Geological Records of Coseismic Shear Localization Along the Yangsan Fault, Korea

Abstract

We conducted outcrop-to-nanoscale structural observations on a principal slip zone (PSZ) of the Yangsan fault, a major strike-slip fault in southeast Korea, and some high-velocity rotary shear tests on the PSZ gouges to understand shear localization and physico-chemical processes in the PSZ during earthquakes. At the fault outcrop, the PSZ appears 1–2 cm thick. At smaller scales, the PSZ is subdivided into Units 1 to 4 based on their microstructures. Microscopic observations confirmed the shear localization in Units 1 (20–500 μm in thickness) and 3 (0.8–3 mm in thickness). Further localization was observed in several μm-thick shear bands. Various structural and mineralogical features that recorded physico-chemical processes in slip zones were found in the PSZ, including gouge fluidization structures relevant to thermal pressurization, solidified frictional melt of clay-rich gouge, and mineralogical changes by frictional heating-induced illitization. The gouge melting and mineralogical changes were reproduced in our experiments at 1.3 m s−1. The gouge fluidization has been observed in previous tests at seismic slip rates. Thus, the observations from the natural and experimental faults indicate that the shear localization in the PSZ was coseismic. In a cataclasite zone next to the PSZ, altered veins of pseudotachylyte were found. All these observations indicate temporal changes of the PSZ materials (e.g., formation of solidified melt, alteration of the melt into clay minerals, friction-induced mineralogical changes). Associated with such changes, coseismic slip zone processes and dominant weakening mechanisms may also vary, even at the same location on a fault.