2021-05-09T22:10:14Zhttps://eprints.lib.hokudai.ac.jp/dspace-oai/requestoai:eprints.lib.hokudai.ac.jp:2115/804152021-02-12T02:46:17Zhdl_2115_20039hdl_2115_116Focal mechanisms and the stress field in the aftershock area of the 2018 Hokkaido Eastern Iburi earthquake (M-JMA=6.7)Susukida, YukiKatsumata, KeiIchiyanagi, MasayoshiOhzono, MakoAoyama, HiroshiTanaka, RyoTakada, MasamitsuYamaguchi, TeruhiroOkada, KazumiTakahashi, HiroakiSakai, Shin'ichiMatsumoto, SatoshiOkada, TomomiMatsuzawa, ToruMiyamachi, HirokiHirano, ShuichiroYamanaka, YoshikoHorikawa, ShinichiroKosuga, MasahiroKatao, HiroshiIio, YoshihisaNagaoka, AiriTsumura, NorikoUeno, TomotakeMiyakawa, KojiTanaka, Shin'ichiAndo, MiwakoUchida, NaokiAzuma, RyosukeTakagi, RyotaYoshida, KeisukeNakayama, TakashiHirahara, SatoshiTerakawa, ToshikoMaeda, YutaMiyamachi, HirokiYakiwara, HiroshiThe Hokkaido Eastern Iburi earthquakeReverse faultAftershock distributionFocal mechanism solutionTemporary seismic networkStress inversion453The tectonic stress field was investigated in and around the aftershock area of the Hokkaido Eastern Iburi earthquake (M-JMA = 6.7) occurred on 6 September 2018. We deployed 26 temporary seismic stations in the aftershock area for approximately 2 months and located 1785 aftershocks precisely. Among these aftershocks, 894 focal mechanism solutions were determined using the first-motion polarity of P wave from the temporary observation and the permanent seismic networks of Hokkaido University, Japan Meteorological Agency (JMA), and High Sensitivity Seismograph Network Japan (Hi-net). We found that (1) the reverse faulting and the strike-slip faulting are dominant in the aftershock area, (2) the average trend of P- and T-axes is 78 degrees +/- 33 degrees and 352 degrees +/- 51 degrees, respectively, and (3) the average plunge of P- and T-axes is 25 degrees +/- 16 degrees and 44 degrees +/- 20 degrees, respectively: the P-axis is close to be horizontal and the T-axis is more vertical than the average of the P-axes. We applied a stress inversion method to the focal mechanism solutions to estimate a stress field in the aftershock area. As a result, we found that the reverse fault type stress field is dominant in the aftershock area. An axis of the maximum principal stress (sigma(1)) has the trend of 72 degrees +/- 7 degrees and the dipping eastward of 19 degrees +/- 4 degrees and an axis of the intermediate principal stress (sigma(2)) has the trend of 131 degrees +/- 73 degrees and the dipping southward of 10 degrees +/- 9 degrees, indicating that both of sigma(1)- and sigma(2)-axes are close to be horizontal. An axis of the minimum principal stress (sigma(3)) has the dipping westward of 67 degrees +/- 6 degrees that is close to be vertical. The results strongly suggest that the reverse-fault-type stress field is predominant as an average over the aftershock area which is in the western boundary of the Hidaka Collision Zone. The average of the stress ratio R = (sigma(1) - sigma(2))/(sigma(1) - sigma(3)) is 0.61 +/- 0.13 in the whole aftershock area. Although not statistically significant, we suggest that R decreases systematically as the depth is getting deep, which is modeled by a quadratic polynomial of depth.SpringerJournal Articlehttp://hdl.handle.net/2115/80415Earth planets and space73112021-12-01enginfo:doi/10.1186/s40623-020-01323-xnone