Heavy rains inflate Mount Fuji, central Japan

Active volcanoes are often deformed by magmatic activity occurring at depth. Here, we report the deformation of Mount Fuji, central Japan, in relation to hydrological activity induced by rain. Through an analysis of the daily coordinates of global navigation satellite system stations deployed around Mount Fuji, we detected transient surface uplift of 1−2 cm correlated with heavy rains and deduced that this is 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 of the summit. The uplift gradually decays with distance and is replaced by large-area subsidence through rainwater loading beyond the end of these lava layers. Understanding such “cold” volcanic inflation assists in the correct interpretation of “hot” changes associated with magmatic forcing mechanisms.