Soil-water response in a volcanic ash hillslope affected by fissures and microtopographic changes caused by the Kumamoto earthquake, in Japan

We examined soil-water response in a hillslope with multi-layered volcanic soils affected by fissures formed by intense shaking during the 2016 Kumamoto earthquake in Japan. Pressure head and volumetric water content responses in a 6 x 20 m hillslope ridgeline plot were monitored by tensiometers and capacitance-based soil moisture sensors, respectively. The plot contained seven seismic fissures (0.3-0.8 m deep and 0.4-2.2 m wide) formed parallel to the ridgeline that exposed underlying soil layers, while areas with topsoil occurred between the fissures. Among the alternating layers, the andisol (ca. 1.0 m deep) consistently experienced high pressure heads (>-150 cm H2O) and volumetric water contents (>0.58 cm(3) cm(-3)), indicating high water-holding capacity. This andisol was a key layer for storing soil water and causing slow drainage into the deeper matrix, resulting in abrupt increases in volumetric water content during storms. Rainwater directly reaching the bottom of fissures without percolating through the topsoil caused rapid soil-water response compared to the soil layer where water percolated via topsoil. Analysis of in-situ soil water retention curves, recession rates and water storage changes revealed that the second andisol had unique characteristics for retaining water in the hillslope. The findings of this study show that spatial and temporal variability of soil water responses varied associated with earthquake-induced fissure formation and soil water characteristics of multi-layered volcanic soils.