Rheology and earthquakes in subduction zones

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The main purpose of this project has been to integrate fault studies in the field and laboratory, experimental studies to determine rheological and transport properties of fault zone and theoretical/numerical analyses of fault motion towards better understanding of mechanisms of subduction-zone earthquakes. The present research grant-in-aid was used to install servo-controlled actuators for axial loading and confining pressure onto our intravessel deformation-fluid-flow apparatus and to modify low-to-high velocity hydrothermal friction apparatus to stabilize confining water-pressure and axial load during high-velocity experiments under pore-water pressure. The extension of intravessel machine has made it possible to complete a-very-easy-to-use system for permeability/porosity measurements. The modification of low-to-high velocity apparatus has made it possible, for the first time in the world, to reproduce seismic fault motion with pore-water pressure at velocities up to several meters … More per second. The two systems will provide numerous unique data on fault-zone properties for years to come. The main scientific outcomes are as follows. (1)Thermal pressurization process, i.e., pore pressure rise due to frictional heating, was analyzed based on measured permeability structures of Median Tectonic Line and Hanaore fault (two papers). This mechanism was shown to yield slip-weakening distance of fault, Dc, of the same order as that of seismically determined values, for the first time. Traditional laboratory data showed Dc smaller that seismic values by a few orders of magnitude, so that our work has filled the gap between fault studies and seismology substantially. (2)Frictional melting processes have been studied in very detail and we showed that the effect of frictional melting processes can be evaluated by solving a Stefan problem, that frictional melt can act as lubricant to fault motion at depths, and that frictional melting processes also yield Dc close to seismically determined order (3 papers). (3)Friction experiments have been conducted using serpentinite recovered from Haha-jima serpentine seamount behind Mariana trench and our results suggests a possibility that great thrust-type earthquakes are suppressed by low-temperature serpentine of serpentinized wedge mantle. (4) Simulation of plate boundary motion using realistic fault properties near the base of seismogenic zone reproduced slow slip events recently recognized in subduction zones. Less