• Economic and Environmental Geology
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• v.49 no.2
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• pp.97-104
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• 2016

Here we firstly present that a timing of reactivated event of Yangsan fault, the major fault in the southeastern Korean Peninsula, by using combined approaches of the optimized illite-polytype quantification, the K-Ar age-dating, and the recently developed illite-age-analysis (IAA) approach for the fault clays from Sangcheon-ri area of Yangsan main fault line. Two chronological record of brittle fault-activation event at about 41.5~43.5 and 50.7 Ma were determined from 3 fault gouges suggesting a crucial reactivation time-scheme. Furthermore, the regional processes that drive tectonics to form and reactivate the Yangsan fault may be explained from the chronological analysis for additional sites along the Yangsan fault.

• Proceedings of the Earthquake Engineering Society of Korea Conference
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• 2001.04a
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• pp.66-73
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• 2001

Yangsan fault in the southeastern part of Korean peninsula has been locally reactivated along a prexisted fault during the late Pleistocene time. Geomorphological evidence of the reactivation is revealed at the northern segment(Yugye-ri, Tosung-ri areas) of the Yangsan fault. The reactivation is distinctively characterized by fault gouge and fracture zone with high frequency in the Yugye-ri area. Obique slip separation of the area is about three meters of the middle terrace. The cumulative vertical displacement is recognized after the formation of the middle terrace. Age of the reactivated faulting is constrained to during the formation of dissected valley deposits. Average vertical slip based on paleo-event is inferred to about 0.5-0.7 meter in this area.

• The Journal of Engineering Geology
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• v.33 no.1
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• pp.121-136
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• 2023

Microstructural characterization, identification of mineral assemblages, and K-Ar age dating of fault gouges from five Quaternary fault sites segmented along the northern Yangsan Fault, SE Korea were performed to understand formation condition and multiple activity of faults. The mean and median sizes of particles of bulk gouges vary among the studied faults: 1.75 ㎛ and 1.43 ㎛ for the Danguri Fault, 1.94 ㎛ and 1.79 ㎛ for the Yukjae Fault, 5.57 ㎛ and 4.16 ㎛ for the Yugye Fault, and 5.55 ㎛ and 2.31 ㎛ for the Bogyeongsa Fault. Fault gouges contain abundant secondary minerals, including smectite, chlorite, illite, kaolinite, laumontite, and mordenite, which are found in association with quartz and feldspar. K-Ar dating of the fault gouges (both bulk samples and separate size fractions) yields ages ranging from 59.1 to 18.8 Ma, with bulk ages of 47.6 Ma for the Yukjae Fault, 59.1 Ma for the Ansim Fault, 39.4 Ma for the Yugye Fault, and 22.6 Ma for the Bogyeongsa Fault. The finer fractions generally have younger K-Ar ages compared with the coarser fractions, and the finest fraction (<0.2 ㎛) is the youngest for each fault. Hydrothermal alteration of the gouges is considered to have occurred under low-temperature (100~200℃) conditions during faulting. Microstructural features and clay mineral assemblages of fault gouges and brecciated rocks should be considered when interpreting fault events and reactivation, in addition to age dating of faulting.

• Economic and Environmental Geology
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• v.45 no.2
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• pp.181-188
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• 2012

The 1M and $2M_1$ stacking sequences are the most frequently encountered in the illite species among the possible 6 polytypes. The $1M_d$, derived from the 1M polytype which exhibits a variable degree of disorder in the stacking sequence, is also observed in illite samples. In this paper, the author introduces and reviews the theoretical background of the quantitative analysis method of illite polytypes, and considers the possibility to determine the fault age and its reactivation age using K/Ar age-dating based on the quantification of illite polytypes in the fault system. For the increase of the accuracy and precision of the illite age analysis method, the occurrence, identification, and mineralogical characterization of illite polytypes should be defined in detail. The broadening effect of (hkl) reflections, due to disordering of 1M polytype and the presence of I/S minerals with expandability, are also considered as the main parameters controlling the quantification of illite polytypes using the WILDFIRE(C)simulation.

• Economic and Environmental Geology
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• v.50 no.2
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• pp.97-104
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• 2017

Here we present the timings of reactivated events from a fault in the northern Pohang area, which should be located at the northern-end of Yangsan fault line, the major fault in the southeastern Korean Peninsula. Recently developed illite-age-analysis (IAA) approach was employed for determining the fault-activated timing, combined with illite-polytype quantification using the optimized full-pattern-fitting (FPF) method, and K-Ar age-dating for each size fraction($<0.1{\mu}m$, $0.1-0.4{\mu}m$, and $0.4-1.0{\mu}m$) of 4 fault clay samples. Two chronological records of brittle fault-activation events were recognized at $19.6{\pm}1.86Ma$ and $26.1{\pm}2.55-27.9{\pm}3.46Ma$. The ages are much younger than those of fault clays from Sangcheon-ri area (41.5~43.5 and 50.7 Ma), the southern part of Yangsan fault line, and are close to the timing of East Sea-opening event. Further chronological analysis for additional sites of the Yangsan fault should be needed to reveal the time-scheme of the tectonic events and their spatial distributions along the fault line.

• Economic and Environmental Geology
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• v.48 no.1
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• pp.41-49
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• 2015

Recently developed illite-age-analysis(IAA) approach was applied to determine the fault-reactivated events for the Inje fault that cut through Precambrian biotite granitic gneiss with NNE-SSW trend in the middle of Korean peninsula. Three distinct fault-reactivated events of shallow crustal regime were recognized using the combined approach of optimized illite-polytype quantification and K-Ar age-dating of clay fractions separated from 4 fault clay samples: $87.0{\pm}0.12Ma$, $65.5{\pm}0.05$ and $66.6{\pm}1.38Ma$, $45.6{\pm}0.15Ma$, respectively. As well, $2M_1$ illite ages of 193~196 Ma and $254.3{\pm}6.96Ma$ were discernible, which may be related to the fault-activated time in the relatively deep crust. The study results suggest that the Inje fault would be firstly formed at $254.3^{\circ}$ ${\ae}6.96Ma$ and sporadically reactivated in shallow regime since about 87 Ma. These reactivation events in shallow regime might be due to the Bulguksa orogeny that would be strongly influenced in Korean peninsula at that time.

• The Journal of the Petrological Society of Korea
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• v.25 no.3
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• pp.195-209
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• 2016

The understanding of geometric complexity of strike-slip Fault system can be an important factor to control fault reactivation and surface rupture propagation under the regional stress regime. The Kumamoto earthquake was caused by dextral reactivation of the Futagawa-Hinagu Fault system under the E-W maximum horizontal principal stress. The earthquakes are a set of earthquakes, including a foreshock earthquake with a magnitude 6.2 at the northern tip of the Hinagu Fault on April 14, 2016 and a magnitude 7.0 mainshock which generated at the intersection of the two faults on April 16, 2016. The hypocenters of the main shock and aftershocks have moved toward NE direction along the Futagawa Fault and terminated at Mt. Aso area. The intersection of the two faults has a similar configuration of ${\lambda}$-fault. The geometries and kinematics, of these faults were comparable to the Yansan-Ulsan Fault system in SE Korea. But slip rate is little different. The results of age dating show that the Quaternary faults distributed along the northern segment of the Yangsan Fault and the Ulsan Fault are younger than those along the southern segment of the Yansan Fault. This result is well consistent with the previous study with Column stress model. Thus, the seismic activity along the middle and northern segment of the Yangsan Fault and the Ulsan Fault might be relatively active compared with that of the southern segment of the Yangsan Fault. Therefore, more detailed seismic hazard and paleoseismic studies should be carried out in this area.