Seismic potential around the 2018 Hokkaido Eastern Iburi earthquake assessed considering the viscoelastic relaxation

Abstract

The 2018 Mj 6.7 Hokkaido Eastern Iburi earthquake (Iburi earthquake) occurred near the eastern boundary fault zone of the Ishikari lowlands, which is composed of a northern and southern fault. Aftershock distribution suggests the existence of a previously unknown fault (the Shallow Iburi (SI) fault) at a shallower extension of the Iburi earthquake fault. In the present study, we examined the seismic potential of the northern, southern, and SI faults based on seismological analysis and numerical simulations. The aftershock focal mechanisms infer the present-day stress field that is characterized by an ENE–SWS compression around the target faults. Slip tendency analysis shows that all target faults originally have high slip potential under the estimated stress field. We, therefore, evaluated earthquake occurrence potential on the target faults influenced by the Iburi earthquake based on the Coulomb stress (ΔCFF). We consider postseismic viscoelastic deformation in the viscoelastic medium with a three-dimensional structure. The present paper shows one possible scenario based on a model incorporating information currently available. The most of the entire SI fault was brought closer to rupture just after the earthquake, indicated by the positive ΔCFF, which is consistent with the activation of seismicity in this region. The ΔCFF continues to increase over many years after the earthquake, which may imply a growing risk of seismic hazards along the SI fault. The distribution of the ΔCFF along the southern fault and the southern half of the northern fault is characterized by a similar depth-dependent pattern. The faults were brought closer to rupture just after the earthquake, indicated by the positive ΔCFF, except for mid-depths along the faults. Then, the ΔCFF increases at shallow depths for a few decades after the earthquake, which suggests a continuous build-up of stress. Although the ΔCFF decreases at the deep depths for a few decades after the earthquake, it is insufficient to return the stress level to that before the earthquake. These results suggest that all target faults are in the state of increasing seismic risk after the Iburi earthquake.[Figure not available: see fulltext.].