What question did this study set out to answer?

This research aims to understand the internal structures of fault zones and how they contribute to fault weakening during earthquakes.

June 29, 2026Open Access

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

TFTatsuru FukutaTohoku Fukushi University THTetsuro HironoOsaka Metropolitan University

Key Points

This research aims to understand the internal structures of fault zones and how they contribute to fault weakening during earthquakes.
Microscopic observations and analyses of the slip zone in a thrust fault in the Hota Group, Japan.
Hydrodynamic modeling of turbulent-flow-like layers to assess fluid viscosity during fluidized granular flow.
Evaluation of layer structures to correlate them with independent slip events.
Identified ultralow viscosity of fault material between 1.2 × 10 −4 and 2.3 × 10 −3 Pa s during seismic events.
The complex layered structure indicates potential for large slip and propagation increases during earthquakes.
Demonstrated that fluidization in fault zones enhances weakening and contributes to larger stress drops.

Abstract

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.

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