Japan Geoscience Union Meeting 2016

Presentation information

International Session (Poster)

Symbol S (Solid Earth Sciences) » S-SS Seismology

[S-SS02] Frontier studies on subduction zone megathrust earthquakes and tsunamis

Tue. May 24, 2016 5:15 PM - 6:30 PM Poster Hall (International Exhibition Hall HALL6)

Convener:*Kyuichi Kanagawa(Graduate School of Science, Chiba University), Demian Saffer(Department of Geosciences, The Pennsylvania State University, USA), Michael Strasser(University of Innsbruck), James Kirkpatrick(McGill University), Shuichi Kodaira(R&D Center for Earthquake and Tsunami Japan Agency for Marine-Earth Science and Technology), Ryota Hino(Graduate School of Science, Tohoku University), Yasuhiro Yamada(Japan Agency for Marine-Earth Science and Technology (JAMSTEC), R&D Center for Ocean Drilling Science (ODS)), Kohtaro Ujiie(Graduate School of Life and Environmental Sciences, University of Tsukuba), Yoshihiro Ito(Disaster Prevention Research Institute, Kyoto University)

5:15 PM - 6:30 PM

[SSS02-P01] Detecting a relative motion across the Japan Trench using precise acoustic ranging

*Ryusuke Yamamoto1, Ryota Hino1, Motoyuki Kido2, Yusaku Ohta1, Tatsuya Kubota1, Fumiaki Tomita1, Kazuaki Ohta3, Yukihito Osada1,4 (1.Graduate School of Science, Tohoku University, 2.International Research Institute of Disaster Science, Tohoku University, 3.Disaster Prevention Research Institute, Kyoto University, 4.GNSS Technologies, Inc.)

Keywords:direct path acoustic ranging, the 2011 Tohoku-oki earthquake, the Japan Trench, postseismic slip, seafloor geodesy

Researchers reported that the 2011 Tohoku-oki earthquake accompanied coseismic slip over 50 m (e.g. Iinuma et al., 2012, JGR). In the Tohoku-oki region, GPS/Acoustic (GPS/A) observation is now ongoing after the Tohoku-oki Eq. Tomita et al. (2015, GRL) revealed the observed acceleration of the Pacific plate can be explained by viscoelastic relaxation modeled by Sun et al. (2014, Nature). However precise nature of localized interplate motion is still unknown only using GPS/A observation. Therefore, we conducted direct path acoustic ranging across the Japan Trench from September 2014 to May 2015. Direct path acoustic ranging can continuously and precisely detect relative motion between a pair of instruments across plate boundary or a fault. In our preliminary examination, precision in 1 cm/yr was achieved (Osada et al., 2014, JpGU).
In the observation, three instruments were deployed across the trench axis at the Miyagi-oki region forming two baselines, 7 km and 10 km, respectively. Acoustic ranging was repeated every 4-hours. Relative ranges can be calculated by multiplying a round trip time of acoustic signal and sound velocity. Sound velocity depends on temperature, pressure and salinity. Then, we concurrently measured in-situ temperature. Pressure was taken from NAO.99Jb tide model (Matsumoto et al., 2000, J. Oceanogr.). Salinity was assumed to be constant because of its stability in deep ocean. Precisions of 2 baselines were less than 2 cm/yr, as same as the pre-observation.
In the observation, we obtained data for about 8 months, and found no relative motion, which indicates subducting rate about 8 cm/yr in global model is not compensated at the trench. Thus, near Miyagi-oki region, postseismic slip does not occur at this moment and is already in the interseismic locking state.
Since September 2015, five instruments were newly installed at the same region for two-years continuous observation. Furthermore, we plan to install additional instruments at Fukushima-oki region, where postseismic slip rate is reported strikingly large (Sun and Wang, 2015, JGR). Integrating these data, it is expected in the near future that deformation pattern along the trench axis would be revealed.