Seafloor Geodetic Observations Around Japan

A0173 Masashi Mochizuki Institute of Industrial Science, University of Tokyo

A0174 Mariko Sato Hydrographic Department, Japan Coast Guard

A0175 Zengo Yoshida Institute of Industrial Science, University of Tokyo

A0176 Tetsuichiro Yabuki Hydrographic Department, Japan Coast Guard

A0177 Akira Asada Institute of Industrial Science, University of Tokyo


Space geodetic techniques have been often used to monitor crustal

deformations on land in this decade. Especially, continuous and dense

geodetic observation networks using Global Positioning System ( GPS )

have achieved the monitoring of crustal movements with centimeter-

level accuracy. They have been playing an important roll in the

earthquake prediction program in Japan. A lot of huge earthquakes

occurred repeatedly around Japan islands beneathocean floor where the

Pacific and the Philippine Sea oceanic plates subduct beneath Japan

islands arc. Geodetic method, which employs not only on-land

observations but also sea floor in situ observations, is desired in

order to understand the mechanism of interplate earthquakes at

convergent plate boundaries in detail. We report on a new seafloor

geodetic observation system which we have been developing. Trials of

monitoring of seafloor crustal deformation in forearc area of Japan

islands arc are also reported in this presentation.

Institute of industrial science, University of Tokyo, has been

developing a method of seafloor geodesy in corporation with Japan

Hydrographic Department. 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 expanded our

observation using the sea floor positioning method to the forearc

areas along Japan island arc in order to monitor how subsea forearc

crust deforms as oceanic plates subduct.

The seafloor reference systems, which consist of three or four

acoustic mirror transponders, were set at nine areas including Kumano

trough on forearc areas of Japan island arc until present. The

seafloor reference system is designed to operate for about five years.

Measurements will be done repeatedly, once or twice a year, for each

seafloor reference system during its lifetime. We already visited all

nine sites at least once to perform the measurement. Occasionally, we

obtained less data than expected due to bad sea condition. Also, a

fast and quickly varying ocean current prevented us from keeping ship

lines as they were planned at the observations. 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