SCG62-12

JpGU-AGU Joint Meeting 2017

プレート沈み込み帯のテクトニック応力場：基本的考え方
Tectonic stress fields in subduction zones: Basic concept*松浦 充宏1、野田 朱美2、寺川 寿子3、深畑 幸俊4*Mitsuhiro Matsu'ura1, Akemi Noda2, Toshiko Terakawa3, Yukitoshi Fukahata4
1. 統計数理研究所、2. 防災科学技術研究所、3. 名古屋大学環境学研究科、4. 京都大学防災研究所
1. Institute of Statistical Mathematics, 2. National Research Institute for Earth Science and Disaster Resilience , 3.
Graduate School of Environmental Studies, Nagoya University, 4. Disaster Prevention Research Institute, Kyoto
UniversityIn subduction zones, one (oceanic) plate moves under another (oceanic or continental) plate against
some frictional resistance and descends into the earth's mantle. Long-term deformation of the overriding
plates varies from mountain building to back-arc spreading, whereas the descending oceanic plates are
only bent convex upward. The patterns of long-term deformation are generally considered to be thereflection of background tectonic stress fields. In the case of subduction zones, not only frictional
resistance at plate interfaces but also steady subduction of oceanic plates itself cause the tectonic stressfields. The frictional resistance fluctuates with the occurrence of large interplate earthquakes, but it canbe regarded as constant on a geological timescale. So, the stress field due to frictional resistance isconstant in time, and its pattern is basically compressional in the direction of plate convergence. On theother hand, the steady plate subduction brings about convex upward bending of both the overriding anddescending plates at a constant rate by the effect of gravity (Fukahata and Matsu'ura, GJI, 2016). So, therates of stress increase due to steady plate subduction is constant in time, and its pattern is basicallytensile (compressional) in the upper (lower) half of plates in the direction of plate convergence. To
evaluate the first type of stress field, we need to know the present distribution of frictional strength alongplate interfaces, which will strongly depend on fault-confined fluid pressure. To evaluate the second typeof stress field, we need to know the past history of plate subduction and the rheological property of theearth's lithosphere, which will control the rate of inelastic deformation (brittle fracture and/or plastic flow)
to release the tectonic stress caused by mechanical interaction at plate interfaces. However, all of these
problems are very difficult to directly solve except one specific case; the stress fields of oceanic plates
produced by steady plate subduction. In this case, the oceanic plate passes through the plate-to-plateinteraction zone within a limited time (1-2 Myr), and so we need not to consider the whole past history ofplate subduction. Furthermore, the rheological property of oceanic plates is much simpler than theoverriding plates. Using the evaluated stress field of an oceanic plate as a reference, we can determine
the frictional strength distribution along a plate interface so as to reproduce the spatial pattern of stresstensor orientation at and below the plate interface, estimated from observed focal mechanism data ofseismic events (Terakawa & Matsu'ura, Tectonics, 2010). In this way, we finally got a starting point toreveal the tectonic stress field and inelastic deformation of the overriding plate.
キーワード：テクトニック応力場、プレートの定常沈み込み、プレート境界での定摩擦抵抗、沈み込む海洋プ
レート
Keywords: Tectonic stress fields, Steady plate subduction, Steady frictional resistance at plate interfaces,Descending oceanic plates©2017. Japan Geoscience Union. All Right Reserved.

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