[J05-1] Crustal dynamics: Multidisciplinary approach to seismogenesis I

[J05-1-02] Spatio-temporal variation of the postseismic deformation of the 2011 off the Pacific coast of Tohoku Earthquake (M9.0) detected by means of terrestrial and seafloor observations

^○Takeshi Iinuma¹, Yusaku Ohta², Satoshi Miura², Jun Muto², Fumiaki Tomita², Motoyuki Kido², Ryota Hino² (1.Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan, 2.Tohoku University, Sendai, Japan)

Clear postseismic deformations are still being detected in terrestrial and seafloor geodetic observations on and around the Japanese Islands. Recently, Tomita et al. [2017, in Review] reported displacement rates at seafloor stations that were newly installed in 2012. We estimated displacement rates at the GNSS continuous stations at the Geospatial Information Authority of Japan and Tohoku University during the period from September 2012 to May 2016 (Period B). The same period in which Tomita et al. [2017] estimated displacement rates at the seafloor stations. Taking a trench-normal profile that runs through the main rupture area of the Tohoku Earthquake, in the forearc region, the trench-normal displacement rates during Period B were as large as one fourth of those during the period from April to December 2011 (Period A). In contrast, the differences between the vertical components in Periods A and B are very clear. The large local subsidence around the Ou Backbone Range observed in Period A had almost vanished in Period B, while the uplift rates in Period B were more than half those in Period A. The low viscosity beneath the Ou Backbone Range hypothesized by Muto et al. [2016] to account for the large local subsidence, also accounts for the rapid decay of local deformation. Because the inland rheological heterogeneity strongly affects the vertical displacement rate field, we estimated the distribution of interplate coupling and postseismic slip based only on horizontal displacement rates and applying Sun et al. [2014]'s model to exclude the effects of viscoelastic relaxation. Preliminary results indicate that postseismic slip occurred at the shallow plate interface off the Fukushima and Ibaraki Prefectures and at the deep portion beneath the Pacific coast of the Iwate Prefecture during Period B, the same as Period A. Estimated back-slip in the main rupture area of the 2011 Tohoku Earthquake indicates interplate coupling at the area has already recovered.