Focal mechanisms and the stress field in the aftershock area of the 2018 Hokkaido Eastern Iburi earthquake (MJMA\u2009=\u20096.7)

Abstract

The tectonic stress field was investigated in and around the aftershock area of the Hokkaido Eastern Iburi earthquake ( M JMA \u2009=\u20096.7) occurred on 6 September 2018. We deployed 26 temporary seismic stations in the aftershock area for approximately 2\xa0months 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°\u2009±\u200933° and 352°\u2009±\u200951°, respectively, and (3) the average plunge of P- and T-axes is 25°\u2009±\u200916° and 44°\u2009±\u200920°, 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°\u2009±\u20097° and the dipping eastward of 19°\u2009±\u20094° and an axis of the intermediate principal stress ( σ 2 ) has the trend of 131°\u2009±\u200973° and the dipping southward of 10°\u2009±\u20099°, 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°\u2009±\u20096° 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 \u2009=\u2009( σ 1 \xa0−\xa0 σ 2 )/( σ 1 \xa0−\xa0 σ 3 ) is 0.61\u2009±\u20090.13 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.