9:45 AM - 10:00 AM
[SSS08-10] Friction experiments on volcanic glass-clay mixtures toward understanding the slip behavior of sediments near trench-arc system
Keywords:Volcanic glass, Subduction zone, Earthquake, Submarine landslide, Clay
Subduction of oceanic plate leads to produce magma beneath the upper plate, and forms island arc and volcanoes. Volcanic materials from such volcanoes deposit on the incoming oceanic plate and subduct at the trench as observed in the Nankai and the Barbados convergent margins. At the Nankai Trough, the subducting sediments comprise approximately 35% volcanic material (Scudder et al., 2018), and many volcanic ash layers are reported within the accretionary prisms (Screaton et al., 2009). The diagenetic reactions of volcanic glass in the volcanic materials to smectite stiffen surrounding sediments at the shallow subduction zone (e.g., White et al., 2011), and amorphous materials like volcanic glass are proposed to be frictionally weak (e.g., silica-gel lubrication; Goldsby & Tullis, 2002). In contrast, as clay minerals like smectite have low friction coefficients and easily induce a large slip (e.g., Faulkner et al., 2011), understanding frictional properties of volcanic glass-clay mixture is important to assess the slip behavior of sediments at a shallow subduction zone.
In this study, we performed a series of friction experiments on the volcanic glass-clay mixtures with different clay contents using a high-velocity rotary shear apparatus at Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (Tanikawa et al., 2012, 2015). All experiments were conducted at the velocity conditions ranged from 10 μm/sec to 1 m/sec under 5 MPa effective normal stress and 10 MPa pore fluid pressure conditions.
For the tests with 100% volcanic glass, friction coefficients of 0.6 to 0.75 were obtained when the velocities were from 10 μm/sec to 30 mm/sec, whereas the friction coefficients dynamically decreased at the velocity conditions higher than 100 mm/sec and dropped to 0.25 at the velocity of 1 m/sec. As the clay content increased, friction coefficients decreased down to 0.1 at the velocities slower than 300 μm/sec for the samples with >50% clay, and at the velocities faster than 1-3 mm/sec for those with >15% clay (Figure).
We found that the friction coefficient of volcanic glass is higher than that of smectite at all velocity conditions, and the velocity-strengthening behavior appears at 1-3 mm/sec that potentially suppresses the evolution to further fast slip rates. On the other hand, the clay content strongly affects the bulk friction coefficients such that the sediment with more than 15% clay easily induce a fast slip due to its low friction and velocity-weakening behavior at 1-3 mm/sec. These results indicate that the friction coefficient of the fault varies with depth depending on the amount of preserved volcanic glass at the shallowest part of the subduction zones such as the Nankai Trough: a fault may have a high frictional strength with velocity-strengthening behavior at a shallower depth due to the preservation of unaltered volcanic glass. Hence, a coseismic slip that is initiated at the seismogenic zone inhibit further slip speed as approaching toward the shallow part of the subductions zone. In contrast, the increase in clay content due to volcanic glass alteration weaken the volcanic ash layer. Such a layer may act as a weak plane to induce submarine landslides. Accordingly, the frictional property of volcanic glass-clay mixture has a strong influence on the fault slip behavior at shallow subduction zone as well as on the mass movement on the seafloor, and therefore plays an important role for tsunamigenesis.
In this study, we performed a series of friction experiments on the volcanic glass-clay mixtures with different clay contents using a high-velocity rotary shear apparatus at Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (Tanikawa et al., 2012, 2015). All experiments were conducted at the velocity conditions ranged from 10 μm/sec to 1 m/sec under 5 MPa effective normal stress and 10 MPa pore fluid pressure conditions.
For the tests with 100% volcanic glass, friction coefficients of 0.6 to 0.75 were obtained when the velocities were from 10 μm/sec to 30 mm/sec, whereas the friction coefficients dynamically decreased at the velocity conditions higher than 100 mm/sec and dropped to 0.25 at the velocity of 1 m/sec. As the clay content increased, friction coefficients decreased down to 0.1 at the velocities slower than 300 μm/sec for the samples with >50% clay, and at the velocities faster than 1-3 mm/sec for those with >15% clay (Figure).
We found that the friction coefficient of volcanic glass is higher than that of smectite at all velocity conditions, and the velocity-strengthening behavior appears at 1-3 mm/sec that potentially suppresses the evolution to further fast slip rates. On the other hand, the clay content strongly affects the bulk friction coefficients such that the sediment with more than 15% clay easily induce a fast slip due to its low friction and velocity-weakening behavior at 1-3 mm/sec. These results indicate that the friction coefficient of the fault varies with depth depending on the amount of preserved volcanic glass at the shallowest part of the subduction zones such as the Nankai Trough: a fault may have a high frictional strength with velocity-strengthening behavior at a shallower depth due to the preservation of unaltered volcanic glass. Hence, a coseismic slip that is initiated at the seismogenic zone inhibit further slip speed as approaching toward the shallow part of the subductions zone. In contrast, the increase in clay content due to volcanic glass alteration weaken the volcanic ash layer. Such a layer may act as a weak plane to induce submarine landslides. Accordingly, the frictional property of volcanic glass-clay mixture has a strong influence on the fault slip behavior at shallow subduction zone as well as on the mass movement on the seafloor, and therefore plays an important role for tsunamigenesis.