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
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.