[S-CG39] Science of slow earthquakes: Toward unified understandings of whole earthquake process

[SCG39-03] Localized megathrust slip controlled by chemical reactions in subduction mélanges

*氏家 恒太郎¹、野呂 和也²、重松 紀生³、Fagereng Ake⁴、西山 直毅¹、Tulley Christopher⁴、増山 春菜²、森 康⁵ (1.筑波大学生命環境系、2.筑波大学生命環境学群、3.産業技術総合研究所活断層・火山研究部門、4.カーディフ大学、5.北九州市立自然史・歴史博物館)

Megathrust slip down-dip of the seismogenic zone is accommodated by either steady creep or episodic slow slip events (SSEs). However, the geological conditions defining the rheology of megathrust slip remain elusive. Field observation and laboratory analyses show that subduction mélanges deformed at ~370–500 °C in warm-slab environments record fluid release and viscous shear localization associated with chemical reactions between juxtaposed metapelitic and metabasaltic rocks. In the mélange deformed near the base of the seismogenic zone, very fine-grained reaction products facilitated grain boundary diffusion creep at lower shear stresses than surroundings, whereas in the mélange deformed near the mantle wedge corner, chemical reactions led to viscous shear at two orders of magnitude faster strain rate than surroundings. We suggest that chemical reactions facilitate localized changes in megathrust slip mode with depth, potentially providing a mechanism for change from viscous creep to SSEs.