Coseismic turbulence-like flow of fault material along the shallow portion of a plate-subduction-related fault

Fault weakening during slip is thought to markedly affect the amount of slip and dynamic rupture propagation during earthquakes. Friction-induced thermal pressurization of interstitial fluid in faults is widely known to be a weakening mechanism, but the internal structure responsible is not well understood. Here we present results from microscopic observations and analyses of the principal slip zone in a major thrust within a fossil accretionary prism, the Hota Group (Emi Group) of the Boso accretionary wedge, Japan, in which the fault rock has experienced thermal pressurization. The slip zone has a complex layered structure, composed of planar-laminated, turbulent-flow-like, breccia, and non-foliated massive layers. The sharp boundaries of each layer indicate that each layer could be correlated with an independent slip event. We performed hydrodynamic modeling of the turbulent-flow-like layers by assuming Couette–Poiseuille flow in the space between two surfaces and determined that the viscosity was ultralow, ranging from 1.2 × 10 −4 to 2.3 × 10 −3 Pa s, during fluidized granular flow induced by thermal pressurization. Such coseismic low viscosity of fluidized fault material could enhance intense fault weakening and promote a large stress drop, probably contributing to larger slip and greater rupture propagation during earthquakes.