High-resolution image on terminus of fault rupture: relationship with volcanic hydrothermal structure

SUMMARY Earthquake-volcano interactions have been discussed to understand the underlying mechanisms of seismic ruptures or eruptions, yet the involvement of volcanic activity and the environment with fault slip termination remains unclear. Here, we present an unprecedented high-resolution image of fault motions and crustal structure at the rupture terminus in volcanic area from the 2016 Kumamoto earthquake by conducting synthetic aperture radar (SAR) data analysis and gravity inversion. We obtained a 3-D displacement field by applying multiple SAR analysis methods: standard SAR interferometry, split-bandwidth interferometry and pixel offset. We successfully mapped the ground displacements with a high-spatial resolution in the Aso caldera which was located on the eastern extension of the Futagawa fault that was the main source fault of this seismic event. We found that the rupture propagating on the Futagawa fault eastward penetrated into the Aso caldera and was divided into two major fault systems: a right-lateral fault system on the northern side and a left-lateral fault system on the southern side. However, they progressively converged immediately after penetrating into the caldera. A gravity-inferred 3-D density contrast structure revealed that a locally distributed low-density body existed in the shallow part (from the subsurface to a depth of ∼3 km) of the western edge of the caldera. The slip distribution model showed that the slips on the bifurcated faults penetrated into the low-density region and subsequently dissipated. A numerical simulation on 3-D dynamic rupture demonstrated that the low-stress state in the caldera played a role in suppressing the rupture evolution. A thermally activated hydrothermal field has developed in the area where the fault slips were attenuated. We interpret that the hydrothermal system may create conditions favourable for low-stress field, and plastic properties in the hydrothermal environment may facilitate a further decrease in rock brittleness owing to the high temperature, resulting in the terminus of fault rupture.