THE 1707 M8.7 HOEI EARTHQUAKE TRIGGERED THE LARGEST HISTORICAL ERUPTION OF MT. FUJI

Studies in magma-tectonics point to a spatio-temporal correlation between M_w >5.0 earthquakes and volcanic eruptions. Research has focused on static and dynamic triggering of eruptions by changes in stress on and the passage of seismic waves through magmatic systems, respectively. Here, we examine the correlation between two great earthquakes in Japan and Mt. Fuji’s most explosive (VEI 5) historical eruption in 1707, the Hoei eruption. The 1703 M_w 8.2 Genroku earthquake occurred along the Sagami Trough east of Fuji. The 1707 M_w >8.7 Hoei earthquake on the Nankai Trough south of Fuji occurred 49 days prior to the eruption. We modeled the static stress change imparted on the Mt. Fuji magmatic system and dilatational strain due to each earthquake to determine if these mechanisms enhanced the potential for eruption. Seismic and petrologic studies suggest a basaltic magma chamber located around 20 km depth as well as a dacitic chamber at ~8 km depth. It is likely that an andesitic chamber lies somewhere in between, as mixed andesite-dacite lavas were erupted in 1707. The majority of dikes and lateral vents on Mt. Fuji are aligned in a NW-SE direction, so the magmatic system was modeled as a single, vertical dike striking 315˚ and extending from the surface to 20 km so as to connect the magma chambers to each other and the surface. The 1703 Genroku and 1707 Hoei earthquakes were modeled using published source parameters. Our results show that the 1703 earthquake resulted in increased normal stress (i.e., clamping) and compression at the 8 and 20 km magma chamber depths. These results suggest that the increased normal stress and compressional strain produced by the 1703 earthquake did not promote eruption. The 1707 earthquake also resulted in compression of both magma chambers and clamping of the dike by 3.71 bars at 8 km depth. However, the dike was unclamped by 1.15 bars at 20 km depth. We hypothesize that the stress change and compressional strain generated by the 1707 earthquake triggered the eruption of Mt. Fuji by permitting opening of the dike and ascent of basaltic magma to the andesitic and dacitic chambers. The injection of basaltic magma into the more evolved magmatic system encouraged mixing of andesitic and dacitic magmas. This pressurized the magma chamber at 8 km and a Plinian eruption ensued.