Trench-parallel crustal anisotropy along the trench in the fore-arc region of Japan

In northeastern Japan, the Pacific plate is descending beneath the North American plate. It is generally understood that trench-normal principal stress is dominant in the crust along the Japanese island arc, because the stress field is controlled by the force of the subducting slab. Observations of shear wave splitting using crustal earthquakes reveal a marked lateral variation in fast-polarization direction with distance from the trench. Trench-normal and trench-parallel fast-polarization directions are observed on the back-arc and fore-arc sides, respectively. In this study, two-dimensional finite element modeling with subducting slab was conducted to investigate the interseismic stress field during earthquake cycles, taking into account linear viscoelasticity. In the model, trench-normal compression is found to dominate in the island arc region. However, an extensional field appears in the shallow upper crust of the fore-arc region during earthquake cycles. The trench-parallel crustal anisotropy can be explained by this extensional field.