Fluid-Driven Injection and Pressurization of Clay-Rich Gouge in the Yangsan Fault: Implications for the Long-Term Seismic Cycle

Elucidating fault zone processes during long-term seismic cycles is critical for mitigating earthquake hazards in intraplate regions. We investigated the hydro-mechanical evolution of a strike-slip branch of the Yangsan Fault, SE Korea, which bounds Triassic and Jurassic granites. By integrating multiscale observation with high-velocity rotary shear experiments and XRD, we characterized the fault architecture, which consists of a <35 m thick damage zone surrounding a <1 m thick core. The core contains breccia and foliated gouge rich in clay minerals (43 wt.%), specifically dominated by illite (21.2 wt.%) and smectite (13.3 wt.%). Shear experiments on the foliated gouge revealed a consistently low friction coefficient (μ_ss<0.17). Notably, instantaneous flash dilation of the mixed smectite/illite gouge was observed at seismic slip rates (1.3 m/s) when total displacement exceeded ~5 m. Microstructural cross-cutting relationships indicate a distinct sequence of events: (1) vigorous injection of pressurized fluids from wall rocks into the densely packed, low-permeability gouge directly; (2) precipitation of fibrous calcite veins along foliation planes and perpendicular to the Y-shear direction; and (3) subsequent injection of fluidized gouge material into the damaged wall rock. These observations suggest that cyclic coseismic and aseismic faulting occurred following the low-temperature formation of expanding clay minerals. We conclude that the dynamic interplay between fluid pressurization and the fluidization properties of clay gouge acts as a primary driver of mechanical instability, playing a key role in the long-term seismic evolution of intraplate granitic fault zones.