09:05 〜 09:30
[SCG54-02] Modeling stress field and deformation associated with the Tohoku-Oki earthquake: Understanding the dynamics of the northeastern Japan islands arc-trench system
★Invited Papers
キーワード：東北沖地震、東北日本島弧海溝系、ジオダイナミクス、応力場、余効変動
After the 2011 earthquake off the Pacific coast of Tohoku (Mw9.0) (Tohoku-Oki earthquake) the dynamics of the northeastern Japan islands arc- trench system were investigated, and as a result, our understanding of it was dramatically improved. Important issues investigated include stress changes due to the Tohoku-Oki earthquake and postseismic deformation that occurred after the Tohoku-Oki earthquake. This presentation reviews the research on stress changes and postseismic deformation after the Tohoku-oki earthquake considering the heterogeneous rheological structure of the islands arc-trench system.
Before the Tohoku-Oki earthquake, east-west compression reverse fault-type earthquakes occurred sparsely under the forearc sea area, but after the Tohoku-Oki earthquake, normal fault-type earthquakes occurred frequently. Hasegawa et al. (2011) pointed out that the amount of compressive stress along the subduction zone was small, and the Tohoku-Oki earthquake released the absolute stress. The sources of the stress field in the arc upper plate are the tensile stress field affected by gravity from the topography and density structure, the compressive stress field affected by the frictional force acting on the plate boundary, the bending stress field caused by steady subduction (e.g., Fukahata and Matsu'ura, 2016), and stress fields associated with mantle wedge convection. Sasajima et al. (2018) considered the effects of four sources by subducting of the oceanic plate beneath Northeastern Japan for 100,000 years, considering a realistic nonlinear elasto-visco-plastic material with reference to the temperature, water content distribution, and serpentinization as simulated by Horiuchi and Iwamori (2016). When low frictional strength (5-11MPa) is applied to the shallow brittle region along the plate boundary, a normal fault stress field of east-west tension in the shallow part of the upper plate under the forearc sea is developed, and so is a reverse fault stress field of east-west compression near the plate boundary. The results suggest that the major changes in the main mechanism before and after the Tohoku-Oki earthquake are the suppression of normal fault activity and the promotion of reverse fault activity during inter-plate coupling between megathrust earthquakes. Then, normal fault activity is promoted and reverse fault activity is suppressed due to the stress change resulting from the Tohoku-Oki earthquake.
During the Tohoku-Oki earthquake, upward movement was observed in the east-southeast direction under the forearc sea area, but in the postseismic deformation, subsidence was observed in the west-northwest movement. Sun et al. (2014) showed that the west-northwest movement was due to the viscoelastic relaxation process of the mantle. Numerous detailed analyses of postseismic deformation after the Tohoku-Oki earthquake were performed (Yamagiwa et al., 2015; Muto et al., 2016; Hu et al., 2016; Iinuma et al., 2016; Suito, 2017; Freed et al., 2017; Noda et al., 2018). Agata et al. (2019) performed a large-scale numerical simulation of postseismic deformation, including afterslip and viscoelastic deformation processes, by applying the rate- and state-dependent friction laws to the plate boundary. They also considered the nonlinear flow law for the viscoelastic element in the Burgers model for the surrounding material. Large stress changes resulting from the Tohoku-Oki earthquake have caused the viscosity to drop sharply and rapid motion to occur due to the non-linear flow law in the asenospheres under the oceanic lithosphere and the forearc. The existence of a low-viscosity layer near the boundary between the oceanic lithosphere and the asenosphere has been pointed out in other studies (Sun et al., 2014), but its existence is inevitable when one considers the nonlinear flow law. In the presentation, we will also discuss the deformation processes during megathrust earthquake cycles.
Before the Tohoku-Oki earthquake, east-west compression reverse fault-type earthquakes occurred sparsely under the forearc sea area, but after the Tohoku-Oki earthquake, normal fault-type earthquakes occurred frequently. Hasegawa et al. (2011) pointed out that the amount of compressive stress along the subduction zone was small, and the Tohoku-Oki earthquake released the absolute stress. The sources of the stress field in the arc upper plate are the tensile stress field affected by gravity from the topography and density structure, the compressive stress field affected by the frictional force acting on the plate boundary, the bending stress field caused by steady subduction (e.g., Fukahata and Matsu'ura, 2016), and stress fields associated with mantle wedge convection. Sasajima et al. (2018) considered the effects of four sources by subducting of the oceanic plate beneath Northeastern Japan for 100,000 years, considering a realistic nonlinear elasto-visco-plastic material with reference to the temperature, water content distribution, and serpentinization as simulated by Horiuchi and Iwamori (2016). When low frictional strength (5-11MPa) is applied to the shallow brittle region along the plate boundary, a normal fault stress field of east-west tension in the shallow part of the upper plate under the forearc sea is developed, and so is a reverse fault stress field of east-west compression near the plate boundary. The results suggest that the major changes in the main mechanism before and after the Tohoku-Oki earthquake are the suppression of normal fault activity and the promotion of reverse fault activity during inter-plate coupling between megathrust earthquakes. Then, normal fault activity is promoted and reverse fault activity is suppressed due to the stress change resulting from the Tohoku-Oki earthquake.
During the Tohoku-Oki earthquake, upward movement was observed in the east-southeast direction under the forearc sea area, but in the postseismic deformation, subsidence was observed in the west-northwest movement. Sun et al. (2014) showed that the west-northwest movement was due to the viscoelastic relaxation process of the mantle. Numerous detailed analyses of postseismic deformation after the Tohoku-Oki earthquake were performed (Yamagiwa et al., 2015; Muto et al., 2016; Hu et al., 2016; Iinuma et al., 2016; Suito, 2017; Freed et al., 2017; Noda et al., 2018). Agata et al. (2019) performed a large-scale numerical simulation of postseismic deformation, including afterslip and viscoelastic deformation processes, by applying the rate- and state-dependent friction laws to the plate boundary. They also considered the nonlinear flow law for the viscoelastic element in the Burgers model for the surrounding material. Large stress changes resulting from the Tohoku-Oki earthquake have caused the viscosity to drop sharply and rapid motion to occur due to the non-linear flow law in the asenospheres under the oceanic lithosphere and the forearc. The existence of a low-viscosity layer near the boundary between the oceanic lithosphere and the asenosphere has been pointed out in other studies (Sun et al., 2014), but its existence is inevitable when one considers the nonlinear flow law. In the presentation, we will also discuss the deformation processes during megathrust earthquake cycles.