Unusual shallow normal‐faulting earthquake sequence in compressional northeast Japan activated after the 2011 off the Pacific coast of Tohoku earthquake

After the occurrence of the 2011 Mw9.0 off the Pacific coast of Tohoku earthquake, an unusual shallow normal‐faulting earthquake sequence occurred near the Pacific coast at the Ibaraki‐Fukushima prefectural border. We have investigated why normal‐faulting earthquakes were activated in northeast (NE) Japan, which is otherwise characterized by E–W compression. We computed the stress changes associated with the mainshock on the basis of a finite fault slip model, which showed that the amount of additional E–W tensional stresses in the study area was up to 1 MPa, which might be too small to generate normal‐faulting earthquakes in the pre‐shock compressional stress regime. We thus determined focal mechanisms of microearthquakes that occurred in the area before the mainshock, which indicated that the pre‐shock stress field in the area showed a normal‐faulting stress regime in contrast to the overall reverse‐faulting regime in NE Japan. We concluded that the 2011 Tohoku earthquake triggered the normal‐faulting earthquake sequence in a limited area in combination with a locally formed pre‐shock normal‐faulting stress regime. We also explored possible mechanisms for localization of a normal‐faulting stress field at the Ibaraki‐Fukushima prefectural border.