Journal
EARTH PLANETS AND SPACE
Volume 73, Issue 1, Pages -Publisher
SPRINGER
DOI: 10.1186/s40623-020-01323-x
Keywords
The Hokkaido Eastern Iburi earthquake; Reverse fault; Aftershock distribution; Focal mechanism solution; Temporary seismic network; Stress inversion
Categories
Funding
- Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under its The Second Earthquake and Volcano Hazards Observation and Research Program (Earthquake and Volcano Hazard Reduction Research)
- MEXT KAKENHI [18K19952]
- Grants-in-Aid for Scientific Research [18K19952] Funding Source: KAKEN
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The tectonic stress field in the aftershock area of the Hokkaido Eastern Iburi earthquake is predominantly characterized by reverse faulting and strike-slip faulting, with the P-axis being close to horizontal and the T-axis more vertical. The stress inversion method applied to focal mechanism solutions revealed that a reverse fault-type stress field is dominant in the aftershock area, with the maximum principal stress axis dipping eastward and the intermediate principal stress axis dipping southward, indicating a predominantly horizontal stress distribution.
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 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.
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