Stress fields in NE Japan before and after the 2011 Tohoku-oki Earthquake

Abstract

We estimated the crustal stress field in NE Japan before and after the 2011 Mw9.0 Tohoku-Oki earthquake based on earthquake focal mechanisms determined using seismograms from temporary and permanent seismic networks deployed in this area. Results before the 2011 Tohoku-oki earthquake show that the arc and back-arc are characterized by spatially uniform margin normal compression. However, the fore-arc has different stress orientations. The Kitakami and Abukuma mountain ranges in the north and south have σ1 axis oriented nearly N-S and vertical, respectively, and the region in the plane between the mountain ranges has a similar stress field to the arc and back-arc. Anomalous stress fields in the mountain ranges of the fore-arc are probably due to gravitational force. Spatially homogeneous margin normal compression is observed throughout the arc and backarc as already mentioned, but the stress field even in those regions might also be influenced by the topography. A clear spatial correlation was found between strike-slip fault stress regime and high mountain ranges in those regions, which again suggests that the prevailing stress field has been influenced by topographic loading. The estimated stress fields changed significantly after the earthquake in northern region of NE Japan, where the orientations of the principal stresses became approximately the same as the orientations of the static stress change associated with the earthquake. This suggests that the stress magnitudes in northern region of NE Japan are very low because the static stress changes are only about 1 MPa of differential stress. Another possibility is that the stress fields in NE Japan are spatially very heterogeneous with the scale < ~ 10 km.