Geological Records of Coseismic Shear Localization Along the Yangsan Fault, Korea

Abstract

In natural fault zones, shear strain is mainly accommodated in fault rocks (or fault materials) that make up the fault core (i.e., a central high-strain zone of a brittle fault; Fossen, 2016). The shear strain may be localized in short-term and long-term scales. The long-term localization has been reported from both seismogenic and aseismic fault zones (e.g., Sibson, 2003). Seismic rupture events may repeatedly occur in a slip zone over a long period, expressed by progressive displacement of geomorphic features (e.g., Lensen, 1968). Also, aseismic fault slip may be accommodated in narrow zones of meters to decimeters in thickness (e.g., the Hayward fault; Bilham & Whitehead, 1997). Aseismic shear localization observed from statically weak faults appears to be promoted by the operation of time-dependent, fluid-assisted deformation mechanisms (e.g., dissolution-precipitation creep) in fault rocks (e.g.,

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

“…Gu et al. (2021) suggested that the structural maturity of the fault appears to fit the intermediate class (Class 2) following the criteria of Manighetti et al. (2007) based on the minimum values of fault length on land (∼200 km), fault initiation age (>70 Ma), maximum long‐term slip rate (∼0.4 cm/yr), and finite dextral offset (>20–30 km).…”

“…Recently, based on a compilation of previous results, Cheon et al (2019) proposed the tectonic evolution of the fault and age constraints, including the following: (a) Late Cretaceous sinistral movement with a component of extensional deformation (e.g., NW-SE-striking extensional structures, and the Dadaepo pull-apart basin between the Yangsan and Dongnae faults; Cheon et al, 2017;Cho et al, 2007Cho et al, , 2016Ha et al, 2016); (2) late Paleogene (∼43-23 Ma) dextral movement, which was the most intense phase of deformation on the fault, with 20-30 km of dextral offset being inferred from the distribution of several piercing points, such as A-type granite bodies (see Figure 1c; Chang et al, 1990;Hwang, Lee, et al, 2007;; and (c) subsequent reactivations along some segments during the late Miocene associated with sinistral kinematics (Chang, 2002;Chang & Chang, 1998;Choi et al, 2009). Gu et al (2021) suggested that the structural maturity of the fault appears to fit the intermediate class (Class 2) following the criteria of Manighetti et al (2007) based on the minimum values of fault length on land (∼200 km), fault initiation age (>70 Ma), maximum long-term slip rate (∼0.4 cm/yr), and finite dextral offset (>20-30 km).…”

“…With respect to the seismic cycle scale, coseismic slip occurs mainly along particular localized slip surfaces within a wide fault zone (e.g., Chester & Chester, 1998; Chester et al., 1993; Sibson, 2003). Therefore, the structural and mineralogical features of a full sequence of mature fault rocks and internal structures can provide critical information for investigating not only long‐term fault zone weakening mechanisms and deformation processes but also short‐term seismic slip that is reactivated along narrow slip zones during earthquakes (e.g., De Paola et al., 2008; Gu et al., 2021). In this study, “long‐term weakening” is defined as a reduction in frictional strength during each syn‐kinematic deformation stage, and “short‐term” corresponds to processes occurring at the seismic cycle scale.…”

Near-surface termination of upward-propagating strike-slip ruptures on the Yangsan Fault, Korea

“…Generally, primary on-fault paleoseismic features along the fault—such as surface faulting-induced sedimentation (e.g., fissure fill and colluvial wedge) and faulted and folded stratigraphic units—indicate seismic slip, because creep, slow slip, or small slip during small to moderate earthquakes generally leave no such signs of observable deformation 17 . In addition, recently reported microscopic features in the PSZ of the Yangsan Fault, such as narrow (a few millimeters) micro-slip zones, sharp surfaces, ultrafine (less than a few micrometers) materials, gouge injections, cortex structures, and spalling fractures of clasts, strongly support the occurrence of rapid seismic slip reaching the surface 25 , 28 , 35 , 45 . Therefore, the thin and straight slip zones that are located mostly along the boundary of the wide inherited weak zone have acted as conduits for repeated earthquake rupture propagations and could also be pathways for future rupture propagation when large slip is initiated along this fault at seismogenic depths 35 .…”

Microstructural comparison in fault gouges and slip surfaces of active and inactive faults: A case study at Watarigawa area in northeastern Yamaguchi Prefecture, SW Japan

“…白水, 1988;Mänttäri et al, 2007) Ga，Sr，Ca が検出された (Fig. 15c) 3) や神城断層 の副次断層でも報告されている (Noda and Shimamoto, 2005;Boullier et al, 2009;Schuck et al, 2018;Komura et al, 2019;Gu et al, 2020;相山ほか, でも報告されている (重富・林, 1999;Otsuki et al, 2003;Noda and Shimamoto, 2005;Boullier et al, 2009;Chen et al, 2013;加藤ほか, 2015;相山ほか, 2017;Schuck et al, 2018;Komura et al, 2019;Gu et al, and Sasaki, T., 2017, Consideration of the activity of a fault based on detailed structural analysis of a fault fracture zone: Case study of the Yamada fault, western Japan. J. Japan Soc.…”