Spatiotemporal Evolution of Postseismic Deformation Following the 2011 Tohoku Earthquake in Northeastern Japan Arc

Thursday, 18 December 2014
Satoshi Miura, Yusaku Ohta, Tomotsugu Demachi, Kenji Tachibana and Toshiya Sato, Tohoku University, Graduate School of Science, Sendai, Japan
The coseismic deformation due to the 2011 Tohoku Earthquake (M9.0) is characterized essentially by tremendous east-west extension, which is completely opposite from the strain field in the inter-seismic period in Northeastern Japan Arc. The postseismic deformation is still dominant at present even though more than 3 years have passed. These cataclysms result in the induced seismic activity and the minor unrest of some volcanoes in the arc. The detailed study on the postseismic deformation, therefore, is a key to unravel the physical process ongoing beneath the area.

The continuous GNSS data obtained by the Geospatial Information Authority of Japan (GSI) and Tohoku University were analyzed using the Precise Point Positioning strategy of GIPSY-OASIS II (Zumberge et al., 1997, JGR) to produce daily site coordinates, which were then converted to strain components utilizing the method of Shen et al. (1996, JGR).

Resulting areal strain distribution in 2013 is characterized by the 2 contractile areas: the Pacific coastal area south of 39.5 degrees N, and the Ou Backbone Range (OBR) south of 40 degrees N, while the other area demonstrates expansion. The former may represent the viscoelastic deformation caused by the 2011 event, and the latter may be related to the local structural anomalies, such as the seismic velocity (Nakajima et al., 2001, Tectonophys; Brenguier et al., 2014, Sci.), and the P wave attenuation (Nakajima et al., 2013, JGR). The OBR is also proposed as a strain concentration zone based on the interseismic strain field obtained by GNSS data (Miura et al., 2004, EPS).

Investigating the time evolution of the postseismic strain field, we notice that the coastal contraction decreases with time, while the OBR contraction keeps its magnitude and extends its area in the north-south direction; the OBR contraction was limited between 38 and 39 degrees N in 2011, between 37.7-39.5 degrees N in 2012. These characteristics suggest the difference in the physical process.