TERRAPUB Earth, Planets and Space

Earth Planets Space, Vol. 64 (No. 12), pp. 1231-1238, 2012

Strain anomalies induced by the 2011 Tohoku Earthquake (Mw 9.0) as observed by a dense GPS network in northeastern Japan

Mako Ohzono1, Yasuo Yabe2, Takeshi Iinuma2,3, Yusaku Ohta2, Satoshi Miura4, Kenji Tachibana2, Toshiya Sato2, and Tomotsugu Demachi2

1Institute of Seismology and Volcanology, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
2Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
3International Research Institute of Disaster Science, Tohoku University, Sendai 980-8578, Japan
4Earthquake Research Institute, the University of Tokyo, Tokyo 113-0032, Japan

(Received December 26, 2011; Revised May 7, 2012; Accepted May 24, 2012; Online published January 28, 2013)

Abstract: We have evaluated an anomalous crustal strain in the Tohoku region, northeastern Japan associated with a step-like stress change induced by the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0). Because the source area of the event was extremely large, the gradient of the observed eastward coseismic displacements that accompanied uniform stress change had a relatively uniform EW extension in northeastern Japan. Accordingly, the deformation anomaly, which is determined by subtracting the predicted displacement in a half-space elastic media from the observed displacement, should reflect the inhomogeneity of the rheology, or stiffness, of the crust. The difference of the EW extension anomaly between the forearc and backarc regions possibly indicates a dissimilarity of stiffness, depending on the crustal structure of the Tohoku region. The Ou-backbone range—a strain concentration zone in the interseismic period—shows an extension deficit compared with predictions. A low viscosity in the lower crust probably induced a relatively small extension. Meanwhile, the northern part of the Niigata-Kobe tectonic zone, another strain concentration zone, indicates an excess of extensional field. This is probably caused by a low elastic moduli of the thick sedimentation layer. The detection of strain anomalies in the coseismic period enables a new interpretation of the deformation process at strain concentration zones.
Key words: Coseismic strain anomaly, 2011 Tohoku Earthquake, structural heterogeneity, strain concentration zone.

Corresponding author E-mail: m.ohzono@mail.sci.hokudai.ac.jp

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