The multiple inverse method applied to heterogeneous focal mechanisms in Ryukyu arc, Japan: determination of stress province in active island arc

Deformation and stress state in active island arcs are important for understanding the dynamics. Especially, forearc stress and deformation are key to understand mechanical interaction of the backarc and subduction. Present crustal stress states are derived from focal mechanism solutions of earthquake. Otsubo and Yamaji (2006 JPGU meeting) proposed a new method which is proposed to separate stresses from heterogeneous data of focal mechanisms of earthquakes. In this study, we apply the technique into the focal mechanisms in Ryukyu arc. Result in Tokara Islands area, northern part of Ryukyu arc is typical one because stress province was likely to be distinguishable using the P-/T-axis of the solutions in domain which sandwiched volcanic line. As a result, present study shows different stress province from one found by P-/T- axes. Two extensional stresses (NNE-SSW and NW-SE) were detected from the solutions. Under regional radial extensional stress state, spatial variation of state stress allows was to divide the Tokara Islands area into two directions normal faulting regimes of stress. The boundary of the stress provinces corresponds to the volcanic front. In backarc area A1, arc- perpendicular (NW--SE) extensional stress is dominant. The dominant focal mechanism in the backarc is strike- slip faulting although the focal mechanisms have a few NE--SW trending normal faults. Stress ratio is low (Φ=0.2). On the other hand, the arc-parallel (NNE--SSW) extensional stress province A2 is found in forearc area. Stress ratio is high (Φ=0.8). The dominant focal mechanism in the forearc is NW--SE trending normal faulting. Many active faults indicate the stresses in the area (e.g. Founier et al, 2001, JGR). Kubo and Fukuyama (2004, EPSL) indicated that the extensional stress province extends to a wide area in the Ryukyu forearc by using P-/T- axes of focal mechanisms. Such forearc deformation and stress provinces are formed in relation to the increasing curvature of arc as a result of backarc extension (Wessel et al., 1994, JGR; Ten Veen and Kleinspehn, 2002, Tectonics).