Underground structures associated with horizontal sliding at Uchinomaki hot springs, Kyushu, Japan, during the 2016 Kumamoto earthquake

The 2016 Kumamoto earthquake (Mw 7.0) caused hot springs in the Uchinomaki area of Aso caldera to become dormant. Geodetic and borehole observations have previously demonstrated that the area around the hot springs slid horizontally similar to 2 m to the northwest during the earthquake. However, the subsurface structure in the area has not been investigated and the mechanism of sliding is unclear. To reveal geological structures in and around the hot spring area, we conducted a seismic microtremor survey at 60 sites and used the Extended Spatial Auto Correlation (ESPAC) method to determine surface-wave dispersion curves from the microtremor data. We then derived S-wave velocity profiles by inversion of the dispersion curves and constructed from them a 3D S-wave velocity model to similar to 100 m depth over the hot springs and surrounding areas. New surface fissures (indicative of extension) that opened during the 2016 earthquake correspond to a boundary in the southeast of the study area between modeled lower velocities (to the northwest) and higher velocities (to the southeast). In the central area of the hot springs, where the largest displacement occurred, the 3D model shows a plume-like high-velocity anomaly, indicative of more-competent sediments there. The lowest S-wave velocities (less-competent rocks) are in paddy fields north of the hot spring area. We interpret the above aspects of the 3D velocity model to indicate that during the 2016 earthquake the relatively competent (higher S-wave velocity) sediments in the central area of the hot springs slid northwestward, causing compressional deformation of the less-competent (lower S-wave velocity) sediments in the northern paddy fields and extensional deformation (fissures) southeast of the sliding block. A distinct increase in S-wave velocity at similar to 50 m depth coincides with the depth of a pumice layer in drillcore from the central hot spring area. Shaking during the 2016 earthquake could have caused a sudden increase in pore pressure in this widely distributed porous layer, thus providing a slip plane for the observed horizontal sliding to the northwest.