Uplift and subsidence by heavy rains: Hydrogeodesy of Mt. Fuji, Japan

Active volcanoes often deform by magmatic activities at depth. Here we report that they deform also by hydrological activities induced by rains. By analyzing the daily coordinates of global navigation satellite system stations deployed around the Fuji volcano, the highest mountain of the country in central Japan, we detected transient surface uplift of 1-2 centimeters correlated with heavy rains. We consider they were caused by the expansion of shallow aquifers within Shin-Fuji lava layers. Such hydrological inflation of the volcano, lasting for a day or two, occurs within ~25 km from the summit. The uplift gradually decays with distance and is replaced with large-area subsidence by rainwater loading beyond the end of these lava layers. Subsidence is proportional to daily rains, rather than cumulative rains, suggesting dynamic equilibrium of precipitation and run-off. Understanding such ‘cold’ deformation of active volcanoes would help us correctly interpret ‘hot’ ones by magmatic activities.