津村 紀子

A seismic array observation across the slow-earthquake source region in the eastern Kii Peninsula, southwest Japan, reveal the detailed structure and seismicity along the upper boundary of the subducted Philippine Sea Plate. Tomography analysis and earthquake reflection imaging clarify the geometry of the upper boundary of the Philippine Sea Plate with a subduction angle of similar to 15 degrees at a depth range of 22-30 km. We observe intraslab low-frequency earthquakes (LFEs) in and around the low-Vp and high-Vp/Vs zones, which are located above the intraslab earthquakes that are controlled by the dehydration embrittlement of serpentine. The intraslab LFE activity may be related to fluid movement. Fluids, which are derived from both dehydration of the serpentinized oceanic mantle and crustal eclogitization, may control the fault slip behavior. Therefore, the fluid behavior around the subducting plate boundary has an important role in controlling the downdip limit of the seismogenic zone.

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.

In the original publication of this article (Iwasaki et al. 2019), the author name ‘Matsubara Makoto’ in author list is not correct. The correct name should be ‘Makoto Matsubara’.