Evaluation of stress state, physical property and rupturing behavior of seismogenic faults by scientific drillings

特集号：沈み込む海洋プレート科学の最前線 --アウターライズ海洋掘削に向けて--To better understand seismic fault rupturing mechanisms, recent research results from scientific ocean drilling projects in seismogenic subduction zones such as Japan Trench and Nankai Trough carried out under the International Ocean Discovery Program (IODP) are reviewed and summarized. Stress states in the vicinity of faults and in wall rocks penetrated by drillings are determined or constrained using borehole wall resistivity images and drill core samples. The stress measurement methods are summarized and the stress results are interpreted. Fluid transportation properties and thermal properties within and around the faults are determined from retrieved core samples. The results are both useful and necessary for examining fault rupturing lubrication mechanisms, e.g., coseismic thermal pressurization. Dynamic frictional behavior of fault gouges is elucidated through rotary frictional experiments over a wide range from intermediate to high velocities covering the real coseismic fault slip velocity at around 1 m/s; the results indicate that frictional coefficients of plate boundary fault gouges in subduction zones are very low. In addition, high-temperature history examined with vitrinite reflectance measurements and minor element movements from isotopic analyses using the fault core samples give important evidence of the frictional heat of faults. Such historic high-temperature signals also enable inversion estimations to reveal past fault-rupturing parameters. It is hoped this review paper will be valuable and helpful for planning and implementing future scientific fault drilling programs