To understand not only the mechanisms of earthquakes, but also the origins of the 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 predictions from (1) to (3) can be fully tested against (4), rather than selecting favorite data from the literature. The 1999 Taiwan Chi-chi earthquake is an ideal example for such integrated studies because the fault motion during the earthquake is clearly analyzed based on very good near-field strong motion data, and because the Chelungpu fault zone, which caused the earthquake, is exposed on land and has been drilled at several places. Also, the IODP drilling project into seismogenic zones in the Nankai Trough will provide rare opportunities for such integrated fault and earthquake studies in the near future.
This paper focuses on high-velocity frictional properties of faults for which frictional heating plays a crucial role, with special reference to dynamic fault motion during large earthquakes. Recent progress in high-velocity friction studies, particularly those in frictional melting, thermal pressurization, and high-velocity gouge behavior, are rapidly filling the gap between field/laboratory studies on faults and seismological/geodetic studies on earthquakes. Representative results from our recent studies are presented, revealing that the field/laboratory data can predict slip weakening distance, Dc, of the same order as determined by seismic data. We also show highlight data on frictional melting and argue that the effect of frictional melting on the dyanimic fault property can be predicted by solving a Stefan problem with moving boundaries. Seismic fault motion may be predicted not so long in the future based on the measured properties of a fault that caused an earthquake. Transition from ordinary friction to high-velocity friction that has been poorly explored to the present, should control the initial phase of earthquake generation and perhaps is critical to an understanding of the physical bases of earthquake prediction. This is perhaps the most important area for systematic studies in the near future.
