Estimation of permeability and degassing depth of Iwodake volcano at Satsuma-Iwojima, Japan, by thermal observation and numerical simulation

Abstract

The thermal activity of the Iwodake volcano at Satsuma-Iwojima, Japan, is characterized by the predominant volcanic gas discharge at the summit crater and heat discharge from the high ground temperature area, which is distributed widely from the summit crater to the mountainside. In this study, the surface temperature distribution was obtained by an airborne survey using an infrared thermal camera to estimate the heat flux from the ground surface to the atmosphere. The heat discharge rate from the high ground temperature area was estimated to be 22 and 84 MW at the summit and the mountainside, respectively. Earlier studies showed that the volcanic gas at the summit crater was of magmatic origin with a discharge rate of 400 kg/s and that the degassing depth was very shallow. The degassing activity of the Iwodake volcano is thought to have continued for more than 800 years. The continuous active degassing at shallow depth probably results in the development of a hydrothermal system within the volcano because the volcanic gas ascending the degassing vent from the top of the magma to the surface is diffused to the surrounding formation. The diffused volcanic gas produces a wide-ranging surface temperature anomaly. The development of such a hydrothermal system was studied using a numerical simulation. The results confirmed that the overall thermal activity of the Iwodake volcano, i.e., the volcanic gas ejection at the summit crater, widely distributed ground temperature anomaly on the mountainside, and hot springs along the coast, could be formed by the degassing activity. Important factors controlling the wide-ranging hydrothermal system are the permeability of the volcanic edifice and the depth of degassing. Simulations indicated that degassing depth from 300 to 500 m below the ground surface, permeability of the mountain edifice above 7 × 10 −14, and vent permeability from 3.8 × 10 −10 to 9.7 × 10 −10 m 2 were suitable conditions for the development of the Iwodake thermal activity.