Recent Ground Deformation at Kuchierabujima Volcano

GPS and precise leveling surveys have been repeated at Kuchierabujima since 1995. GPS benchmarks are installed around the summit crater and at the flank of the volcano. Continuous GPS station is located at the northwest flank.Leveling route is developed at the western flank with 8km length. Horizontal displacements during the period from 1995/96 to 2000 by GPS measurements show radial outward pattern from the summit crater. The horizontal displacements are 2-4 cm near the summit crater and 1-3 cm at the flank. Vertical displacements detected by the precise leveling are less than 5mm during the period from 1996 to 2001. Although the vertical displacements are much smaller than horizontal ones, it seems that the ground toward to the summit crater was inflated.Both of horizontal and vertical deformation occurred during the period from 1995/96 to 1999 and no remarkable deformation was not detected after 2000. The seismicity at Kuchierabujima volcano increased in August 1999. The ground deformation maybe related with increase in seismicity.Location of a pressure source inducing the ground deformation is estimated based on Mogi's model, using horizontal displacement vector from 1995/96 to 2000. It is located 500m east of the summit crater at a shallow depth of 100m beneath the sea level.Volume increase in the source is estimated to be 1.7x105m3. Tensile crack is an alternative model to explain the ground deformation as inferred from the topography of a north-south fissure east of the summit crater. The tensile crack is similarly located 500m east of the summit crater at a shallow depth of 500m beneath the sea level. Strike is north-south and the dip angle of the crack is 30ﾟ. Volume increase in the source is estimated to be 4. 1x105m3. The location of the pressure source coincides with that of geomagnetic anomaly detected by airborne survey.The ground deformation maybe induced by the inflation of hydrothermal reservoir at a shallow depth east of the summit crater.