단층 손상대 내 분쇄암의 진화: 양산 금산리 제4기 단층 사례

초록

Earthquakes are generated by the dynamic propagation of ruptures along faults, and the resulting deformation structures can be preserved within fault cores and damage zones. This study investigates the formation of seismic slip indicators and the development and evolution of pulverized rocks in the Quaternary strike-slip fault zone at two localities in Geumsan-ri, Yangsan, Korea. At Site 1, a fault with an orientation of N26°E/83°NW juxtaposes Quaternary sediments in the southeast against Cretaceous granite in the northwest. Clay-rich gouge is injected from the slip zone into the sediments, with a width-to-length ratio (aspect ratio) of approximately 0.3. Within the granite on the northwestern side of the fault, healed fractures and open fractures occur at high density, together with deformation bands and folded deformation bands. At Site 2, a fault with an orientation of N60°E/87°SE is developed within Cretaceous granite. Here, pulverized rocks are concentrated in the northwestern side, containing both healed and open microfractures, whereas only healed fractures are developed in the southeastern side. The gouge injection structure at Site 1, with its high aspect ratio, is interpreted to have formed not only by fluid overpressure induced by frictional heating but also through the contribution of dynamic tensile stresses. Pulverized rocks at both sites evolved through three pathways during repeated seismic slip: (1) pulverization-successive pulverization, (2) pulverization-fracture healing, and (3) pulverization-fracture healing-pulverization. Fracture healing during interseismic periods served as a critical factor in determining the occurrence and location of subsequent pulverization. The asymmetric distribution of pulverized rocks is attributed to bimaterial interface effects and lithological property contrasts, but it can also occur in homogeneous rocks depending on rupture directivity and stress field distribution. This study suggests that fracture healing within damage zones can control subsequent pulverization and location, thereby contributing to the development of long-term fault zone evolution models.

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