Seismically Inferred Rupture Process of the 2011 Tohoku-Oki Earthquake by Using Data-Validated 3D and 2.5D Green's Tensor Waveforms

Wednesday, 17 December 2014
Taro Okamoto1, Hiroshi Takenaka2, Tatsuhiko Hara3, Takeshi Nakamura4 and Takayuki Aoki1, (1)Tokyo Institute of Technology, Tokyo, Japan, (2)Okayama University, Okayama, Japan, (3)Building Research Institute, Tsukuba, Japan, (4)Japan Agency for Marine–Earth Science and Technology, Yokohama, Japan
We analyze seismic rupture process of the March 11, 2011 Tohoku-Oki earthquake (GCMT Mw9.1) by using a non-linear multi-time-window waveform inversion method. We incorporate the effect of the near-source laterally heterogeneous structure on the synthetic Green's tensor waveforms; otherwise the analysis may result in erroneous solutions [1]. To increase the resolution we use teleseismic and strong-motion seismograms jointly because the one-sided distribution of strong-motion station may cause reduced resolution near the trench axis [2]. We use a 2.5D FDM [3] for teleseismic P-waves and a full 3D FDM that incorporates topography, oceanic water layer, 3D heterogeneity and attenuation for strong-motions [4]. We apply multi-GPU acceleration by using the TSUBAME supercomputer in Tokyo Institute of Technology [5]. We "validated" the Greens tensor waveforms with a point-source moment tensor inversion analysis for a small (Mw5.8) shallow event: we confirm the observed waveforms are reproduced well with the synthetics.

The inferred slip distribution using the 2.5D and 3D Green's functions has large slips (max. 37 m) near the hypocenter and small slips near the trench (figure). Also an isolated slip region is identified close to Fukushima prefecture. These features are similar to those obtained by our preliminary study [4]. The land-ward large slips and trench-ward small slips have also been reported by [2]. It is remarkable that we confirmed these features by using data-validated Green's functions. On the other hand very large slips are inferred close to the trench when we apply 1D Greens functions that do not incorporate the lateral heterogeneity. Our result suggests the trench-ward large deformation that caused large tsunamis did not radiate strong seismic waves. Very slow slips (e.g., the tsunami earthquake), delayed slips and anelastic deformation are among the candidates of the physical processes of the deformation.

[1] Okamoto and Takenaka, EPS, 61, e17-e20, 2009.

[2] Yokota et al., GRL, doi:10.1029/2011GL050098, 2011.

[3] Takenaka and Okamoto, in Seismic Waves, Intech, 2012.

[4] Okamoto et al., 2012.http://www.jaee.gr.jp/event/seminar2012/eqsympo/pdf/papers/94.pdf

[5] Okamoto et al,2013. http://link.springer.com/chapter/10.1007/978-3-642-16405-7_24#page-1