[S-SS02] Frontier studies on subduction zone megathrust earthquakes and tsunamis

[SSS02-25] 準高速すべりに伴うスメクタイト質断層の動的強度弱化

*大橋 聖和¹、廣瀬 丈洋²、高橋 美紀³、谷川 亘² (1.山口大学大学院理工学研究科、2.独立行政法人海洋研究開発機構　高知コア研究所、3.独立行政法人産業技術総合研究所活断層・火山研究部門)

キーワード：海底堆積物, 摩擦実験, 速度弱化, デコルマ, 巨大分岐断層, 津波地震

The hydrous clay mineral smectite, which is pervasive in sediments on subducting oceanic plates, is thought to weaken and stabilize subduction thrust faults. However, these frictional properties of smectite alone cannot explain the large coseismic slip in the vicinity of a trench. Here, we performed friction experiments to demonstrate the rate dependence of friction at slip rates from 30 μm/s to 1.3 m/s for water-saturated smectite-quartz mixtures with various smectite contents, so as to shed light on the frictional response of smectite-bearing faults at intermediate to high slip rates. At slip rates of 30 to 150 μm/s, the friction coefficients decreased gradually from 0.5-0.6 to 0.1 with an increase in smectite content from 20 to 50 wt%. In contrast, at slip rates higher than 1.3 mm/s, friction exhibited marked slip weakening, resulting in low friction coefficients of 0.1-0.05, even for low smectite contents (roughly <30 wt%). Drastic slip weakening occurred at smectite contents of 10-30 wt% at slip rates of ~10 mm/s, which is one to two orders of magnitude lower than the slip rate at which slip weakening was observed in previous experiments on various rock types. The intermediate-velocity weakening could be attributed to a rise in pore pressure caused by both shear-enhanced compaction and microscopic thermal pressurization of pore fluids. This process could weaken the fault even below seismic slip rates, leading to an acceleration of fault motion and potentially facilitating large coseismic slip and a stress drop in the vicinity of a trench.

[Acknowledgments]
We thank Kyuichi Kanagawa, Masaya Suzuki, Osamu Tadai, and Hiroko Kitajima for constructive discussions and technical help. This work was supported by a JSPS Grant-in-Aid for JSPS fellows (25-04960) to K.O., a JSPS Grant-in-Aid for Scientific Research (25287135) to T.H., and MEXT KANAME grant #21107004.