10:00 AM - 10:15 AM
[SCG57-05] Structural Heterogeneity in and around the Source Region of the 2018 Hokkaido Eastern Iburi Earthquake, Central Hokkaido, Japan
Keywords:Hokkaido Island, Hokkaido Eastern Iburi Earthquake, Crust and upper mantle structure, Controlled-source seismic expeditions, Seismic tomography, Fold-and-thrust belt
The seismic reflection processing imaged for the first time the NE Japan Arc underthrust eastward beneath the thick sedimentary package. The refraction/wide-angle reflection data were re-interpreted by tomography and ray-tracing methods. The latter analysis with aid of the new structural information from the reflection processing indicate that the crust is very thin (15-23 km) and the Moho depth is 22-30 km. The Pn velocity is a normal value of around 8 km/s. These structural features are well consistent with those in the offshore area of NE Japan, which belongs to the same geological unit. Based on the structural information mentioned above, the aftershock distribution was carefully investigated. Aftershocks relocated by the 3D structure model (Matsubara, 2017) with appropriate station corrections are distributed in a depth range of 7-45 km showing a steep geometry facing to the east. We cannot find any evidence that the aftershock distribution is related with shallow west-verging active faults developed in the fold-and-thrust belt. We think that the high-angle fault of this earthquake was formed by a certain tectonic event before the collision, maybe in Late Oligocene to Middle Miocene. The relocated aftershocks are separated into the upper (7-20 km) and lower (22-45 km) groups. The seismicity gap between them corresponds to the lower crust of our velocity model, probably representing its ductile property. According to our results, more than 80 % of relocated events occurred within the uppermost mantle of our structure model. Possible explanation for this high seismic activity is the cold crust delaminated from the Kuril Arc side, whose existence was is confirmed by a series of seismic reflection profiling in the southern part of the HCZ (Arita et al., 1998; Tsumura et al., 1999; Ito, 2000, 2002) and seismic tomography by Matsubara et al. (2017). This delaminated crustal body prevents the thermal circulation and upward migration of dehydrated fluid to cool the forearc mantle and generate more favorable environment for the brittle failure.