Stress field in the northeastern Japan and its relationship with faults of recent earthquakes

Abstract Inversion tectonics, an old normal fault that acts as a reverse fault in the current stress field, is frequently observed in northeastern Japan (Tohoku District). Furthermore, new reverse faults that formed in the current stress field are distributed in this area; however, the conditions that control these fault activities remain unclear. To investigate the condition of fault activity and its regional variation in the current stress field, the stress field in Tohoku District and the likelihood of fault activities are estimated in this study using slip tendency (ST) analysis. In the eastern margin of the Japan Sea (EMJS) area, the reverse fault type of the stress field is dominant. Therefore, the maximum horizontal direction changes clockwise from E-W to NW-SE, from the northern to the southern region. In addition, it changes counterclockwise from NW-SE to EW from the Japan Sea area to the inland area. In the Tohoku inland area, the estimated direction of the maximum horizontal axis changed before and after the 2011 Tohoku-Oki earthquake. Before the 2011 Tohoku-Oki earthquake, it was E-W to WNW-ESE. Therefore, only the stress field before the 2011 Tohoku-Oki earthquake was used to calculate the ST values for seven events in the EMJS and four events in the Tohoku inland area. The results of the ST analysis showed eastward-dipping fault planes with low dip angles (approximately 30°–45°) and large ST values (approximately > 0.7). A large ST value indicates that the fault is favorable for slip in the stress field. A fault plane with a large ST value is consistent with the actual fault plane in the EMJS area. However, in the Tohoku inland area and southern part of the 1993 Hokkaido Nansei-Oki earthquake, the fault planes with large ST values were inconsistent with the actual fault plane, indicating that fault planes are unfavorable for slipping under the current stress field. These regional differences are consistent with the volcano distribution; therefore, the fluid supply from volcanic activity may help the fault slip under difficult stress conditions.