## 1 Introduction  
### Scientific Background and Motivation  
The accumulation and release of tectonic stress within the Earth's crust are fundamental processes that govern earthquake occurrence. While seismic activity provides direct evidence of stress release, it is the interseismic phase--characterized by subtle yet measurable crustal deformation--that offers critical insight into stress accumulation. With the advent of high-precision Global Navigation Satellite System (GNSS) networks, researchers have been able to monitor these strain dynamics with unprecedented temporal and spatial resolution. Such geodetic techniques are increasingly recognized as essential tools for seismic hazard assessment and earthquake forecasting in both interplate and intraplate settings [1, 2]. In particular, GNSS-derived strain rates have proven effective in informing probabilistic earthquake forecasts. For example, ref. [1] converted strain-rate fields into estimates of seismic moment accumulation in southwestern Japan, successfully correlating zones of elevated strain with historical seismicity. Ref. [2] further emphasized the importance of separating long-term tectonic trends from transient postseismic signals, warning that uncorrected geodetic anomalies can mislead hazard models--especially in slowly deforming regions. While the utility of strain monitoring is well established in active plate boundary zones, its application in stable continental interiors remains limited. The Korean Peninsula, situated in an intraplate setting but with a history of moderate to large earthquakes, presents a compelling case. Of particular significance is the Yangsan Fault, a structurally complex, right-lateral strike-slip fault extending approximately 170-200 km in southeastern Korea. The fault transects both sedimentary and crystalline rocks, exhibiting wide, distributed fault zones in weaker lithologies and narrow, localized slip zones in more competent units [3]. Tectonic analysis has identified at least four major movement phases since the Late Cretaceous: an initial sinistral phase, intense Paleogene dextral activity, weak Miocene sinistral reactivation, and Quaternary dextral slip with a reverse component [4]. These phases reflect broader changes in the tectonic regime of East Asia and have resulted in a structurally segmented fault zone with differential mechanical properties and slip behaviors along strike. Recent advances in paleoseismology have clarified the Yangsan Fault's seismogenic potential. Ref. [5] documented at least five to six surface-rupturing earthquakes during the Holocene using three-dimensional trenching data, estimating recurrence intervals between 10,000 and 13,000 years. Ref. [6] demonstrated significant along-strike variability in rupture history, with multiple Holocene ruptures in the northern and central segments and a notable lack of such events in the south. Offshore investigations have further revealed active faulting, with [7] estimating slip rates between 0.13 and 0.73 mm/yr in the Riedel shear zones, suggesting a recurrence interval as short as 550-1500 years. The fault's internal architecture also supports its long-term activity and complexity. Ref. [8] reconstructed its deformation history using 3-D shape preferred orientation and illite age dating, revealing multiple reactivation episodes consistent with episodic fluid infiltration and clay mineral growth [9]. Ref. [3] found that long-term weakening processes--such as mineral alteration, fault gouge development, and clay-rich core formation--create conditions for both seismic and aseismic slip. These features promote strain localization along narrow principal slip zones, allowing stress to accumulate between major rupture events. Such localized deformation patterns are characteristic of broader active tectonic zones, where long-term stress accumulation drives complex rupture behavior. Active tectonic zones exhibit intense crustal deformation associated with faults, folds, and fault-bounded basins.