First Observation of Coseismic Seafloor Crustal Deformation due to M7 Class Earthquakes in the Philippine Sea Plate

The Mw7.3 and 7.5 earthquakes (Off Kii-Peninsula Earthquakes) occurred close to the source region of the anticipated Tonankai Trough in September 5, 2004. The focal mechanisms of the two earthquakes have no low angle nodal planes, which shows that the earthquakes are intraplate earthquakes in the Philippine Sea Plate. We observed coseismic horizontal displacement due to the Off Kii-Peninsula Earthquakes by means of a system for observing seafloor crustal deformation, which is the first observation of coseismic seafloor displacement in the world. We have developed a system for observing seafloor crustal deformation. The observation system is composed of 1) acoustic measurement between a ship transducer and sea-bottom transponders, and 2) kinematic GPS positioning of the observation vessel. We have installed a seafloor benchmark close to the epicenters of the Off Kii-Peninsula Earthquakes. The benchmark is composed of three sea-bottom transponders. The location of benchmark is defined as the weight center of the three transponders. We can determine the location of benchmark with an accuracy of about 5 cm at each observation. We have repeatedly measured the seafloor benchmark six times up to now: 1) July 12-16 and 21-22, 2004, 2) November 9-10, 3) January 19, 2005, 4) May 18-20, 5) July 19-20, and 6) August 18-19 and 29-30. The Off Kii-Peninsula Earthquakes occurred during the above monitoring period. The coseismic horizontal displacement of about 21 cm toward SSE was observed at our seafloor benchmark. The displacement is 3.5 times as large as the maximum displacement observed by on land GPS network in Japan, GEONET. The monitoring of seafloor crustal deformation is effective to detect the deformations associated with earthquakes occurring in ocean areas. This study is promoted by "Research Revolution 2002" of Ministry of Education, Culture, Sports, Science and Technology, Japan. We are grateful to the captain and crews of Research Vessel, Asama, of Mie Prefectural Science and Technology Promotion Center, Japan.