Crustal Deformation Associated With the 2000 Eruption and Degassing Process of Miyakejima, Izu Islands, Japan

Miyakejima is located in the northern part of the Izu Islands lying along the boundary between the Pacific plate and the Philippine Sea plate. Miyakejima volcano erupted on Miyakejima is located in the northern part of the Izu Islands which are a chain of volcanoes lying along the boundary between the Pacific plate and the Philippine Sea plate. Miyakejima volcano erupted on June 27, 2000 after the quiescence of 17 years. First eruption is a small submarine eruption 1.5km off the western coast of Miyakejima. Subsequently, several summit eruptions as tephra ejecta occurred in July and August 2000. The summit collapsed just after the first summit eruption and a caldera was formed for 40 days. Collapsed volume and erupted volume are estimated to be 0.6km³ and 0.02km³, respectively. In September 2000, the collapse caldera started emitting a large amount of volcanic gasses. A peak amount of degassing SO₂ is ~70000 ton/day in the period from October to December 2000. Amount of volcanic gas is decreasing gradually and is 15000 ton/day (SO₂ ) now. However, it is still larger than other active volcanoes. Permanent GPS data reveals the spatial pattern and time evolution of ground deformation. Inflation of Miyakejima was observed by continuous GPS and leveling before the 2000 eruption. The observed displacements associated with the 2000 eruption show radial pattern suggesting shrinking of the island and subsidence. This pattern continues for 14 months from July 2000. Though the rate of crustal deformation is almost constant from July to August 2000, it is decreasing exponentially with a time constant of ~150days from September 2000. We assumed a point deflation source and inverted the observed displacement to estimate parameters of the point source. Volume decrease and depth of the deflation source is 0.12km³ and 4.2km from July to August 2000. We interpret that it is the squeezing of magma from a magma chamber of Miyakejima volcano. The displacement observed in neighbor islands suggests that the squeezed magma and collapsed material of Miyakejima migrated northwestward by as much as 30km in the form of a dike intrusion. Volume decrease and depth of the deflation source is 2.9km and 0.016km³ from September 2000 to May 2001. Decreasing rate of the deflation source is an order of magnitude smaller than that of the previous period. We considered that degassing from magma mainly causes the deflation of magma chamber after September 2000. The decreasing rate estimated from crustal deformation was quantitatively consistent with that estimated from the rate of the emitting volcanic gasses (Kazahaya, personal communication). This consideration suggests that the magma does not migrate any more and that the magma chamber is in closed system. However, recent rate of the deformation is much smaller than that calculated from the degassing rate. This implies that new magma is supplied to the magma chamber from deeper region or that the depth of magma head becomes deeper in a conduit from the magma chamber to the caldera.