Bonded nanoparticles restrengthen faults during earthquake slip

Abstract

Faults are the primary sources of seismicity worldwide, yet the mechanisms of fault weakening and recovery remain controversial. This study examines the microstructures and nanostructures of fault rock from a seismogenic normal fault within chert-banded dolostones. The fault slip surface exhibits various slip-related structures, including slickenlines, truncated clasts and nanoparticles/fragments. These nanoparticles on the fault slip surface are presented into two forms, single spherulitic nanoparticles (ranging in size from 50 to 300 nm) and agglomerated nanoparticles (ranging from 300 to 500 nm). The principal slip zone is characterized by cataclasites and micron-scale foliations. The cataclasite layer comprises a yellow-greyish matrix, grain-supported, and angular to sub-rounded coarser clasts which are composed primarily of dolomite, with a few clasts of quartz and calcite. The micron-scale foliations are defined by fine-grained fragments ranging from 1 to 20 μm. The microstructural investigations suggest that the single spherulitic nanoparticles may result from thermal decomposition of dolomite along the principal slip surface during fault slip or earthquake. Nano powder lubrication, facilitated by the rolling of single spherulitic nanoparticles, significantly weakens the fault during carbonate fault slip. The DEM simulation results indicate that the shear strength increases exponentially with the increasing volume percent of bonded nanoparticles. The transformation from single spherulitic nanoparticles into agglomerated/bonded nanoparticles through sintering can result in the recovery of frictional strength at the fault plane. The thin foliations in the slip zone are likely the results of laminar grain flow, possibly induced by CO₂ degassing. We inferred that nanoparticles can form through thermal decomposition on fault surfaces, which first facilitate and then inhibit earthquake behavior in thermally unstable rocks such as dolomite. The post-seismic strength recovery can be partly attributed to the formation of agglomerated nanoparticles.