Long-Term Weakening Processes and Short-Term Seismic Slip Behavior of an Intraplate Mature Fault Zone: A Case Study of the Yangsan Fault, SE Korea

Abstract

An intraplate tectonic fault evolves with repeated seismic cycles that may exhibit multi-mode slip behaviors (continuous aseismic sliding, slow slip with silent earthquakes, and earthquakes resulting from stick-slip frictional instability). Here, we show long-term (geological time scale) fault zone weakening processes and short-term (seismic cycle scale) fault behavior of the Yangsan Fault, SE Korea, based on structural and mineralogical observations of an across-strike fault zone sequence retrieved by inclined borehole drilling. Since the Late Cretaceous, physical and chemical weakening processes have formed a wide and complex fault zone characterized by interconnected clay-rich fault core strands and fractured lenses. The deformation is more concentrated in the eastern block (Cretaceous sedimentary rock with felsic dikes) of the fault zone than in the western block (Paleogene granite) of the fault zone. The asymmetrical deformation pattern is controlled by the distinct properties of the different host rocks and differential fluid-assisted reaction softening involving hydrothermal processes. Of the five main strands and numerous subsidiary strands in the wide fault zone, strand S1 between the sedimentary rock and granite has accommodated the largest displacements. In particular, the smectite-rich, <2-cm-wide principal slip zone within S1 has acted as the major pathway for past seismic slip. Our findings consider that slow deformation in the sedimentary rocks formed the asymmetrical wide fault zone over long geological time, whereas that the preferential propagation of repeated seismic slip events propagated preferentially along the narrow principal slip zone of S1.