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Tectonics is broadly accepted as one of the main factors controlling long-term landscape evolution. The impact of tectonics on short timescales is most often observed through earthquake rupturings that produce localized, metric-scale deformations. Although these deformations significantly affect the landscape, it remains challenging to precisely correlate major landscape changes with these localized earthquake deformations. Therefore, linking instantaneous deformation to long-term morphological changes often involves a thought experiment with potentially limited temporal resolution. At a paleoseismological site along the slow-moving strike-slip Yangsan Fault in South Korea, we employed optically stimulated luminescence (OSL) dating and detrital zircon analysis on all exposed unconsolidated layers in trench walls. The results reveal a significant provenance shift in the sediments accumulating in the trench, indicating a major reorganization of the drainage network. Based on our OSL and detrital zircon data, we estimate that this change occurred around 70 ka. We propose that this drastic drainage reorganization was caused by a combination of very slow, yet continuous, earthquake activity and a temporary reduction in river erosion during the onset of the cold, dry spell characteristic of the MIS4 stage across the Korean Peninsula.

We present a new example of the termination of strike-slip paleoearthquake ruptures in near-surface regions on the Yangsan Fault, Korea, based on multi-scale structural observations. Paleoearthquake ruptures occur mostly along the boundary between the inherited fault core and damage zone (N10–20°E/> 75°SE). The ruptures propagated upward to the shallow subsurface along a < 3-cm-wide specific slip zone with dextral-slip sense, along which the deformation mechanism is characterized mainly by granular flow in near-surface region. The ruptures either reach the surface or are terminated in unconsolidated sediment below the surface. In the latter case, the rupture splays show westward bifurcation, and their geometry and kinematics show a change to NNW-strike with low-angle dip and dextral-reverse oblique-slip sense in the strata. We suggest that the upward termination of the contractional strike-slip ruptures is controlled by the inherited fault geometry that is unfavorable with respect to the stress field (ENE–WSW σ Hmax ) at basement depths in terms of movement on the fault, and the lack of extension of the fault into shallow subsurface; a depth-dependent change in stress from σ Hmax  > σ v  > σ Hmin to σ Hmax  > σ Hmin  > σ v at depth of a ~ 200 m; and the physical properties of unconsolidated sediment, which have low inter-granular cohesion, resulting in distributed deformation.

Detailed geological mapping and observations of various structural elements were made in order to determine the geometry and kinematics of the Ocheon Fault System (OFS) along the boundary between the Early Miocene Janggi and the Middle Miocene Pohang basins, SE Korea, and to reveal its roles on the basin evolutions. The OFS is a NE-trending relayed fault system composed of a number of NE or NNE-trending normal-slip and sinistral-normal oblique-slip faults, and has a scissor fault geometry decreasing in vertical offset southwestward. The constituent faults created independent grabens or half-grabens on the hanging-walls for the deposition of the Early or Middle Miocene strata. The OFS was initially the northwestern border fault of the Janggi Basin which acted as normal faults by the WNW-ESE tensional stress associated with the NNW-directed dextral simple shear caused by the East Sea opening. Afterwards, it experienced clockwise rotation with change of slip sense from normal-slip to sinistral-normal oblique-slip in response to the progressive dextral simple shear. At about 17 Ma, the shear stress propagating westward was released rapidly by the dextral strike-slip faulting of the NNW-trending Yeonil Tectonic Line (YTL) and the normal faulting of the NNE-trending western border faults of the Pohang Basin. At that time, the depocenter suddenly migrated northward and the depositional environment also changed rapidly from terrestrial to marine due to dramatic subsidence of the Pohang Basin. The Pohang Basin is interpreted to be a pull-apart basin extended at releasing bend/overstep between two PDZs (Principal Displacement Zones), i.e., the YTL and probably the East Korea Fault. The OFS was also reactivated as the eastern border faults of the Pohang Basin. In contrast to the western border faults, the OFS was rotated clockwise and could not be linked with the YTL because of its scissor fault geometry. Our results suggest that the NNW-trending regional dextral shear stress persisted for a considerable period of time in SE Korea during the East Sea opening, supporting the pull-apart opening of the East Sea rather than the fan-shaped opening. Most of the previous studies advocating the pull-apart opening emphasize the role of the NNE-trending strikeslip faults, like the Yangsan fault and OFS, as PDZs. In contrast, this study suggests that the NNE-trending faults in SE Korea acted as major normal faults at releasing bends or stepovers in the NNW-trending dextral fault system during the East Sea opening.

The main goal of active fault investigation is obtaining evidence of the Quaternary fault activity through trenching. To accomplish this, electrical resistivity surveys are widely utilized to accurately detect faults buried beneath alluvium. In this study, 2D and 3D electrical resistivity surveys were conducted at the Miho site of the southern Yangsan Fault, the Quaternary active fault area already well-studied via trench investigations. The efficacy of electrical resistivity surveys for fault detection was examined by comparing the resistivity distributions against data in the trench logs. At the Miho site, three lines were installed for the 2D electrical resistivity survey and 22 lines were set within a 27 × 27 m square area for the 3D electrical resistivity survey. The length of each survey line was 27 m with an electrode spacing of 1 m. A dipole-dipole array was used to measure the potential difference between each measurement electrode using the same transmitted current and voltage. To derive the 2D and 3D resistivity distributions from the data acquired in the field, the inversion programs DC_2DPRO and DC_3DPRO were employed. Comparison of the 2D resistivity distribution with trench log data shows that the zone of dacitic welded tuff west of the Quaternary fault plane has relatively high resistivity, while the zone of foliate gouge and breccia derived from sedimentary rocks east of the Quaternary fault plane has a low resistivity of less than 40 Ω·m. From these results, it is evident that the method is effective, particularly when different rock types are distributed on either side of the fault boundary or when highly conductive materials, such as clay-rich fault gouges, are present within the fault zone. The resistivity distribution in the 3D survey area can be depicted using both block diagrams and depth-specific slices, facilitating a spatial understanding of the continuity of fault (or fracture) zones. Consequently, this study demonstrates that 3D surveys offer numerous advantages over 2D surveys by accurately capturing planar structures and enabling spatial interpretation based on 3D resistivity distribution.

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Some parts of the Yangsan Fault, a prominent mature intraplate fault on the Korean Peninsula, are still active. However, structural and paleoseismic investigations are limited because a large portion of the fault zone is covered by Quaternary sediments. To characterize the northern Yangsan Fault (NYF) and its paleoseismic features, we conducted topographic analyses, geological mapping, electrical resistivity surveys, borehole drilling, SHRIMP U–Pb age dating, a trench survey, and optically-stimulated-luminescence age dating (OSL). This multidisciplinary approach shows that the NYF is expressed as a nearly straight incised valley and a ridge-disrupting topographic lineament with anastomosing multiple core zones at outcrops. The NYF in Yeonghae area exhibits a NNE-striking eastern strand and a NNW- to N–S-striking western strand. Between these, a Jurassic granite (> 300-m-wide) is distributed as an enclosed lens, bounded by Precambrian metamorphic rocks to the east and Cretaceous sedimentary rocks to the west. The eastern strand likely passes S–N through the offshore area (East Sea) to onshore Pyeonghae area. A trench survey identified faults transecting Quaternary strata, providing the first paleoseismic record along the NYF. Stratigraphic features and OSL ages show that the most recent rupture occurred after 97 ± 7 ka, with the rupture along the western boundary of the mature fault core. Although older structures are prominent, paleoseismic records are few—a limitation for our onshore investigations. To reveal YF neotectonic activities under the East Sea, we need further information about off-fault damage (landslide, turbidite, tsunami records) within marine deposits as well as on-fault damage.

The Yangsan Fault, a long-lived intracontinental fault in SE Korea, exhibits various slip behaviors, including coseismic slip and aseismic creep. However, there is insufficient knowledge of deformation microstructures to reveal the deformation mechanisms operating within the fault. In this study, we present an analysis of the mechanical behaviors displayed by the Byeokgye section of the Yangsan Fault over seismic cycles. Our results are based on detailed microscopic observations of drillcore samples recovered from the Byeokgye section, using an electron backscattered diffraction (EBSD) technique. In injected calcite veins located close to the principal slip zone (PSZ) of < 2 cm in width, plastic deformation (including dynamic recrystallization by subgrain rotation and deformation twins) is concentrated in the blocky calcite grains. In a narrow microbrecciated slip zone (< 1 cm wide) within the granitic damage zone, we observed mechanical Dauphiné twins associated with fractures and microfaults in quartz, as well as intergranular pressure solution (IPS) in the quartz fragments. Given that dynamic recrystallization and IPS are indicative of mechanical behavior of aseismic creep, it is possible that aseismic creep occurs upon the fault during interseismic periods. Conversely, the presence of mechanical Dauphiné twins, coupled with the nature of the PSZ, gouge injections, and the blocky structure of calcite veins, suggests the exposure of the fault section to local seismic stresses during coseismic slip. In conclusion, various deformation processes have operated upon the Yangsan Fault at the studied section throughout multiple seismic cycles. Furthermore, our study demonstrates that EBSD analysis is an effective technique for elucidating the mechanical behavior of fault zones.

We studied the Quaternary incised fills drilled at the northern Yangsan Fault having multiple deformation histories since Late Cretaceous or Paleogene to determine tectonic influence on development of incised valley and its sedimentation. Incised valley fills were deposited during and after the Last Glacial Maximum and are composed of fluvial lag, debris flow deposits interbedded with fluvial sediments, shallow marine sandy deposits, and fluvial sediments from bottom to top. These fills show lateral changes in sediment thickness from 44 to 11.5 m over a short distance of 230 m, implying sediment stacking in a deep and steeply inclined valley. Fluvial lag and debris flow deposits are common in the thalweg of a valley. Despite small drainage basin (195.9 km2), the development of deep incised valley is interpreted to have resulted from fluvial downcutting on erodible basement during sea level fall as a consequence of dense development and fault and fracture networks in the pre-Quaternary rocks caused by multiple movements of Yangsan Fault. With steep gradient, the damaged rocks led to frequent slope failure and forceful accumulation of debris flow deposits on the valley’s axis at the time. In addition, stacking of debris flow deposits resulted in decrease of longitudinal gradient of incised valley, promoting rapid transgression during sea level rise (9 to 7 ka). This resulted in insufficient time for the central basin mud to be accumulated, which explains why the studied fills lack central estuarine mud that is common in incised valleys fills deposited during transgression.