T43G-07
Postseismic deformation of the 2011 Tohoku-Oki earthquake revealed by repeated GPS/Acoustic observations
Thursday, 17 December 2015: 15:10
104 (Moscone South)
Fumiaki Tomita1, Motoyuki Kido2, Yusaku Ohta1, Ryota Hino1, Yukihito Osada1 and Takeshi Iinuma3, (1)Tohoku University, Graduate School of Science, Sendai, Japan, (2)Tohoku University, International Research Institute of Disaster Science, Sendai, Japan, (3)JAMSTEC Japan Agency for Marine-Earth Science and Technology, Kanagawa, Japan
Abstract:
Afterslip and viscoelastic relaxation are the major mechanisms of postseismic deformations caused by large earthquakes and hence provide us information on the mechanical and rheological properties of subduction zones. A number of studies have investigated the postseismic deformations of past huge earthquakes. For the 2011 Tohoku-Oki earthquake, Sun et al. [2014, Nature] indicated that viscoelastic relaxation is dominant based on their numerical model that explains both off-shore and on-shore geodetic data. However, the spatial extent of their off-shore geodetic data was limited in the area around the main rupture of the Tohoku-Oki earthquake, and was not sufficient to reveal the whole postseismic deformation. To fill the gap in the site distribution, we have constructed 20 GPS/acoustic seafloor observation sites along the Japan Trench in Sep. 2012 and have repeatedly conducted surveys for 2.5 years since then. Estimated positions for individual surveys are weighted by their error and linearly fitted to get a displacement rate of the site. Our results clearly show spatial variation: trench-ward low rates (-5 cm/yr) in the north region of the main rupture zone, land-ward high rates (~10 cm/yr) in the main rupture zone, and trench-ward high rates (5-15 cm/yr) in the south region of the main rupture zone. Our results in the north region and the main rupture zone are roughly explained by the viscoelastic responses expected from the model of Sun et al. [2014]. However, slight systematic difference between them are also seen, which indicates the necessity of minor improvement of the viscoelastic model itself. Meanwhile, the displacements in the south region significantly differ from the viscoelastic response indicating significant contribution of afterslip near the trench. Thus, our observational facts provide new insight to the postseismic deformation process of the 2011 Tohoku-Oki earthquake and will be utilized for improving the existing models.
