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

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 818 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 are 78° ± 33° and 357° ± 52°, respectively, and (3) the average plunge of P- and T-axes are 25 ° ± 16° and 46° ± 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 73° ± 8° and the dipping eastward of 17° ± 6° and an axis of the intermediate principal stress ( σ 2 ) has the trend of 126° ± 91° and the dipping southward of 16° ± 13°, 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 64° ± 9° 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 0.6 ± 0.2 in the whole aftershock area. Although not statistically significant, we suggested that R decreases systematically as the depth is getting deep, which is modeled by a quadratic polynomial of depth.