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

<jats:title>Abstract</jats:title><jats:p>The tectonic stress field was investigated in and around the aftershock area of the Hokkaido Eastern Iburi earthquake (<jats:italic>M</jats:italic><jats:sub>JMA</jats:sub> = 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 (<jats:italic>σ</jats:italic><jats:sub>1</jats:sub>) has the trend of 72° ± 7° and the dipping eastward of 19° ± 4° and an axis of the intermediate principal stress (<jats:italic>σ</jats:italic><jats:sub>2</jats:sub>) has the trend of 131° ± 73° and the dipping southward of 10° ± 9°, indicating that both of <jats:italic>σ</jats:italic><jats:sub>1</jats:sub>- and <jats:italic>σ</jats:italic><jats:sub>2</jats:sub>-axes are close to be horizontal. An axis of the minimum principal stress (<jats:italic>σ</jats:italic><jats:sub>3</jats:sub>) 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 <jats:italic>R</jats:italic> = (<jats:italic>σ</jats:italic><jats:sub>1</jats:sub> − <jats:italic>σ</jats:italic><jats:sub>2</jats:sub>)/(<jats:italic>σ</jats:italic><jats:sub>1</jats:sub> − <jats:italic>σ</jats:italic><jats:sub>3</jats:sub>) is 0.61 ± 0.13 in the whole aftershock area. Although not statistically significant, we suggest that <jats:italic>R</jats:italic> decreases systematically as the depth is getting deep, which is modeled by a quadratic polynomial of depth.</jats:p>