seismogenic zone.
Here, we studied the transitions in frictional properties associated with the S-I transition along the Kumano transect in the Nankai Trough. Spatial variation of illite content in the S-I mixture (I% in S-I) can be modeled (Hüpers et al., 2019) via the kinetic expression of the S-I transition (Pytte & Reynolds, 1989), evolution of the accretionary prism (Screaton et al., 1990), and thermal structure (Sugihara et al., 2014). Based on the modeled S-I content, we selected five locations along the plate boundary (the bottom of the accretionary prism) whose I% in S-I were 30-50-70-90-95%. Simulated fault gouge materials having in-situ I% in S-I for given locations were prepared by mixing quartz (20%), albite (10%), orthoclase (10%), and the S-I mixture (60%). The gouge material was sandwiched by two porous alumina spacers with 30-degree saw-cut. Shear deformation was applied to the gouge with axial loading rates of 0.01-100 μm/s under the in-situ hydrostatic pressure and in-situ temperature conditions for the selected location. Friction experiments were performed with the gas-medium high-temperature-high-pressure triaxial deformation apparatus in GSJ, AIST.
Friction coefficient of the gouge showed an increase trend in accordance with the increase in illite content in the S-I mixture. The Reuss average of friction coefficients of mineral components well-explained the observed increase in the friction coefficient. The (a−b) values were always positive up to the temperature conditions of 150℃ (I% in S-I = 90%) whereas those at 0.1-10 μm/s and 171℃ (I% in S-I = 95%) were negative.
The shallow earthquakes (VLFEs and SSEs) in the Nankai Trough locate from 10 to 30 km landward from the trench where the temperature conditions are less than 150℃ (I% in S-I is less than 90%). As the (a−b) values were positive at all velocity conditions in this temperature range, and as the illitization is almost completed at 30 km landward from the trench, the experimental results indicate that the S-I transition does not have a significant influence on the (a−b) values. On the other hand, the observed transition in (a−b) from positive to negative values at 171℃ (~40 km landward from the trench) may be associated with the temperature dependence of frictional behavior of illite (den Hartog et al., 2012). As the hypocenter of the 2016 Off-Mie earthquake (Mw 5.9) located at ~40-50 km landward from the trench, this transition in (a−b) due to the frictional properties of illite rather than the S-I transition could correspond to the updip limit of the seismogenic zone.