Structural records and mechanical characteristics of seismic slip along an active fault crosscutting unconsolidated Quaternary sediments: Suryum fault, SE Korea

Abstract

The Suryum fault is an active fault in southeast Korea crosscutting the Quaternary sediment deposit. We conducted outcrop-scale to microscale observations of the fault slip zone and performed shear experiments on the fault gouge at subseismic (3–30 µm/s) and seismic slip rates (0.53 m/s). The gouge is rich in clay minerals (> 58%), particularly in expandable clay minerals (40%). In the gouge zone, a very narrow (20–150 µm thick) principal slip zone (PSZ), identified by the strong alignment of clay minerals, was developed. At several places along the fault, the gouge is observed to have been injected into the Quaternary wallrock sediments. The development of the narrow PSZ is incompatible with the velocity-strengthening behavior of the Suryum fault gouge observed in the shear tests, and it may indicate that seismic rupture could be propagated along the PSZ in the clay-rich gouge. Slip localization into the PSZ was presumably possible because of significant dynamic fault weakening due to thermal pressurization (or buildup of pore pressure caused by frictional heating) in the low-permeability clay gouge during fast slip. The gouge injections along the Suryum fault may be the geological record of gouge fluidization caused by thermal pressurization. This idea is supported by observations of very low friction of the Suryum fault gouge when sheared experimentally at the seismic slip rate and of the gouge injections in the sheared gouge. The dynamic weakness of the Suryum fault implies that large displacement and low-frequency ground motion at near-surface depths would be possible when it is reactivated.