An emerging field of high-velocity friction and its implication for dynamic fault motion during large earthquakes

In order to understand not only the mechanisms of earthquakes, but also the origin of diverse behavior of faults and plate boundaries, one must integrate (1) field studies on faults to understand deep intrafault processes, (2) laboratory work to reproduce those processes and determine mechanical and transport properties of fault zones, (3) theoretical and numerical studies analyzing fault motion, including earthquake generation processes, based on the constitutive properties determined by laboratory studies, and (4) seismological and geodetic studies revealing dynamic fault motion during earthquakes and diverse aseismic fault behavior. Ideally, such integrated studies should be carried out for a selected fault that produced an earthquake with good seismic/geodetic records so the prediction from (1) to (3) can be fully tested with (4), rather than selecting favorite data in the literature. Present session is organized to promote such integrated fault and earthquake studies. This presentation will focus on high-velocity frictional properties of faults for which frictional heating plays crucial roles, with special reference to dynamic fault motion during large earthquakes. Recent progress in high-velocity friction studies on (1) frictional melting, (2) thermal pressurization and (3) high-velocity weakening of fault gouge are rapidly filling the gap between field/laboratory studies on faults and seismological/geodetic studies on earthquakes. Permeability and concentration of shearing deformation within fault zones determines relative significance of those processes. Accumulation of data on transport properties of fault zones has made it possible to perform realistic calculation of thermal pressurization processes, with predicted Dc values in quantitative agreement with seismically determined values. I also show highlight data on frictional melting and argue that effect of frictional melting on dynamic fault property can be predicted by solving a Stefan problem with moving boundaries [Hirose and Shimamoto, 2004; Satomi and Shirono, 2003, 2004; Matsuzawa and Takeo, 2004]. Remaining task is to include incipient frictional melting, characterized by melt-patches formation, and melt loss into fractures in the host rocks in the analyses of frictional melting [Hirose and Shimamoto, this session]. High-velocity friction data on Nojima fault gouge data [Mizoguchi and Shimamoto, this session] and intermediate-velocity data on rock-on-rock friction by the Brown group have revealed that there are unknown slip-weakening mechanisms, besides those two well-studied mechanisms. Tribochemical effects on high-velocity friction, i.e., the effects of interfacial chemical changes promoted by frictional heating under fluid-rich environments, are very important area for future systematic studies. Despite these unexplored areas, seismic fault motion will be predicted not so long in the future based on the measured properties on a fault that caused an earthquake. Transition from ordinary friction to high-velocity friction, poorly explored at present, should control the initial phase of earthquake generation and perhaps is critical to understand the physical bases of earthquake prediction. This is probably the most important area for systematic studies in the near future.