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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.

We analyzed shear‐wave splitting of small earthquakes around the Atotsugawa fault, central Japan, to infer a spatial variation in the direction of maximum horizontal compression (Shmax). While the angle, ϕ, between the Shmax direction and fault strike ranges approximately from 55° to 80° at the stations 2 to 8 km from the fault, it approaches 45° at the stations within 1 km from the fault. The present result indicates the local stress accumulation due to the localized flow or slip along the deep extension of the fault. The Finite Element Modeling of the stress field around the fault suggests that the present existing stress field can be explained by the superposition of the stress field associated with regional deformation and the stable slip along the deep extension of the fault.