In November 2017, an $Mw$ 5.4 earthquake with a shallow (⁠$∼4km$⁠) hypocenter occurred in the Pohang area, one of the most developing industrial cities in South Korea. This shock resulted in the costliest earthquake damages in modern Korean history. Immediately after the event, it was not easy to recognize the source fault due to the lack of clear coseismic surface ruptures but also the absence of previously mapped active faults around the epicenter. Based on satellite‐ and unmanned aerial vehicle (UAV)‐derived images, with ground resolution higher than 0.5 m, and field observations, we investigate surface deformation such as sand blows and ground failures associated with the Pohang earthquake as well as bedrock fault exposures in the epicentral area. To better constrain the source fault and deformation mechanism, we combine both moment tensors with aftershock relocations and surface displacements estimated by Interferometric Synthetic Aperture Radar (InSAR). Our results indicate that (1) various types and distributions of secondary ground deformation were controlled by blind oblique‐slip rupture; characterized by right‐lateral and reverse‐slip components, on a reactivated pre‐existing segmented normal‐fault system, and (2) local ground conditions could trigger or amplify ground deformations and their related earthquake damage. Our study highlights the importance of mapping the extent of coseismic deformation as well as the trace of active faults on improving seismic hazard assessments in the Korean Peninsula.

Rock permeability, an important factor in subsurface fluid migration, can be influenced by microcracks and chemical weathering due to water–rock interactions. Understanding the relationship between permeability, chemical weathering, and microcracks is crucial for assessing fluid flow in rocks. This study focuses on the hydrogeological characteristics of granite and gneiss, potential host rocks for high-level radioactive waste disposal in South Korea. Samples were analyzed for permeability, porosity, P-wave velocity, and chemical weathering indices. Regression analysis revealed a weak correlation between permeability and both porosity and rock density, while an inverse correlation was observed between permeability and chemical weathering indices. Interestingly, some samples showed low permeability (10⁻²¹ to 10⁻²² m²) despite high weathering, while others showed high permeability (10⁻¹⁸ to 10⁻¹⁹ m²) despite low weathering. SEM-EDS analysis indicated the presence of microcracks within the rocks or the filling of these cracks with secondary minerals. The findings suggest that chemical weathering generally increases pore size and porosity, but actual permeability can vary depending on the presence and connectivity of microcracks and the extent to which they are filled with secondary minerals. Therefore, both chemical weathering and microcrack connectivity must be considered when evaluating the hydrogeological characteristics of crystalline rocks.

The Gyeokpori Formation in the Buan volcanic area primarily contains siliciclastic rocks interbedded with volcanoclastics. These sediments are characterized by a variety of soft-sediment deformation structures (SSDS). The SSDS in the Gyeokpori Formation are embedded in poorly sorted conglomerates; slump folds are also present in the formation. The deformation mechanisms and triggers causing the deformation are not yet clear. In the present study, the trigger of the SSDS in the Gyeokpori Formation was investigated using facies analysis. This included evaluation of the reworking process of both cohesive and non-cohesive sediments. The analysis indicates that the SSDS are directly or indirectly associated with the alternation of conglomerates and mud layers with clasts. These layers underwent non-cohesive and cohesive deformation, respectively, which promoted SSDS formation. The slump folds were controlled by the extent of cohesive and non-cohesive deformation experienced by the sediment layers in the slope environment. The SSDS deformation style and morphology differ, particularly in the case of reworking by slump activity. This study contributes to the understanding of lacustrine slope-related soft-sediment deformation structures.

Cretaceous volcano-sedimentary basins and successions in the Korean Peninsula are located along NE-SW- and NNE-SSW-trending sinistral strike–slip fault systems. Soft-sediment deformation structures (SSDS) of lacustrine sedimentary strata occur in the Wido, Buan, and Haenam areas of the southwestern Korean Peninsula. In this study, systematic geological, geochronological, and geochemical investigations of the volcanic-sedimentary successions were conducted to constrain the origin and timing of SSDS-bearing lacustrine strata. The SSDS-bearing strata is conformably underlain and overlain by volcanic rocks, and it contains much volcaniclastic sediment and is interbedded with tuffs. The studied SSDSs were interpreted to have formed by ground shaking during syndepositional earthquakes. U-Pb zircon ages of volcanic and volcaniclastic rocks within the studied volcano-sedimentary successions were ca. 87–84 Ma, indicating that active volcanism was concurrent with lacustrine sedimentation. Geochemical characteristics indicate that these mostly rhyolitic rocks are similar to subduction-related calc-alkaline volcanic rocks from an active continental margin. This suggests that the SSDSs in the study area were formed by earthquakes related to proximal volcanic activity due to the oblique subduction of the Paleo-Pacific Plate during the Late Cretaceous.

Intrastratal flow in the Cretaceous Gyeokpori Formation (SW South Korea)

Geological disposal of High-Level Radioactive Waste (HLRW) is considered to be one of the best disposal methods that can stably and semi-permanently isolate high-level radioactive waste from the biosphere. In this study, three types of potential host rocks for HLRW disposal were selected and the hydrogeological characteristics were investigated using deep drilling cores collected at about 50 m intervals in the borehole of a depth of 1 km. The rocks used in this study were granites and gneiss which are crystalline rock, and mudstone which is a sedimentary rock. The results of the study showed that the average porosity of granite was 0.48% and the permeability ranged from 7.87 × 10-19 m2 to 1.39 × 10-21 m2 except for samples outside the measurement limit (4.04 × 10-22 m2). The average porosity of gneiss was 0.49% and the permeability ranged from 3.62 × 10-18 m2 to 4.58 × 10-22 m2. The average porosity of mudstones was 3.62% and the values of permeability for most mudstone samples were lower than the measurement limit. For SEM-EDS analysis, many microcracks were observed in the crystalline rock samples having high permeability. On the other hand, there were almost no microcracks in crystalline rock samples having low permeability, and even if there were cracks, the cracks were filled with fillers such as clay minerals. These results indicate that the presence of microcracks or the filling of cracks in crystalline rocks has a significant effect on the flow of groundwater through the host rock.