How clay delamination supports aseismic slip

• Published in: The American mineralogist Vol. 108; no. 1; pp. 87 - 99
• Main Authors: Zhou Huijun, Zhou Huijun, Chen Meng, Chen Meng, Zhu Runliang, Zhu Runliang, Zhu Jianxi, Zhu Jianxi, He Hongping, He Hongping
• Format: Journal Article
• Language: English
• Published: Washington Mineralogical Society of America 03.01.2023; Walter de Gruyter GmbH
• Subjects: aseismic slip; Bilayers; clastic sediments; Clay; Clay minerals; delamination; Earthquakes; experimental studies; faults; Fluid pressure; Friction; gouge; hydration; interlayer; Interlayers; Lubricants; Mechanics; Mineralogy; Minerals; Molecular dynamics; Monolayers; pore pressure; pressure; sediments; Seismic activity; Shear; shear strength; sheet silicates; silicates; simulation; Sliding; Slip velocity; Slumping; Smectite; strain; Strengthening; Structural geology; temperature; Velocity; Viscous fluids; Water
• ISSN: 0003-004X; 1945-3027
• DOI: 10.2138/am-2022-8195

Aseismic slip is a stable fault slip, which allows strain to be relieved smoothly. Aseismic slip prevents the earthquake propagation, but it could nucleate an earthquake elsewhere. Understanding the mechanism of aseismic slip is promising in revealing the seismic cycle. Experimental evidence showed clay-rich fault gouge bears a low-friction strength, and the friction is strengthened with slip velocity (velocity-strengthening), which was thought to support aseismic slip. Clay minerals are comprised of platy crystalline layers with water intercalated between them, which may act as a lubricant. Sliding between clay layers was suspected to support aseismic slip but lacked a clarified mechanistic insight. We use non-equilibrium molecular dynamics simulations to show that shear-induced interlayer sliding is frictionally weak and velocity-strengthening, which evidences the role of clay minerals in aseismic slip. We find that interlayer water is a viscous fluid at most times, which explains the shear response of interlayer sliding. Depending on temperature and pressure conditions, intercalated water can be monolayer or bilayer, fluidic or ice like. Shear induces ice-like water to transform into fluidic water, which happens as a stick-slip phenomenon reflecting a first-order transition. Increased pore fluid pressure leads to the transformation from monolayer to bilayer intercalated water, resulting in a lower friction strength and enhanced velocity-strengthening behavior. Our work suggests that disclosing the hydration state of a clay mineral is preliminary when studying fault mechanics.