Keywords:the Hokkaido Eastern Iburi earthquake, aftershock distribution, focal mechanism solution
The Hokkaido Eastern Iburi earthquake (MJMA = 6.7) occurred on 6 September 2018 in the Hokkaido-corner region where the Kurile and the northeastern Japan island arcs meet. We relocated aftershocks of this intraplate event immediately after the mainshock by using data from a permanent local seismic network and found that the aftershocks concentrated in depths from 20 to 40 km, which is extraordinarily deep when comparing with other shallow intraplate earthquakes in the inland area of Honshu and Kyushu, Japan. Moreover, we found that the aftershock area consists of three segments. The first one is located in the northern part of the aftershock area, the second one is located in the southern part, and the third one is located in a stepover segment between the first one and the second one. The hypocenter of the mainshock, which is a rupture initiation point, is located in the stepover segment. The centroid moment tensor solution for the mainshock indicates a reverse faulting, whereas the focal mechanism solution determined by using a first motion polarity of P wave indicates a strike-slip faulting. To explain this mismatch qualitatively, we present a model that the rupture started as a small strike-slip faulting in the stepover segment of the aftershock area and it is followed by the large rupture of two reverse faulting in the northern and the southern segments.