11:15 AM - 11:30 AM
[SVC31-09] Reconstruction of past phreatic eruptions in Midagahara volcano
Keywords:phreatic eruption, Midagahara volcano, explosion crater
Midagahara volcano has had repeated phreatic eruptions over the past 40,000 years. Though eruptions are identified from air-fall tephra, little is known about what happened in these eruptions. Understanding past eruptions is essential for preparing for future eruptions. We reconstruct past phreatic eruptions in Midagahara volcano from the reported erupted volume. We assume that the erupted volume is equal to the volume of an explosion crater to estimate its diameter and depth. From the diameter, we estimate the explosion energy through the scaling law of explosion craters. From the depth, we estimate the temperature of water just before the explosion. Assuming that the explosion energy is equal to the decrease in the internal energy of water, we estimate the volume of water. The eruption volumes of 3×104 m3 (small eruption) and 3×106 m3 (large eruption) respectively require the temperature of 170ºC and 250ºC and the volume of 2.1×104 m3 and 1.8×106 m3 of water. Considering the extremely low-density body (< 2.1 g/cm3) at 4-8 km depth beneath the Hida mountains that is detected with geophysical observations, we assume a hydrothermal reservoir at 4 km depth and conduct numerical experiments on the ascent of a water-filled crack from the reservoir. The reservoir is assumed to be in an oblate spheroidal shape with the minor axis in the vertical direction. When the initial length of a crack is 1 m, the condition for the crack propagation requires the length of major axis of 1 km and 4 km for small and large eruptions, respectively. The ascent time is estimated to be 2 hours and 15 minutes for small and large eruptions. The volume of reservoir controls the magnitude of eruption. Understanding reservoirs beneath Midagahara volcano is critical for preparing future eruptions.