Seafloor Geodetic Observations West off Miyake-jima Island During January to April, 2001

An intensive earthquake swarm started under Miyake-jima Island, 180km south off Honshu, Japan, on June 26, 2000. The earthquake swarm migrated towards northwest off from Miyake-jima Island, where numerous earthquakes, more than 100,000, were detected within about two months and an extensive crustal deformation was observed by on-land geodetic observations. We started seafloor geodetic observation in this area to monitor seafloor deformation for the better understandings of underground magmatic activities. This poster presents summary of the observations and preliminary results from them. IIS has been developing a method of seafloor geodesy in corporation with JHD. A combination of kinematic GPS measurements and precise acoustic ranging techniques is employed to achieve centimeter-level seafloor geodesy. First observation site using the method was Kumano trough, where the Philippine Sea Plate subducts beneath Japan Islands arc. It was confirmed that the method could locate horizontal position of the seafloor reference points within 4 cm standard deviation ( Asada and Yabuki, 2001). We apply this seafloor positioning method to the observations conducted in the area west off Miyake-jima Island. Three seafloor reference systems ( Stations A, B, and C ), which consist of three or four acoustic mirror transponders, were built in triangle area surrounded by three islands, Miyake-jima, Nii-jima and Koudu-jima Islands, in November and December, 2000. This area would be deformed remarkably due to underground magma movement including magma injections from deeper part. Distances among three reference systems were set about 15 km. Stations A and B were located on the two sides of NW-SE trending seismically active area. The observations have been conducted three times until present, in January, February and April 2001. We obtained less data than we had expected due to bad sea condition in January and February observations. Also, a fast and quickly varying ocean current prevented us from keeping ship lines as they were planned at the observation in April. Although the amount and quality of the data might be less and worse than projected, analyses are going on by improving software suitable for each data set in order to extract as much information as possible from available data.