Observation of Sea-floor Deformation in Tokai-Nankai Region, Japan

We have developed an observation system for observing sea-floor crustal deformation. The observation system is composed of 1) acoustic measurement between a ship transducer and sea-floor transponders, and 2) kinematic GPS positioning of the observation vessel. A sea-floor positioning test has revealed that our system can obtain the horizontal location of a sea-floor station within 5-6 cm (95-% confidence interval) [Tadokoro et al., 2001].

\ \ We installed transponder networks at Tokai region (Suguga trough), and Nankai trough, Japan, with water depths of 1000-2200 m. The transponders are set in a 13-inchs glass sphere, and equipped batteries for five-years-measurement. Large subduction earthquakes are expected to occur in these regions during early this century. It is necessary to monitor spatial distribution of coupling regions and their temporal changes for predicting these earthquakes and disaster prevention. The sea-floor transponder network is expected to be a useful tool to accomplish them. Each network is composed of two to four transponder arrays. We plan to monitor sea-floor deformation in the regions five years.

\ \ The GPS positioning causes the major error in our whole system. An experiment on kinematic GPS positioning with several base lines and a moving antenna shows that larger base lines, especially for those about 50 km or longer, causes larger GPS positioning error [Sato et al., 2001].

\ \ Temporal and spatial variations of sound speed structure in seawater is also a possibility of error source. We repeatedly measured sound speeds in regions of about 4 square-miles by using CTD profilers. The results are as follows:\1) The long-term (seasonal and annual) change in sound speed is up to 15 m/s (1 %) in a portion shallower than about 600 m.\2) The short-term (within several hours to one day) change is 3-5 m/s at several layers.

\ \ We also performed CTD profiler measurements with two vessels so that we could detect correct lateral variations of sound speed. The distances between the two vessels are 2, 1, 0.6, 0.3, and 0.15 miles. The two vessels were located on lines parallel and perpendicular to the orientation of ship drift.

\ \ We also performed CTD profiler measurements with two vessels so that we could detect correct lateral variations of sound speed. The distances between the two vessels are 2, 1, 0.6, 0.3, and 0.15 miles. The two vessels were located on lines parallel and perpendicular to the orientation of ship drift.