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

Earth, Planets and Space

The Group for the Aftershock Observations of the 2018 Hokkaido Eastern Iburi Earthquake¹  Noriko Tsumura²  Airi Nagaoka³  Hiroshi Katao³  Yoshihisa Iio³  Shin’ichi Sakai⁴  Masahiro Kosuga⁵  Shinichiro Horikawa⁶  Yoshiko Yamanaka⁶  Kazumi Okada⁷  Hiroshi Aoyama⁷  Ryo Tanaka⁷  Kei Katsumata⁷  Masayoshi Ichiyanagi⁷  Hiroaki Takahashi⁷  Mako Ohzono⁷  Masamitsu Takada⁷  Yuki Susukida⁷  Teruhiro Yamaguchi⁷  Hiroki Miyamachi⁸  Satoshi Matsumoto⁹  Shuichiro Hirano^1,10  Tomotake Ueno^1,11  Tomomi Okada^1,12  Toru Matsuzawa^1,12 

[1] ;Department of Earth Science, Faculty of Science, Chiba University;Disaster Prevention Research Institute, Kyoto University;Earthquake Research Institute, University of Tokyo;Earthquake and Volcano Observatory, Hirosaki University;Earthquake and Volcano Research Center, Graduate School of Environmental Studies, Nagoya University;Faculty of Science, Institute of Seismology and Volcanology, Hokkaido University;Graduate School of Science and Engineering, Kagoshima University;Institute of Seismology and Volcanology, Kyushu University;Nansei-Toko Observatory for Earthquakes and Volcanoes, Research and Education Center for Natural Hazards, Kagoshima University;National Research Institute for Earth Science and Disaster Resilience;Research Center for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University;

关键词: The Hokkaido Eastern Iburi earthquake;    Reverse fault;    Aftershock distribution;    Focal mechanism solution;    Temporary seismic network;    Stress inversion;   

DOI  :  10.1186/s40623-020-01323-x

来源: DOAJ

【 摘 要 】

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.

【 授权许可】

Unknown