Relative Precise Gravity Survey at Izu-Oshima Volcano in the Eruption Preparation Period

Abstract

   In order to investigate the magma accumulation process of Izu-Oshima Volcano in the eruption preparation period, we conducted 14 relative microgravity surveys from 2004 to 2009. The gravity changes tend to decrease near the northern margin of the summit caldera, and rates of negative gravity changes reached as much as 0.015 mgal/year. Assuming the Mogi model, we estimated the pressure increase at a depth of 3.65 km with increasing volume of 7.6 × 10⁶ m³/year, which could account for the observed data. However, the uplift rate calculated with the estimated increasing volume would be five times as large as the observed uplift rate by the GPS network, so these estimated parameters cannot explain both gravity changes and uplift changes simultaneously.
   Izu-Oshima Island could be a volcano having an extraordinary magma accumulation system that cannot be expressed as a simple physical model. More accurate microgravity survey with the uplift observation should be conducted in the future to clarify the magma accumulation system of this complicated volcano.