Seismic velocity structure of Unzen Volcano, Japan, and relationship to the magma ascent route during eruptions in 1990-1995

Subsurface structures may control the migration of magma beneath a volcano. We used high-resolution seismic tomography to image a low- P-wave velocity (Vp) zone beneath Unzen Volcano, Japan, at depths of 3-16 km beneath sea level. The top of this low-Vp zone is located beneath Mt. Fugendake of Unzen volcano, which emitted 0.21 km(3) of dacitic magma as lava domes and pyroclastic flows during eruptions in 1990-1995. Based on hypocenter migrations prior to the 1990-1995 eruptions and modeled pressure source locations for recorded crustal deformation, we conclude that the magma for the 1990-1995 eruptions migrated obliquely upward along the top of the low-Vp zone. As tectonic earthquakes occurred above the deeper part of the low-Vp zone, the deep low-Vp zone is interpreted to be a high-temperature region (> 400 degrees C) overlying the brittle-ductile transition. By further considering Vs and Vp/Vs structures, we suggest that the deeper part of the low-Vp zone constitutes a highly crystalized magma-mush reservoir, and the shallower part a volatile-rich zone.