Earth, Planets and Space (Jan 2021)

Focal mechanisms and the stress field in the aftershock area of the 2018 Hokkaido Eastern Iburi earthquake (M JMA = 6.7)

  • Yuki Susukida,
  • Kei Katsumata,
  • Masayoshi Ichiyanagi,
  • Mako Ohzono,
  • Hiroshi Aoyama,
  • Ryo Tanaka,
  • Masamitsu Takada,
  • Teruhiro Yamaguchi,
  • Kazumi Okada,
  • Hiroaki Takahashi,
  • Shin’ichi Sakai,
  • Satoshi Matsumoto,
  • Tomomi Okada,
  • Toru Matsuzawa,
  • Hiroki Miyamachi,
  • Shuichiro Hirano,
  • Yoshiko Yamanaka,
  • Shinichiro Horikawa,
  • Masahiro Kosuga,
  • Hiroshi Katao,
  • Yoshihisa Iio,
  • Airi Nagaoka,
  • Noriko Tsumura,
  • Tomotake Ueno,
  • The Group for the Aftershock Observations of the 2018 Hokkaido Eastern Iburi Earthquake

DOI
https://doi.org/10.1186/s40623-020-01323-x
Journal volume & issue
Vol. 73, no. 1
pp. 1 – 13

Abstract

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Abstract The 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° ± 33° and 352° ± 51°, respectively, and (3) the average plunge of P- and T-axes is 25° ± 16° and 44° ± 20°, 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 (σ 1) has the trend of 72° ± 7° and the dipping eastward of 19° ± 4° and an axis of the intermediate principal stress (σ 2) has the trend of 131° ± 73° and the dipping southward of 10° ± 9°, indicating that both of σ 1- and σ 2-axes are close to be horizontal. An axis of the minimum principal stress (σ 3) has the dipping westward of 67° ± 6° 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 = (σ 1 − σ 2)/(σ 1 − σ 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.

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