2:15 PM - 2:30 PM
[SSS07-03] Hydrothermal friction experiments on simulated basaltic fault gouge and implications for megathrust earthquakes
★Invited Papers
Keywords:Seismogenic zone, Oceanic crust, Altered basalt, Friction experiment
Megathrust earthquakes in subduction zones generally source in the depth range of ~5-35 km corresponding to the temperature range of ~150-350℃. The extent of this “seismogenic zone” is widely attributed to variations in the frictional characteristics of materials present along the plate boundary fault (décollement), for example to the velocity-weakening behavior of weak, illite-rich sediments (den Hartog et al., 2012) or to changes in the extent of their lithification (Ikari & Hüpers, 2021). However, recent seismological and geological studies suggest that the décollement at the updip limit of the seismogenic zone develops within the subducting oceanic crust (e.g., Kimura et al., 2012). This means that altered basaltic rocks may also be important for seismogenesis at these levels. Despite the possible importance for megathrust earthquakes, studies of basaltic fault rock are limited, especially regarding temperature dependence.
In this study, we used altered basalt samples taken from outcrops of the Mugi mélange in the Shimanto accretionary complex, Japan. The basalt has been initially formed as mid-ocean ridge basalt, and then subducted and exhumed from the seismogenic zone of a former subduction plate boundary. The altered basalt samples show intersertal texture of albitized plagioclase, clinopyroxene, and chlorite. Samples were crushed and sieved to obtain a simulated gouge material with a grain size of <100 μm. Laboratory friction experiments were conducted on the gouge using the hydrothermal ring shear apparatus at Utrecht University, at 100 MPa effective normal stress, 100 MPa pore fluid pressure, and at temperatures of 100-550℃, to simulate seismogenic zone conditions. Velocity stepping friction tests were performed at velocities of 1-3-10-30-100 μm/s to explore frictional stability at fixed temperature conditions. Some constant velocity experiments were also conducted for microstructural observations.
Friction coefficient increased from 0.55 to 0.85 at all temperatures investigated, due to slip hardening over the total displacement of 47.5 mm. Conditionally unstable frictional behavior that can lead to an earthquake (i.e., velocity weakening) was observed at temperatures between 100 to 450℃. Although the friction coefficients were higher than clay-rich sedimentary materials at any given temperature (cf., den Hartog et al., 2012), the temperature range of velocity-weakening behavior was substantially wider than found for illite-rich, granitic, or unaltered basaltic gouges. Microstructural observations suggested that pressure solution of albitized plagioclase may play a primary role in controlling the observed frictional stability. Since albitization likely occurs at shallow subduction levels (Moore et al., 2007), our results point to a possible role of altered oceanic crust in controlling seismogenesis on portions of subduction megathrusts that are hosted in such material.
In this study, we used altered basalt samples taken from outcrops of the Mugi mélange in the Shimanto accretionary complex, Japan. The basalt has been initially formed as mid-ocean ridge basalt, and then subducted and exhumed from the seismogenic zone of a former subduction plate boundary. The altered basalt samples show intersertal texture of albitized plagioclase, clinopyroxene, and chlorite. Samples were crushed and sieved to obtain a simulated gouge material with a grain size of <100 μm. Laboratory friction experiments were conducted on the gouge using the hydrothermal ring shear apparatus at Utrecht University, at 100 MPa effective normal stress, 100 MPa pore fluid pressure, and at temperatures of 100-550℃, to simulate seismogenic zone conditions. Velocity stepping friction tests were performed at velocities of 1-3-10-30-100 μm/s to explore frictional stability at fixed temperature conditions. Some constant velocity experiments were also conducted for microstructural observations.
Friction coefficient increased from 0.55 to 0.85 at all temperatures investigated, due to slip hardening over the total displacement of 47.5 mm. Conditionally unstable frictional behavior that can lead to an earthquake (i.e., velocity weakening) was observed at temperatures between 100 to 450℃. Although the friction coefficients were higher than clay-rich sedimentary materials at any given temperature (cf., den Hartog et al., 2012), the temperature range of velocity-weakening behavior was substantially wider than found for illite-rich, granitic, or unaltered basaltic gouges. Microstructural observations suggested that pressure solution of albitized plagioclase may play a primary role in controlling the observed frictional stability. Since albitization likely occurs at shallow subduction levels (Moore et al., 2007), our results point to a possible role of altered oceanic crust in controlling seismogenesis on portions of subduction megathrusts that are hosted in such material.