Global positioning system measurements of ground deformation caused by magma intrusion and lava discharge: the 1990-1995 eruption at Unzendake volcano, Kyushu, Japan

Global positioning system (GPS) measurements made around Unzendake volcano, Kyushu, Japan, since January 1991 have detected ground deformation caused by magma intrusion and lava discharge. In the intermittent phreatic and phr eatomagmatic eruption stage, the ground was inflating. After growth of the lava dome and following frequent pyroclastic flows at Unzendake volcano, th e ground began deflating. These ground deformations are explained by the in flation and deflation of a Mogi's source model (a point source model) locat ed about 6 km west of the active crater at a depth of 11 km, at an aseismic region. The observed horizontal displacement vectors pointed radially away from the estimated pressure source during inflation and pointed to the pre ssure source during deflation. The horizontal displacements at the referenc e GPS station calculated from contraction of the estimated pressure source coincide well with the actual horizontal displacements observed from other GPS baseline systems. These observations validate our estimates for the pre ssure source. Based on the relation between the deformation volume of the g round surface and the discharged volume of the lava, it is estimated that d uring the eruption there was magma supply from the deeper portion as well a s magma discharge at the crater. Magma is estimated to be supplied to the r eservoir at an average rate of 1.1x10(5) m(3)/day; magma intrusion began in December 1989 at the latest and continued for 1.9 x 10(3) days. (C) 1999 E lsevier Science B.V. All rights reserved,.