Anomalous Groundwater-level Changes in Date City Associated with the 2000 Eruption of Usu Volcano, Japan(Special Section：The 2000 Eruption of Usu Volcano)

Abstract

Remarkable hydrological anomalies occurred in relation to the 2000 eruption of Usu volcano, Japan. The groundwater level at DT1 in Date City rose more than 4 m, and the groundwater discharged one day before the eruption. The groundwater level at DT2 rose 0.95 m before the eruption. Based on the well's response to tide, the rises of groundwater level are corresponded to the crustal strain changes caused by a spherical inflation source with a volumetric change of 0.3-1.7×10⁷ m³ (Matsumoto et al., 2002). The depth of the spherical source on March 29 is estimated at above 4.3 km by using the ratio of groundwater-level changes at DT1 to DT2. Assuming a vertical line source with the bottom depth of 10 km, the depth of the top on March 29 is estimated to be above 1.6 km. The ratio of groundwater-level changes gradually increased before the eruption, which might show the rising process of the magma.