Abstract
By interpreting the 2D/3D seismic survey data acquired in the surrounding ocean areas of the Northeast (NE) Japan Arc, we clarified the detailed geological structure and demonstrated that the basic structure in the hanging-wall plate of the subduction system consists of many structural blocks (segments) separated by NW–SE trending large transcurrent faults (strike-slip faults). This structural configuration showed a close relationship with the distribution of foreshocks, mainshock, and aftershocks, coseismic slip models of the 2011 M9.0 Tohoku-Oki megathrust earthquake, coseismic slip area of M-7 class earthquakes, quasi-static slip rates, back slip rate, and seismic tomography images. In addition, the coseismic slip models revealed that the trenchward forearc of the structural blocks between the Offshore Hidaka tectonic line and the Honjo-Sendai tectonic line fitted well with the coseismic slip area of the 2011 Tohoku-Oki earthquake. These findings suggest that the structural blocks bounded by these two tectonic lines slipped rapidly trenchward when the mainshock occurred. The M7 earthquakes were also concentrated along these two tectonic lines, thereby suggesting a close relationship between seismic activity and the inherited geological structure of the overriding plate in the NE Japan forearc.
Highlights
Yoshida et al (2017) reviewed the 2011 Tohoku-Oki earthquake, and suggested that it might have occurred below a segmented upper plate disrupted by large, steep, NW–SE trending transcurrent faults with the fault blocks clamped together by the NE–SW compressional stress arising from the SW indentation of the forearc sliver of the Kuril Arc (Kimura 1986; Acocella et al 2008)
We have revealed the close relationship between the geological structure and the rupture process of the 2011 Tohoku-Oki earthquake
Using offshore 2D/3D seismic survey data and exploratory well data obtained from all over the NE Japan Arc, we created a revised map of major geological structures in the forearc, demonstrating that the basic assemblage consists of segmented structural blocks defined by a series of NW–SE large transcurrent faults
Summary
Yoshida et al (2017) reviewed the 2011 Tohoku-Oki earthquake, and suggested that it might have occurred below a segmented upper plate disrupted by large, steep, NW–SE trending transcurrent faults with the fault blocks clamped together by the NE–SW compressional stress arising from the SW indentation of the forearc sliver of the Kuril Arc (Kimura 1986; Acocella et al 2008). The abovementioned scales suggest that these large transcurrent faults can deeply penetrate the mantle or crosscut through the entire lithosphere This possibility is corroborated by the seismic Q structure and the temperature distribution in the mantle wedge bounded by Faults N to T (Tsumura et al 2000; Nakajima and Hasegawa 2003), the change in the down-dip limit of interplate earthquakes (Uchida and Matsuzawa 2013), and the back slip rate distribution (see Additional file 1: Fig. S8). Two days prior to the mainshock, the largest foreshock (Mw 7.3) and the associated post-seismic slow-slip began at the shallow end of the slab-mantle contact zone around Fault G (Kato et al 2012; Ito et al 2013) This short-term preceding slow-slip broke the strongly coupled plate interface of Block C and propagated southwestward toward the hypocenter of the mainshock of the 2011 Tohoku-Oki earthquake, and led to the M9.0 main rupture (see Fig. 6).
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