Spatial Distribution of Crustal Anisotropy in and around the Atotsugawa Fault, Japan -To Infer the Frictional Coefficient of Active Fault-

The spatial variation of frictional coefficient along active fault can be inferred from detail observation of stress field around the fault. We infer the spatial variation of maximum compressional stress axis around the active faults from dense observation of crustal anisotropy. The major possible cause of seismic anisotropy in the crust is the preferred orientation of micro cracks directed parallel to the axis of maximum compressional stress. Seismic anisotropy data are integrated effects from hypocenter to observatory, similar to travel time analysis, so we can obtain detailed images of the stress field by a tomographic approach if huge amounts of anisotropy data are available. The resolution of stress field imaged by the observation of anisotropy is expected to be higher than that studied by focal mechanisms or geodetic measurements such as GPS observation. In this study, we analyzed detailed anisotropy structure in and around the Atotsugawa fault, central Japan. The Atotsugawa fault, 60 km in length and N 60° E of strike, is located along the Niigata-Kobe tectonic zone, which is recognized as a deformation zone with a high strain rate. GPS observation indicates that the central part of the Atotsugawa fault is creeping and the other parts of the fault is locked to a depth of 15 km. Therefore, we can expect that the axis of maximum compressional stress is varied along the fault. Our observation indicates that crack orientation in and around the Atotsugawa fault is nearly consistent with the direction of the regional maximum stress in this area (NW-SE). On the western part, at stations 5 km away from the fault, crack orientation almost correspond to the regional maximum compressional direction. However, the direction of maximum compressional stress that inferred from anisotropy seems to have variation. At the western part of the fault, the direction of maximum compressional stress is estimated to be WNW-ESE. At the eastern part of the fault, the angle of maximum compressional axis makes more acute with the fault. This result may imply that frictional coefficient at the eastern part of Atotsugawa fault is higher than that of the western part.