岩手山および三ッ石山周辺の1998年〜2002年の山体変動について

抄録

Ground deformation around Iwate and Mitsuishi-yama volcanoes was monitored by Electro-optical distance measurement (EDM), global positioning system (GPS), and leveling from 1998 to 2002. Repeated GPS observation data, which showed a four to six centimeter eastward movement of the summit of Iwate volcano in 1998 and 1999, are concordant with the interferometry SAR analysis by the NASDA, which showed the expansion around the Mitsuishi-yama in 1998. The leveling on the north side of Iwate and Mitsuishi-yama volcanoes also showed the upheaval around the Mitsuishi-yama between 1998 and 1999, and depression between 1999 and 2000. The automatic continuous GPS observation and data transfer system was developed in this study, and the data showed the depression of the Mitsuishi-yama area from 2000 to 2002. The direction of the depression was not straight downward, but the south or south-south-east at the rate of one to three centimeters a year. According to the extent of the deformation area, their source is thought to be at a depth around eight kilometers. A higher temperature granitic body of five hundred degrees centigrade or more has been already detected directly by drilling in the Kakkonda geothermal field, just south of Mitsuishi-yama. The addition of new magma to the granitic body or separation of gas from the magma possibly caused the expansion in this area in 1998 and 1999, but it may be difficult to explain the depression of this area from 2000 to 2002.