[S-CG66_30AM1] プレート収束帯における地殻変形運動の統合的理解

[SCG66-04] プレート境界の摩擦強度が支配する沈み込み帯のテクトニック応力場

*松浦 充宏¹、野田 朱美²、寺川 寿子³、深畑 幸俊⁴ (1.統計数理研究所、2.構造計画研究所、3.名古屋大学環境学研究科、4.京都大学防災研究所)

キーワード：沈み込み帯, テクトニック応力場, プレート境界, 摩擦強度, 造山運動, 背弧拡大

Tectonic crustal motion in plate convergence zones varies from mountain building (e.g., Himalaya) to back-arc spreading (e.g., Mariana) [1, 2, 3]. Such difference in tectonic crustal motion reflects the diversity of tectonic stress fields. So our question is what causes the diversity of tectonic stress fields in plate convergence zones. Recently, from a theoretical study [4], we revealed that the tectonic stress field consists of basically two different sorts of stress fields; one of which is a horizontally compressional stress field due to frictional resistance at plate interfaces, and another is a horizontally tensile stress field due to steady plate subduction. On a geological timescale, the former can be regarded as constant in time, but the latter increases with time. So, if the earth's crust were infinitely strong, tectonic stress fields in plate convergence zones would become tensile in time everywhere. Actually, the earth's crust includes a number of defects with low strength, over which inelastic deformation (brittle fracture and/or plastic flow) occurs so as to release the tectonic stress caused by mechanical interaction at plate interfaces. From these considerations, we may conclude as follows. When the plate interface is very weak in comparison with the earth's crust, a horizontally tensile stress field becomes dominant, which causes back-arc spreading as in the case of Mariana. When the plate interface is very strong, a horizontal compressional stress field becomes dominant, which causes mountain building as in the case of Himalaya. Tectonic stress fields in most subduction zones, where the strength of plate interfaces are comparable to that of the earth's crust, are between these two extreme cases.References[1] Takada, Y. and M. Matsu'ura, 2004. A unified interpretation of vertical movement in Himalaya and horizontal deformation in Tibet on the basis of elastic and viscoelastic dislocation theory, Tectonophysics, 383, 105-131.[2] Hashimoto, C. and M. Matsu'ura, 2006. 3-D simulation of tectonic loading at convergent plate boundary zones: Internal stress fields in northeast Japan, Pure Appl. Geophys., 163, 1803-1817.[3] Hashima, A., Y. Fukahata, and M. Matsu'ura, 2008. 3-D simulation of tectonic evolution of the Mariana arc-back-arc system with a coupled model of plate subduction and back-arc spreading, Tectonophysics, 458, 127-136.[4] Matsu'ura, M., A. Noda, and T. Terakawa, 2013. Strength of plate interfaces and tectonic stress fields in subduction zones, Seismological Society of Japan 2013 Annual Meeting, D22-08, Yokohama.