Abstract
The source rupture process of the 2018 Hokkaido Eastern Iburi earthquake was investigated by performing a joint inversion analysis using the strong motion and geodetic data. A fault model that consists of two fault planes was constructed by considering the relocated aftershock distribution and the focal mechanisms. The inversion result showed that the large slip occurred at approximately 22 km depth, which was much shallower than the hypocentral depth. Our results showed that the rupture initiated on the minor fault plane around the hypocenter and the major fault plane started to rupture 4–6 s after the rupture initiation. Although the shape of the minor fault plane has not been clearly determined, the major fault plane appears to be high-angle, east-dipping. An additional inversion with only the major fault plane showed that the strong ground motion near the source was mainly generated from the major fault plane. The total seismic moment estimated by the inversion with the two fault-plane model was 1.1 × 1019 Nm, which yielded an Mw of 6.6. Using the inversion result of the two fault-plane model, we simulated the ground surface and borehole waveforms at strong motion station IBUH03 where the large velocity pulse was observed on the ground surface. The simulations suggest that this large velocity pulse was generated from the combination of the large slip of the source and large site amplification of the velocity structure between the ground surface and borehole seismometers.
Highlights
On September 5, 2018, at 18:08 (UTC), an MJMA (Japan Meteorological Agency (JMA) magnitude) 6.7 earthquake occurred in the eastern Iburi region in Hokkaido (Fig. 1)
We excluded a UD component at station IBUH03 whose sensor was broken and a UD component at station 47004 that seemed to contain a large amount of noise; we used 31 components of 11 strong motion stations
On the major fault plane, a maximum slip of 2.9 m was obtained at a depth of around 22 km, which is approximately 19 km shallower than the hypocenter. This large slip region was again located at a similar depth even when Vr is faster than 2.8 km/s (Additional file 1: Figure S2)
Summary
On September 5, 2018, at 18:08 (UTC), an MJMA (Japan Meteorological Agency (JMA) magnitude) 6.7 earthquake occurred in the eastern Iburi region in Hokkaido (Fig. 1). Data and methods We selected 11 strong motion stations from K-NET, KiKnet, and JMA stations considering hypocentral distance and azimuthal distribution (Fig. 1). We constructed an averaged 1D velocity structure model of the area shown in Fig. 1 from the JIVSM and used it for all GEONET stations.
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