[S-CG46] スラブ内地震とその発生メカニズム

[SCG46-P03] Stress accumulation in viscoelastic splay fault and subducting oceanic crust: its effect to intraslab earthquakes

*村本 智也^1,2、伊藤 喜宏¹、宮川 歩夢³、古市 紀之² (1.京都大学防災研究所、2.産業技術総合研究所 計量標準総合センター、3.産業技術総合研究所 地質調査総合センター)

キーワード：レオロジー、分岐断層、粘弾性、有限要素法、スラブ内地震

Recent seafloor observations have revealed the complexity of the stress accumulation and relaxation model during the earthquake cycle. The quantification of the accumulation and relaxation of strain and stress around the branching fault during earthquake cycles is crucial for understanding the fundamental physics of the subduction system. On the other hand, no previous studies have accurately accounted for the influence of rheological properties with bi- or tri- materials. In the bi- or tri-materials, for instance, we should consider differences among hanging-wall and foot-wall, or upper plate and subducting oceanic crust. To consider the temporal evolution of strain and stress and their relaxation process, especially on the differences in effective viscosity (effect of bi- or tri-viscoelastic material) and viscoelasticity should be assumed (Fukahata and Matsu’ura, 2006; Sun et al., 2014). This study constructs a viscoelastic finite element model to represent the stress–strtain field surrounding a branching fault and demonstrated the accumulation processes of both strain and stress around the branching fault. We consider the case of multiple fault dislocations, construct the model as a function of effective viscosity in the media, and investigate the influence of effective viscosity on the strain and stress accumulation patterns. Strain and stress tend to accumulate along the foot-wall side of the branching fault and the subducting oceanic crust. In a viscoelastic medium, the accumulation and relaxation of strain and stress occur simultaneously, and the accumulation rate varies with the effective viscosity. These suggest that the strain and stress concentrated at the ends of branching faults could contribute to intraslab earthquakes such as those observed at the Hikurangi subduction margin, and that the mode of occurrence depends on effective viscosity. Our model suggests that a more representative model can be constructed by systematically and quantitatively examining the individual effects of each element.