Seismic slip zone characteristics in near-surface unconsolidated sediments: Low-angle reverse paleo-rupture in Central Korea

Abstract

Fault materials and deformed sediments formed by coseismic rupturing events in the near-surface region show various deformation characteristics. To gain insights into rupture propagation and termination processes close to the surface, we analyzed the slip zone and deformed sediments/granite of a paleoearthquake rupture in central Korea. Here, we list our structural and mineralogical findings of the low-angle reverse rupture. (1) The slip zone mainly contains microbreccia and intercalated illuvial clay. The kaolinite-rich illuvial clays migrated downward with water, accumulate in spaces, and have a distinct texture and color. They show strong alignments and injection structures indicating that seismic slip occurred after clay infiltration in the open spaces along the pre-formed rupture. (2) The dip angle of the slip zone where it cuts unconsolidated strata tends to decrease, showing a branching geometry and termination. The slip zone changes in thickness where it goes through the different alternating sedimentary units (sands and clays) due to their physical properties. Based on these features, we conclude that the type and distribution of infilling materials in the slip zone and of surrounding materials transected by ruptures are related to the interruption and termination of the propagating rupture within the unlithified strata at near-surface depths.