Spatial variations of present crustal stresses in the Japan island arc by using stress tensor difference map technique

Stress tensor inversions determine an optimal stress tensor from a population of fault-slip data or earthquake focal mechanism data (double couple component of focal mechanisms). Otsubo et al. (2013, RS2013 proceedings) proposed a stress tensor difference map technique to provide a visual representation of stress heterogeneity patterns in the crust based on the stress tensors. The technique displays calculated stress differences between stress tensors estimated from focal mechanism data processed using the stress tensor inversion and an established reference stress. The technique procedure consists of three steps: (1) We chose locations of earthquake event as grid points; (2) We treat states of stress that activates earthquake events at each location as stress tensors at each grid point; (3) We then map the difference between each tensor and a reference stress tensor. States of stress can be detected by the stress tensor inversions (e.g., Otsubo et al., 2008, Tectonophysics 457, 150-160). The technique uses the "stress difference" defined by Orife and Lisle (2003; Journal of Structural Geology 25, 949-957) to show the similarity or dissimilarity between the stress tensors, in which each stress tensor is represented by a point in the four-dimensional parameter space (i.e., the three Euler angles for the principal stress axes and the stress ratio (s2-s3/s1-s3)). We apply the technique to natural earthquake data in Japan island arc that is situated in a subduction zone where four tectonic plates converge. In this study, we show the stress map on the different spatial scales (regional and local scales). In the regional scale, the E-W compressional stress characterized in NE Japan arc is also mapped in central Japan, whose region is concordant with the high strain rate zone (i.e., Niigata-Kobe Tectonic zone). Results in the local scale indicate the spatial stress heterogeneity acting on fault planes of the active faults. The stress mapping technique provides important potential to compare the stress state and spatial geophysical information (e.g. geodesic data, seismic velocity structure and gravity anomaly).