[S-CG46] 岩石―流体相互作用の新展開：表層から沈み込み帯深部まで

[SCG46-P20] 沈み込み帯浅部断層の膨潤作用

*亀田 純¹、向吉 秀樹²、中元 啓輔³、濱田 洋平⁴、増本 広和³ (1.岡山大学惑星物質研究所、2.島根大学総合理工学部、3.北海道大学理学研究院、4.海洋研究開発機構高知コア研究所)

キーワード：沈み込み帯、スメクタイト、断層膨潤

Recent geophysical observations have shown that faults in shallow subduction zones exhibit a wide variety of slip behavior, ranging from very slow plate motions to fast regular earthquake slips, and intermediate-speed slips between these two end-members. One factor for this diversity is fluid pressure acting on a fault via its influence on fault strength and frictional properties. This study focuses on the swelling behavior of clay (smectite) as an alternative pressure medium and evaluates its effect on fault slip processes. Using a mechanistic model of smectite swelling, we quantitatively determine in situ swelling pressures in incoming deposits and plate-boundary faults at the Japan Trench, the North Sumatra Subduction Zone, and the northern Hikurangi Subduction Zone. Results indicate that high swelling pressure comparable to vertical effective stress may occur on the plate-boundary fault in the former two subduction zones, whereas negligible swelling pressure occurs in the Sumatra margin. Faults with very low effective stress due to smectite swelling are prone to viscoplastic flow deformation, which not only acts as a low barrier against seismic slip from depth and facilitates its propagation but may also spontaneously generate slow earthquakes such as those observed in the Japan Trench and the Hikurangi subduction zone.