Internal architecture and earthquake rupture behavior of a long-lived intraplate strike–slip fault: A case study from the Southern Yangsan Fault, Korea

Abstract

Abstract Deciphering the internal composition of large-scale fault zones helps to better understand the various geologic factors that govern their seismic rupture behavior. We used new detailed geologic, structural, and geomorphic map of an excellent natural laboratory in the Southern Yangsan Fault (SYF) in the southeastern part of the Korean Peninsula to elucidate the fault zone architecture and its controls on the rupture processes of neotectonic earthquakes. The Yangsan Fault is a major strike–slip fault, and a subsidiary fault within the fault zone ruptured during the 2016 ML 5.8 Gyeongju earthquake which is the largest instrumental record in the intraplate Korean Peninsula. The 12-km-long study area in the SYF shows along-strike variations in wall rock lithology and fault zone architecture. At the south of the study area, the fault zone surrounded by crystalline rocks consists of a single major fault core. In contrast, at the north, the fault zone is sandwiched between crystalline rocks and sedimentary rocks and exhibits multiple major fault cores bounding broken-up wall rock blocks. The width of the across-fault damage zone, which was estimated from the cumulative fracture frequency with the distance from the major fault core, widens northward, indicating the distributed deformation of the northern fault zone over a long history of fault evolution. The geometry of paleoearthquake surface ruptures traced by the geomorphic and stratigraphic evidence of deformed young deposits diverge to multiple strands in the northern fault zone but converge to a single strand in the southern fault zone, directly reflecting its internal architecture. We propose that the long-lived rupturing patterns have been primarily controlled by intrinsic discontinuity- and mineralogy-dependent deformation processes in terms of strain-hardening vs. strain-weakening, along with propagation arrest and rupture tip damage due to a fault-bend geometry (geometric barrier) and an abrupt change in wall rock lithology (relaxation barrier).