[S-CG46] New Insights of Fluid-Rock Interactions: From Surface to Deep Subduction Zone

[SCG46-P20] Seismic fault slip variation caused by smectite swelling in shallow subduction zones

*Jun Kameda¹, Hideki Mukoyoshi², Keisuke Nakamoto³, Yohei Hamada⁴, Hirokazu Masumoto³ (1.Institute for Planetary Materials, Okayama University, 2.Department of Earth Science, Shimane University, 3.Department of Earth and Planetary Sciences, Hokkaido University, 4.Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology)

Keywords:Subduction zone, Smectite, Fault swelling

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.