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.
<p>S wave reflectors in the crust may be caused by strong heterogeneous structures such as ones containing &#160;fluid. Especially, fluid around an earthquake fault could play an important role for initiation of the earthquake rupture as a mechanism for reducing fault strength. The location and geometry of the reflector can be determined from the travel time of the reflected phases. For detecting the reflections, we need to observe at stations located close to the hypocentre because of sufficient phase separation of the small lapse time of the reflected phases due to a reflector in the crust from direct S wave. In this study, we attempted to detect reflected waves in observed seismograms at the seismic stations in and around the 2016 Kaikoura earthquake (Mw7.6). Seismic records were obtained from the permanent GeoNet stations as well as from seismic stations deployed before the Kaikoura earthquake in the northern South Island. We applied reflection seismology techniques to the data obtained by the network. We used seismograms with smaller epicentral distance than 30 km and obtained dip move-out sections for each station. We detected several reflectors in the mid and lower crust from the sections. Strong reflected phases were observed at the southern edge of the focal area (from a reflector with depth about 20 km). Weak reflectors were detected in/beneath the aftershock area (in the mid- to lower-crust). In addition, the subducting slab might be imaged with dipping angle 20 degree. &#160;Reflectors parallel to the slab were also found below the interface.</p>
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