Impact of inherited fault zone and its surrounding rocks on earthquake rupture behaviors: a case study of the Yangsan strike-slip fault, South Korea

Abstract:

Faults zone is composed of deformed rock with complex internal structures as result of fault evolution over long periods of geologic time. The structural complexity in inherited fault zones influences earthquake rupture behaviors but their relations have so far rarely studied. Here we test the impact of pre-existing fault zone architectures on paleoearthquakes involving surface ruptures along the Yangsan strike-slip fault, South Korea. Geomorphological and geological survey were conducted on the Southern Yangsan fault along ~12 km long section, especially on fault zone outcrops along four rivers crossing the roughly north-south trending fault valley. Fault-related topographic features including offset terraces of the fault-crossing rivers were used as geomorphic indicators to trace surface ruptures associated with paleoearthquakes. Our results show that there are remarkable differences in wall rock, fault zone architecture, and paleoearthquake rupture trace between northern and southern parts of the studied Southern Yangsan Fault as follows: 1) fault zone was surrounded by sedimentary rocks and granite in North but granite on both sides in South, 2) the maximum width of fault zone was ~1,400 m in North but ~850 m in South, 3) fault zone architectures has high complexity with at least three fault cores that enclose fractured blocks of various types of rocks in North but relatively low complexity with single fault core in South, and 4) paleoearthquake surface ruptures were traced as sub-parallel multiple strands in North but a single strand in South. We note that, in North, the traces of paleoearthquake surface rupture does not correspond with the traces of main fault cores. Our observations imply that localized or distributed trend of deformation is a persistent property depending on localities along the fault strike. We attribute the distinguishable properties to behavior pattern and pre-existing weak zone on different type of surrounding rocks as well as lithological discontinuity along fault-strike that could control earthquake rupture propagation.