Abstract
The 2016 Kumamoto earthquake, including two large (Mw ≥ 6.0) foreshocks and an Mw 7.0 mainshock, occurred in the Hinagu and Futagawa fault zones in the middle of Kyushu island, Japan. Here, we obtain the complex coseismic deformation field associated with this earthquake from Advanced Land Observation Satellite-2 (ALOS-2) and Sentinel-1A Interferometric Synthetic Aperture Radar (InSAR) data. These InSAR data, in combination with available Global Positioning System (GPS) data, are then used to determine an optimal four-segment fault geometry with the jRi method, which considers both data misfit and the perturbation error from data noise. Our preferred slip distribution model indicates that the rupture is dominated by right-lateral strike-slip, with a significant normal slip component. The largest asperity is located on the northern segment of the Futagawa fault, with a maximum slip of 5.6 m at a 5–6 km depth. The estimated shallow slips along the Futagawa fault and northern Hinagu fault are consistent with the displacements of surface ruptures from the field investigation, suggesting a shallow slip deficit. The total geodetic moment release is estimated to be 4.89 × 1019 Nm (Mw 7.09), which is slightly larger than seismological estimates. The calculated static Coulomb stress changes induced by the preferred slip distribution model cannot completely explain the spatial distribution of aftershocks. Sensitivity analysis of Coulomb stress change implies that aftershocks in the stress shadow area may be driven by aseismic creep or triggered by dynamic stress transfer, requiring further investigation.
Highlights
In April 2016, a series of shallow earthquakes struck Kumamoto prefecture of Kyushu island, southwest Japan, causing more than 100 fatalities, more than 1000 injuries, and severe destruction to houses and infrastructure
The surface ruptures, active fault traces, Interferometric Synthetic Aperture Radar (InSAR) deformation maps, and seismicity indicate that multi-segment fault planes ruptured during the earthquake
In order to avoid the effect of the imprecise estimation of the slip distribution on the fault planes during searching for optimal dip angles, each fault segment was further discretized into 2 km × 2 km sub-faults
Summary
In April 2016, a series of shallow earthquakes struck Kumamoto prefecture of Kyushu island, southwest Japan, causing more than 100 fatalities, more than 1000 injuries, and severe destruction to houses and infrastructure. Moment Tensor (GCMT) [4], United States Geological Survey (USGS) [5], and National Research This sequence produced approximately 34 km-long surface ruptures, mainly along the Futagawa fault zone and its eastern extension to the caldera of the Aso volcano, and the northernmost part of the Hinagu fault zone [7]. The Futagawa and Hinagu fault zones belong to part of the western extension of the Median Tectonic Line (MTL), known as the longest right-lateral strike-slip active fault in Japan These fault zones are located within the Beppu–Shimabara graben, which is dominantly controlled by north–south extension and some northeast–southwest shear modifications from MTL [8]. Yue et al [3] performed the joint inversion of GPS, strong-motion, Synthetic Aperture Radar (SAR) images, and surface offset data with a curved multi-segment fault model Their results showed the maximum slip exceeding 10 m, which is rare for an. The Coulomb stress change calculation and its sensitivity analysis are carried out to investigate the possible triggering mechanisms
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