## 1 Introduction
### Study Areas  
This study focuses on the central to southern segment of the Yangsan Fault Zone in the Gyeongsang Basin, southeastern Korea. This region is particularly well-suited for investigating crustal strain and fault-zone behavior due to the combination of active faulting, complex lithological variation, and clear structural expression. As shown in Figure 1a [25], the study area lies on the eastern margin of the Eurasian Plate, adjacent to complex tectonic boundaries involving the Philippine Sea Plate and Pacific Plate.  
The regional geological framework (Figure 1b) is based on the geological maps from [25; 26], showing a distribution of Mesozoic to Cenozoic volcanic, plutonic, and sedimentary rocks dissected by the Yangsan and Ulsan Fault systems. The Yangsan Fault, in particular, cuts across Late Cretaceous to Paleogene rock units, indicating a long-lived and complex deformation history.  
A detailed geological map of the study area (Figure 1c), revised from the foundational works of [27; 28; 29], reveals that the fault traverses various lithologies, including the Eonyang granite (Ceg), Chusan andesitic rocks (Csa), and Daegu formation (Cd). These lithological variations are closely linked to differences in fault architecture and deformation style observed along the fault zone.  
A high-resolution digital elevation model (DEM; Figure 1d) further illustrates the geomorphic expression of the fault and its influence on the drainage network. Major streams, such as the Hangsancheon River, align with mapped and inferred fault traces, suggesting a strong tectonic control on surface morphology. The DEM serves as a critical dataset for geomorphic and structural analyses in this study.  
Figure 1b illustrates the Yangsan and Ulsan faults, both of which are major left-lateral strike-slip faults that transect a variety of geological units across the region. The map reveals the spatial distribution of distinct lithological domains, including middle Miocene sedimentary rocks (light blue), Late Cretaceous plutonic rocks (pink), volcanic rocks (purple), Early to Late Cretaceous sedimentary rocks (light green), and pre-Cretaceous metamorphic and plutonic rocks (gray). The crosscutting relationships between these geological units and the fault traces highlight the complex structural framework and the spatiotemporal evolution of fault activity in the region.  
Figure 1: (**a**) Simplified tectonic map around the Korean Peninsula. (**b**) Regional geological map around the Gyeongsang Basin, SE Korea (modified from [25; 26]). (**c**) Detailed geological map of the central-southern Yangsan Fault (modified from [27; 28; 29]), showing lithology and major fault traces. (**d**) Digital elevation model (DEM) displaying the drainage system and fault distribution in the study area. Investigation sites A, B, C, and D marked in both (**c**,**d**), are locations where surface displacements associated with the Yangsan Fault were previously investigated by [30].  
Figure 1c depicts the heterogeneous distribution of Cretaceous and Paleogene igneous and sedimentary rocks, offering key insights into intrusive and extrusive magmatic processes. Notably, lithological boundaries are sharply defined along fault zones, underscoring the role of these faults as structural boundaries. This spatial contrast in rock assemblages across the faults reflects the tectono-thermal history of the area and provides critical evidence for interpreting the regional crustal deformation and structural segmentation associated with intraplate fault systems.