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.

中文翻译：

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2018年北海道东部伊武里地震（MJMA=6.7）余震区震源机制及应力场

对 2018 年 9 月 6 日发生的北海道东伊武里地震（M JMA = 6.7）余震区及其周围构造应力场进行了调查。我们在余震区部署了 26 个临时地震台站，历时约 2 个月，精确定位了 1785 次余震。在这些余震中，894次震源机制解是利用北海道大学、日本气象厅（JMA）和日本高灵敏度地震台网（Hi-net）临时观测和永久地震台网的P波初动极性确定的. 我们发现（1）余震区以逆断层和走滑断层为主，（2）P轴和T轴的平均趋势分别为78°±33°和352°±51°， (3) P-轴和T-轴的平均倾角为25°±16°和44°±20°，分别为：P 轴接近水平，T 轴比 P 轴的平均值更垂直。我们将应力反演方法应用于震源机制解，以估计余震区域的应力场。结果发现，余震区以逆断层型应力场为主。最大主应力（ σ 1 ）的轴有72° ± 7°的趋势，向东倾斜19° ± 4°，中间主应力（ σ 2 ）的轴有131° ± 73的趋势。 °和向南倾斜10°±9°，表明σ 1 - 和σ 2 - 轴都接近水平。最小主应力（σ 3 ）的轴向西倾斜67°±6°，接近垂直。结果有力地表明，在日高碰撞带西部边界的余震区，逆断层型应力场作为平均值占主导地位。整个余震区应力比R=(σ1-σ2)/(σ1-σ3)的平均值为0.61±0.13。尽管在统计上不显着，但我们建议 R 随着深度变深而系统地减小，这是由深度的二次多项式建模的。