• Journal of the Mineralogical Society of Korea
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• v.18 no.3 s.45
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• pp.165-181
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• 2005

The granitic rocks distributed in the southern part of the Yangsan Fault are classified into five distinct rock facies based on the field relation, petrography and geochemical characteristics. These five different rock facies can be grouped into two considering their origins. Group I, which reveals various evidences of magma mixing, includes three rock facies of granodiorite, enclave-rich porphyritic granite, and enclave-poor porphyritic granite. Group H intruding Croup I includes equigranular granite and micrographic granite with no evidence of magma mixing. It is suggested that the distinctively different trace element and isotopic chemistries between group I and II, support evolution from the different parental magma. It is suggested that the three rock facies in group I were generated by different degrees of magma mixing in addition to fractionation of plagioclase. MMEs experienced fractionation of biotite. The two facies in group H seem to have been generated from different parent magma from group I and evolved by fractionation of K-feldspar. The Rb-Sr whole-rock ages of the group I rocks yield $59.2\~58.9Ma$, and those of the group II rocks give 53. $3\~51.7Ma$, regardless of their distribution whether they occur in the eastern or western parts of the Yangsan Fault. Based on Sm-Nd isotope compositions, depleted mantle model ages $(T_2DM)$ of the group I range $0.8\~0.9Ga$, while those of the group II$0.6\~0.7Ga$.

• The Journal of Engineering Geology
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• v.13 no.1
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• pp.107-127
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• 2003

After detailed geological mapping, structural and fracture-density data were collected and analyzed in the vicinity of Cheonseong and Jeongjok Mts., Gyeongsangnam-do. A extensive dextral strike-slip fault (Beopgi Fault) Parallel to Yangsan and Dongrae Faults, a dextral-transtensional-NW fault, and a few intermittent faults have been found in the study area. Based on strike and frequency, fracture system has been divided into three sets such as NNE-trending J1 ($NS-40^{\circ}E$), WNW-trending J2 ($N50^{\circ}-80^{\circ}W$), and ENE-trending J3 ($N60^{\circ}-90^{\circ}E$). According to analysis of fracture density, it is revealed as follows: (a) Jl is the combination of Y-, P-, and R-shear fractures due to the dextral strike-slip of the Beopgi Fault. (b) J2 is the preexisted fracture zone conducting the intrusion of granite. Two tensional fractures dipping to NNE and SSW respectively have been induced by intrusion of granite and followed crustal uplift. (c) J3 is the tensional fracture developed between Yangsan and Dongrae Faults having NNE trend and dextral strike-slip sense. This study aims to reduce environmental impact and insure stability of underground facilities and tunnels.

• The Journal of Engineering Geology
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• v.29 no.1
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• pp.1-12
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• 2019

Earthquake can change underground stress condition around the hypocenter and affect the fracture systems of the rocks. In Korea, the M5.8 Gyeongju earthquake on September 12, 2016 and M5.4 Pohang earthquake on November 15, 2017 occurred inside the Yangsan fault zone and possibly affected the fracture systems in the Yangsan fault zone and nearby rock masses. In this study, the characteristics of the fracture system (fracture orientation, number of the fractures, fracture spacing and aperture, dip angle, fracture density along depth, and relative rock strength) of the rocks in the low/intermediate level radioactive waste repository site located in the coastal area of the East Sea are analyzed by the impact of the Gyeongju and Pohang earthquakes using acoustic televiewer data taken from the boreholes at the radioactive waste repository site in 2005 and 2018. As a result of acoustic televiewer logging analysis, the fracture numbers, fracture aperture, and fracture density along depth overall increased in 2018 comparing to those in 2005. This increase tendency may be due to changes in the fracture system due to the impact of the earthquakes, or due to weathering of the wall of the boreholes for a long period longer than 10 years after the installation of the boreholes in 2005. In the borehole KB-14, on the whole, the orientation of the fractures and the average fracture spacing are slightly different between 2005 and 2018, while dip angle and relative rock strength in 2005 and 2018 are similar each other.

• Journal of the Korean earth science society
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• v.43 no.1
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• pp.151-164
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• 2022

According to the records of historical and instrumental earthquakes, the southeastern part of the Korean Peninsula is considered the highest seismic activity area. Owing to recent reports of numerous Quaternary faults along the Yangsan and Ulsan fault zones, paleoseismological studies are being actively conducted in these areas. The study area is located in the central part of the Ulsan fault zone, where the largest number of active faults have been reported. Based on lineament and geomorphic analysis using LiDAR images and aerial photographs, fault-related landforms showing topographic relief were observed and a trench survey was conducted. The trench length 20 m, width 5 m, depth 5 m is located approximately 300 m away to the northeast from the previously reported Malbang fault. From the trench section, we interpreted the geometric and kinematic characteristics of the fault based on the deformed features of the Quaternary sedimentary layers. The attitude of the reverse fault, N26°W/33°NE, is similar to those of the reported faults distributed along the Ulsan fault zone. Although a single apparent displacement of approximately 40 cm has been observed, the true displacement could not be calculated due to the absence of the slickenline on the fault plane. Based on the geochronological results of the cryogenic structure proposed in a previous study, the most recent faulting event has been estimated as being earlier than the late Wurm glaciation. We interpreted the thrust fault system of the study area as an imbrication structure based on the previous studies and the fault geometry obtained in this additional trench. Although several previous investigations including many trench surveys have been conducted, they found limited success in obtaining the information on fault parameters, which could be due to complex characteristics of the reverse fault system. Additional paleoseismic studies will contribute to solving the mentioned problems and the comprehensive fault evolution.

• The Journal of Engineering Geology
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• v.11 no.1
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• pp.101-113
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• 2001

Structural and geological engineering properties of the rock mass distributed in the Yokmang mountain area were investigated to detenninc the usable cut-out volume and quarrying efficiency. The study area is located in the southern tip of the Yangsan fault system which controls the geological structure of the Kvungsang basin. As a result, the study area is mainly composed of andesicic. rhyolitic. and granitic rocks of the Cretaceous Kyungsang Supergroup and a series of right-handed strike-slip faults is developed along NNE-SSW direction. These regional faults significantly affect the spatial and meclwnical characteristics of joints such as spacing, frequency, and compressive strength. The joint frequency is highest along the fault zones and decreases toward the remote region. Based on the geological information obtained from the field survey, the detailed structure of the Yokmang mountain was analyzed and the volume of the rock mass was assessed. Considering the minimum rock block size required for the construction of a coastal dumping site, potential cut-out volume is then estimated to be 4,018,000m$^3$ the volume % of which is 48% of Yokmang mountain including the soil and weathered rock and 61% of the unweathered rock mass.

• The Journal of the Petrological Society of Korea
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• v.23 no.2
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• pp.75-103
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• 2014

The Quaternary faults are extensively observed along major inherited fault zones (i.e. Yangsan Fault System, Ulsan Fault, Yeonil Tectonic Line, Ocheon Fault System) in SE Korea. Their geometry and kinematics provide a very useful piece of information about the Quaternary crustal deformation and stress field in and around Korean Peninsula. Using magnetic fabrics (AMS), we attempted to determine the slip senses of Jinti, Mohwa, Suseongji2, and Wangsan faults and then interpreted the fabric development process of fault gouge and the characteristics of stress field during the Quaternary. All the magnetic fabrics of the faults, except the Wangsan Fault, consistently indicate a dominant reverse-slip sense with weak strike-slip component. Most of the oblate fabrics are nearly parallel to the fault surface and the anisotropy degrees generally increase in proportion to the oblatenesses. These results suggest that the fabrics of the fault gouges resulted from a progressive deformation due to continuous simple shear during the last reactivation stage as reverse faulting. It is also interpreted that the pre-existing fabrics were overwhelmed and obliterated by the re-activated faulting. Paleostress field calculated from the fault slip data indicates an ENE-WNW compressive stress, which is in accord with those determined from previous fault tectonic analysis, focal mechanism solution, and hydraulic fracturing test in and around Korean Peninsula.

• Economic and Environmental Geology
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• v.23 no.1
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• pp.59-71
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• 1990

Magnetic anisotropy of a total of 213 independently oriented Tertiary rock samples from Pohang-Ulsan area has been studied. The sampled strata comprise basalts, tuffs and black shale, and range in age from Eocene to Miocene. The previous palaeomagnetic studies indicate that their magnetic carrier minerals are titanomagnetites. Among 23 sampled sites, 11 sites were found to preserve magnetic load foliation parallel to the bedding plane caused by the Iithostatic load of the overlying strata. Other 4 sites showed magnetic lineation indicating the flow direction of lava and tuffs. The remaining 8 sites revealed the magnetic tectonic foliation nearly vertical to the bedding plane. This magnetic foliation is interpreted to be generated by tectonic compression which acted nearly horizontally during the solidification stage of the strata. The compression directions deduced from the tectonic foliation of the 8 sites can be grouped into internally very consistent two group: a N-S trending one and the other WNW-ESE trending one. It is interpreted that the former N-S compression was associated with the N-S spreading of the East Sea(Sea of Japan) and the dextral strike-slip movement of the Yangsan-Ulsan fault system. The latter WNW-ESE compression is interpreted to represent the folding and reverse faulting activity in the Korean and Tsushima straits during middle/late Miocene times.

• Economic and Environmental Geology
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• v.51 no.4
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• pp.393-400
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• 2018

Despite some skeptical views on the possibility of earthquake prediction, observation and evaluation of precursory changes have been continued throughout the world. In Korea, the public concern on the earthquake prediction has been increased after 2016 $M_L5.8$ and 2017 $M_L5.4$ earthquakes occurred in Gyeongju and Pohang, the southeastern part in Korea, respectively. In this study, the abnormal increase of groundwater level was observed before the 2016 $M_L5.8$ Gyeongju earthquake in a borehole located in 52 km away from the epicenter. The well was installed in the Yangsan fault zone, and equipped for the earthquake surveillance. The abnormal change in the well would seem to be a precursor, considering the hydrogeological condition and the observations from previous studies. It is necessary to set up a specialized council to support and evaluate the earthquake prediction and related researches for the preparation of future earthquake hazards.

• Economic and Environmental Geology
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• v.54 no.2
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• pp.177-186
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• 2021

For disaster management and mitigation of earthquakes in Korea Peninsula, active fault investigation has been conducted for the past 5 years. In particular, investigation of sediment-covered active faults integrates geomorphological analysis on airborne LiDAR data, surface geological survey, and geophysical exploration, and unearths subsurface active faults by trench survey. However, the fault traces revealed by trench surveys are only available for investigation during a limited time and restored to the previous condition. Thus, the geological data describing the fault trench sites remain as the qualitative data in terms of research articles and reports. To extend the limitations due to temporal nature of geological studies, we utilized a terrestrial LiDAR to produce 3D point clouds for the fault trench sites and restored them in a digital space. The terrestrial LiDAR scanning was conducted at two trench sites located near the Yangsan Fault and acquired amplitude and reflectance from the surveyed area as well as color information by combining photogrammetry with the LiDAR system. The scanned data were merged to form the 3D point clouds having the average geometric error of 0.003 m, which exhibited the sufficient accuracy to restore the details of the surveyed trench sites. However, we found more post-processing on the scanned data would be necessary because the amplitudes and reflectances of the point clouds varied depending on the scan positions and the colors of the trench surfaces were captured differently depending on the light exposures available at the time. Such point clouds are pretty large in size and visualized through a limited set of softwares, which limits data sharing among researchers. As an alternative, we suggested Potree, an open-source web-based platform, to visualize the point clouds of the trench sites. In this study, as a result, we identified that terrestrial LiDAR data can be practical to increase reproducibility of geological field studies and easily accessible by researchers and students in Earth Sciences.

• Economic and Environmental Geology
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• v.39 no.4 s.179
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• pp.507-524
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• 2006

3-D seismic tomographic inversion is applied to investigation on velocity structure in and near Korean Peninsula. Firstly, it is applied to the region in southeastern Korean Peninsula. According to the results low-velocity zone seems to be clearly appeared in the so called Gyeongsang sedimentary basin and high-velocity zone is shown at the section of 7.5 km depth it implies the inclusion of plutonic rocks at the sedimentary basin. At the depth about $20{\sim}30$ km existence of low-velocity zone seems to be related with the development of Yangsan fault system. Secondly it is applied to the region not only in Korean Peninsula but also East Sea using data from both Korean Peninsula and Japan Islands. Accorging to the results, subduction zone starting from eastern part of Japan seems to be extended to the region beneath the East Sea.