2018 年北海道胆振東部地震の初期の破壊過程

The rupture process at the beginning stage of the 2018 Hokkaido Eastern Iburi, Japan, earthquake (MJ 6.7) is investigated by analyzing P-wave records from local strong-motion stations. The records show about 3 s of small amplitude arrival (so-called "initial rupture phase") followed by the onset of the main energy release ("main rupture phase"). In this paper we address the issue: where the seismic energy release corresponding to the main rupture phase started at the primary stage of the main rupture, by locating the main rupture onset position. For this aim we applied the 2D and the 3D search methods to estimate the relative location of the onset of the main rupture with respect to the hypocenter, and the time difference between them. The 2D method assumes a plausible initial fault plane and locates the main rupture onset position on the initial fault plane, while the 3D method does not require such assumed fault planes. In the 2D method we employed each of the nodal planes of the first-motion focal mechanism as the initial rupture plane. The 3D method was able to give a better solution than the 2D one. It suggests that the main rupture initiation point might not be on the same fault plane as the hypocenter is. The solution shows that the main rupture onset point is 5.8 km southward and 2.5 km upward from the hypocenter, with the rupture time of 3.3 s from the origin time. It is consistent with the previous studies on the relationship between the magnitude and the initial rupture duration.