GPS/Acoustic Observations Along the Japan Trench for Postseismic Deformation After the 2011 Tohoku-Oki Earthquake

After the 2011 Tohoku-Oki Earthquake, we realized that the interplate coupling can be extended to the trench even for subducting slab of great age. Monitoring the recovering process of the coupling after the earthquake may provide important knowledge on the total perspective of this great earthquake. For this purpose, Japanese government decided to strengthen the monitoring systems by means of seafloor geodesy. Constructing a GPS/Acoustic network along the trench is one of these projects. Before the earthquake, only five (Japan Coast Guard) and three (Tohoku Univ.) benchmarks were working off-Tohoku area, however, in 2012, we have newly installed up to 20 benchmarks along the trench, especially close to the trench. We took place GPS/Acoustic surveys after the installation and have carried out four times of campaign surveys until 2013 (FY). Not all the benchmarks were measured in each campaign, however, three times of campaigns were made for most of the benchmarks ranging 1 or 1.5 years. Because of such a short period of data accumulation and hard conditions on the accuracy due to great depth (>5000 m) than before, the estimated errors in the obtained displacement vectors are typically 5-10 cm for most of the benchmarks, which are not enough to clearly address the postseismic deformation. Nevertheless, some benchmarks, one is at relatively close to the coast and the other is on the incoming Pacific plate off-Miyagi, show significant WNW-ward movements up to 17 cm/yr, which is much larger than interseismic subducting rate. This gives a strong constraint to evaluate the viscoelastic relaxation process after the earthquake. More campaign surveys are planned in the next year to accumulate the data for improvement of the total accuracy in the displacement vectors for all the benchmarks. In addition, we are dedicated in improving the analytic technique to reduce error sources, such as acoustic signal processing and sound speed correction to lessen the uncertainty in each campaign data. In the presentation, we will introduce our measured results above and additional benchmarks applying the analytical improvement.