Korea has been recognized as an earthquake-safe zone, but over recent decades, several earthquakes, at a medium scale or higher, have occurred in succession in and around the major fault zones, hence there is a need for studying active faults to mitigate earthquake risks. In Korea, research on active faults has been challenging owing to urbanization, high precipitation, and erosion rates, and relatively low earthquake activity compared to the countries on plate boundaries. To overcome these difficulties, the use of aerial light detection and ranging (LiDAR) techniques providing high-resolution images and digital elevation models (DEM) that filter vegetation cover has been introduced. Multiple active fault outcrops have been reported along the Yangsan Fault, which is in the southeastern area of the Korean Peninsula. This study aimed to detect active faults by performing a detailed topographic analysis of aerial LiDAR images in the central segment of the Yangsan Fault. The aerial LiDAR image covered an area of 4.5 km by 15 km and had an average ground point density of 3.5 points per m², which produced high-resolution images and DEMs at greater than 20 cm. Using LiDAR images and DEMs, we identified a 2–4 m high fault scarp and 50–150 m deflected streams with dextral offset. Based on the image analysis, we further conducted a trench field investigation and successfully located the active fault that cut the Quaternary deposits. The N–S to NNE-striking fault surfaces cut unconsolidated deposits comprising nine units, and the observed slickenlines indicated dextral reverse strike-slip. The optically stimulated luminescence (OSL) age dating results of the unconsolidated deposits indicate that the last earthquake occurred 3200 years ago, which is one of the most recent along the Yangsan Fault.

An intraplate tectonic fault evolves with repeated seismic cycles that may exhibit multi‐mode slip behaviors (continuous aseismic sliding, slow slip with silent earthquakes, and earthquakes resulting from stick‐slip frictional instability). Here, we show long‐term (geological time scale) fault zone weakening processes and short‐term (seismic cycle scale) fault behavior of the Yangsan Fault, SE Korea, based on structural and mineralogical observations of an across‐strike fault zone sequence retrieved by inclined borehole drilling. Since the Late Cretaceous, physical and chemical weakening processes have formed a wide and complex fault zone characterized by interconnected clay‐rich fault core strands and fractured lenses. The deformation is more concentrated in the eastern block (Cretaceous sedimentary rock with felsic dikes) of the fault zone than in the western block (Paleogene granite) of the fault zone. The asymmetrical deformation pattern is controlled by the distinct properties of the different host rocks and differential fluid‐assisted reaction softening involving hydrothermal processes. Of the five main strands and numerous subsidiary strands in the wide fault zone, strand S1 between the sedimentary rock and granite has accommodated the largest displacements. In particular, the smectite‐rich, <2‐cm‐wide principal slip zone within S1 has acted as the major pathway for past seismic slip. Our findings consider that slow deformation in the sedimentary rocks formed the asymmetrical wide fault zone over long geological time, whereas that the preferential propagation of repeated seismic slip events propagated preferentially along the narrow principal slip zone of S1.

Earthquake prediction in intraplate regions, such as the Korean Peninsula, is challenging due to the complexity of fault distributions. This study employed diverse methods and data sources to investigate Quaternary surface rupturing along the Yangsan Fault, aiming to understand its long-term earthquake behavior. Paleoseismic data from the Byeokgye section (7.6 km) of the Yangsan Fault are analyzed to provide insights into earthquake parameters (i.e., timing, displacement, and recurrence intervals) as well as structural patterns. Observations from five trench sites indicate at least six faulting events during the Quaternary, with the most recent surface rupturing occurring approximately 3000 years ago. These events resulted in a cumulative horizontal displacement of 76 m and a maximum estimated magnitude of M_w 6.7–7.1. The average slip rate of 0.13 ± 0.1 mm yr⁻¹ suggests a quasi-periodic model with possible recurrence intervals exceeding 13 000 years. Structural patterns indicate the reactivation of a pre-existing fault core with top-to-the-west geometry, causing a dextral slip with a minor reverse component. This study underscores the several surface ruptures with large earthquakes along the inherited mature Yangsan Fault, since at least the Early Pleistocene, offering critical insights for seismic hazard and a broader understanding of intraplate earthquake dynamics, enhancing earthquake prediction efforts.

Quantifying today's topography can provide insights into landscape evolution and its controls, since present topography represents a cumulative expression of past and present surface processes. The Ulsan Fault Zone (UFZ) is an active fault zone on the southeastern Korean Peninsula that was reactivated as a reverse fault around 5 Ma. The UFZ strikes NNW–SSE and dips eastward. This study investigates the relative tectonic activity along the UFZ and the landscape evolution of the hanging-wall side of the UFZ, focusing on neotectonic perturbations using ¹⁰Be-derived catchment-averaged denudation rates and bedrock incision rates, topographic metrics, and a landscape evolution model. Five geological segments were identified along the fault, based on their relative tectonic activity and fault geometry. We simulated four cases of landscape evolution to investigate the geomorphic processes and accompanying topographic changes in the study area in response to fault movement. Model results reveal that the geomorphic processes and the patterns of topographic metrics (e.g., χ anomalies) depend on inherited topography (i.e., the topography that existed prior to reverse fault reactivation of the UFZ). On the basis of this important model finding and additional topographic metrics, we interpret the tectono-geomorphic history of the study area as follows: (1) the northern part of the UFZ has been in a transient state and is in topographic and geometric disequilibrium, so this segment underwent asymmetric uplift (westward tilting) prior to reverse faulting on the UFZ around 5 Ma, and (2) its southern part was negligibly influenced by the asymmetric uplift before reverse faulting. Our study demonstrates the utility of topographic metrics as reliable criteria for resolving fault segments. Together with landscape evolution modeling, topographic metrics provide powerful tools for examining the influence of inherited topography on present topography and for the elucidation of tectono-geomorphic histories.

Sungchan Choi, Sung-Wook Kim, Eun-Kyeong Choi, Young-Cheol Lee, Sangmin Ha. Econ. Environ. Geol. 2021;54:91-103. https://doi.org/10.9719/EEG.2021.54.1.91