Seafloor crustal movement observed off Miyagi after the 2011 Tohoku Earthquake using GPS-Acoustic observation system

Large interplate earthquake repeatedly occurred in Japan Trench. Recently, the detail crustal deformation revealed by the nation-wide inland GPS network called as GEONET by GSI. However, the maximum displacement region for interplate earthquake is mainly located offshore region. Based on this background, we has been developed a GPS-Acoustic observation (GPS-A) system for the seafloor crustal deformation monitoring. A major earthquake struck in Japan Trench on March 11, 2011, named as the "2011 off the Pacific coast of Tohoku earthquake" (here after 2011 Tohoku earthquake). Kido et al (2011) investigated the coseismic seafloor deformation by the GPS-A system. They reported the 15 and 31m coseismic displacements in GJT4, and GJT3, respectively. Both sites moved toward ESE direction, which is opposite direction of the plate subduction. These results suggested the heterogeneity of the coseismic slip distribution in the plate interface [e.g. Iinuma et al. 2012]. After such large earthquake, the large postseismic deformation is also expected which may be caused by the afterslip for short (~several year) time period. The spatial coverage of the GPS/A sites is still not enough because of we have only two sites in and around the focal area. For more detail information for the postseismic deformation, we has been deployed one more GPS/A site (GJT1) from 2011 May. We observed 4 times for the GJT3 for postseismic deformation in 2011 April, August, October, and 2012 July. As the result of 3 campaigns, we obtained 1.2 m displacement toward the ENE direction in GJT3. It is clearly larger than the displacement expected from the postseismic slip inferred from inland GEONET time series (Ozawa et al., 2012). It suggests that the estimated postseismic slip based on the inland GPS data may underestimate the actual postseismic slip amount in the offshore region. Iinuma et al. (this meeting) constructed a postseismic slip distribution using onshore and seafloor displacement (GPS/Acoustic observations, pressure gauges). They pointed out the large postseismic on the shallow part of plate boundary. We discuss the postseismic slip between 2011 April and 2012 July in the meeting.