• 한국지구물리탐사학회:학술대회논문집
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• 2005.05a
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• pp.9-14
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• 2005

For the understanding the locus of the Quinling-Dabie-Sulu continental collision's boundary and the underground structure of the sedimentray basin in the Yellow Sea, three dimensional density modelling is carrid out by using gravity dataset (Free Air Anomaly), which is measured by Tamhae 2, KIGAM in a period 2000 - 2002. The measured gravity anomaly in the investigations area is mainly responsed by depth distribution of the sedimentary basin. After comparing the sea-measured gravity data to CHAMP-GRACE satellite gravity data, I suggested that the high density model bodies extend mainly from the southern part of China to the middle-western part of the Korean Peninsula., which might be emplaced along the continental collision's boundary. The total volume of very low density bodies modified by modelling might be about $20000\;km^3$.

• Journal of the Korean Geophysical Society
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• v.2 no.2
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• pp.123-134
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• 1999

Magnetotelluric responses may be affected by strong anisotropy of the high-conductivity layers (HCL) in the upper mantle or lower crust. We have studied two-dimensional anisotropy MT modelling to examine the effect of high anisotropic media. Electrical properties of a homogeneous anisotropic body are defined by a symmetric conductivity tensor and the problem is described by coupled diffusion equation in the frequency domain. In two-dimensional anisotropic environments, diagonal elements of the impedance tensor have higher values than those in isotropic environments. In some cases, TM mode phases reach more than 90°and apparent resistivities decrease for some frequency range because of telluric distortion. GB decomposition may be used to recover regional responses, but can be affected by the regional anisotropic effect. Considering these results, BC87 dataset was interpreted with a modified anisotropic model.

• Geophysics and Geophysical Exploration
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• v.16 no.1
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• pp.18-26
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• 2013

Shear-wave velocity ($v_s$) structures beneath two seismic stations, JJU and JJB on the flanks of the volcano Halla on Jeju island, Korea, were estimated by receiver-function inversion and H-${\kappa}$ stacking applied to 150 teleseismic events ($M_W{\geq}5.5$) recorded since 2007. $P_S$ waves converted at the Moho discontinuity does not appear clearly for northwesterly back-azimuths ($207{\sim}409^{\circ}$, average $308^{\circ}$) at station JJU and southeasterly back-azimuths ($119{\sim}207^{\circ}C$, average $163^{\circ}$) at station JJB. This may be due to a gradual velocity increase at Moho or heterogeneity within the crust. The $v_s$ models derived by inversion of receiver functions indicate a distinct low velocity layer ($v_s{\leq}3.5km/s$; LVL) within the crust and a gradual increase in $v_s$ in the depth interval of 30 to 40 km. Within the radius of 18 km beneath station JJB, the LVL occurs at depths of 14 ~ 26 km and the 'Moho' ($v_s{\geq}4.3km/s$) is at 34 km depth. Ten kilometers to the west, within the radius of 16 km beneath station JJU, both the LVL and the Moho are significantly shallower, at depths of 14 to 24 km and 30 km, respectively. H-${\kappa}$ analyses for stations JJU and JJB yield estimated crustal thickness of 29 and 33 km and $v_p/v_s$ ratios of 1.64 and 1.75, respectively. The lesser $v_p/v_s$ ratio was derived for rocks nearest to th peak of the volcano.

• The Journal of Engineering Geology
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• v.32 no.1
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• pp.113-126
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• 2022

The tectonic history of the Chukchi Abyssal Plain in the Amerasia Basin, Arctic Ocean, has not been fully explored due to the harsh conditions of sea ice preventing detailed observation. Existing models of the tectonic history of the region provide contrasting interpretation of the timing of formation of the crust (Mesozoic to Cenozoic), crust type (from hyper-extended continental crust to oceanic crust), and formation process (from parallel/fan-shaped rifting to transformation faulting). To help determine the age of the oceanic crust, the geothermal gradient was measured at three stations in the south of abyssal plain at depth of 2,160-2,250 m below sea level. Heat flow measurement stations were located perpendicular to the spreading axis over a 40 km-long transect. In-situ thermal conductivity measurement, corrected by the laboratory test, gave observed marine heat flows of 55 to 61 mW/m². All measurements were taken during Arctic expeditions in 2018 (ARA09C expedition) and 2021 (ARA12C expedition) by the Korean ice-breaking research vessel (IBRV) Araon. Given the assumption of oceanic crust, the results correspond to formation in the Late Cretaceous (Mesozoic). The inferred age supports the hypothesis of formation activated by the opening of the Makarov Basin during the Late Mesozoic-Cenozoic. This would make it contemporaneous with rifting of the Chukchi Border Land immediately east of the abyssal plain. The heat flow data indicate the base of the gas hydrate stability zone is located 332-367 m below the seafloor, this will help to identify the gas hydrate-related bottom simulating reflector in the future seismic survey, as already identified on the Chukchi Plateau. Further geophysical surveys, including heat flow measurements, are required to increase our understanding of the formation process and thermal mantle structure of the abyssal plain.

• Journal of the Korean Geophysical Society
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• v.1 no.1
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• pp.3-22
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• 1998

Korea boasts of abundant historical earthquake records of almost 1900 events. The epicenters and intensities of these earthquakes are determined on the basis of descriptions and felt areas of the events. It turns out that most of the earthquakes occurred on major faults or tectonic boundaries of the peninsula except for the northeastern part which had been the least disrupted by tectonic disturbances during the Mesozoic. It appears that the crustal layers of the southern and northwestern parts of the peninsula had been severely ruptured during the Mesozoic disturbances and some of the faults thus generated have been active since. The seismicity of the peninsula had been rather low from the first to the fourteenth century, but unusually high from the fifteenth to the eighteenth century, and have been rather low since. This period of unusually high seismicity of the peninsula coincides with that of the northeastern part of China, suggesting the two areas are seismologically closely connected. It appears that most of the seismicity of the peninsula results from the high stress propagating from the Himalayas where the Eurasian and Indian plates collide. The data file of Korean historical earthquakes is not yet complete and supplementary studies are under way. The main purpose of this paper is to provide the data file of Korean historical earthquakes analyzed up to date for geoscientists and engineers in need of this file.

• The Journal of the Petrological Society of Korea
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• v.25 no.1
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• pp.85-88
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• 2016

The new rocks of the oceanic crust, like basalt, are created in the mid-oceanic ridge, and the magnetic polarities of the rocks are supposed to be oriented as following the Earth's magnetic field. An extensive magnetic survey of total field at sea level reveals mainly unusual north-south magnetic stripes parallel to the axis of the mid-oceanic ridge, especially in the Atlantic Ocean. From this stripes the Earth's magnetic field is considered as repeatedly 'flipped'(the N pole becoming the S pole, and vice versa) and many times over geological time. The discovery of stripes of alternately normal and reversed-magnetized rocks forming the ocean floor has been a key evidence for the sea-floor spreading, continental drift, and plate tectonics. This study introduces a simple apparatus to explain a possible mechanism of the magnetic reversal in the new oceanic crust, which makes a magnetic stripe adjacent to the mid-oceanic ridge. The apparatus shows a bar magnet effect of adjoined stripes to have a special magnetic polarity on the rocks in the center of the mid-oceanic ridge. The new magnetic stripe seems to be generated not only by Earth's magnetic field, but also by neighbored stripes in the mid-oceanic ridge, acting as a bar magnet.

• Journal of the Korean Geophysical Society
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• v.8 no.1
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• pp.45-48
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• 2005

In order to investigate the velocity structure of the southern part of the Korean peninsula, exploded seismic signals were recorded for 120 s along a 294-km WNW-ESE line and 150 s along a 335-km NNW-SSE line in 2002 and 2004, respectively. Velocity tomograms were derived from inverting first arrival times. One-dimensional velocity models derived by joint analyses of teleseismic receiver functions and surface wave dispersion at several stations near the profiles were uesd to build initial models. The raypaths indicate several midcrust interfaces including ones at approximate depths of 2.0 and 14.9 km with refraction velocities of approximately 6.0 and 7.1 km/s, respectively. The deepest significant interface varies in depth from 30.8 km to 36.1 km. The critically refracting velocity varies from 7.8 to 8.1 km/s along this interface which may correspond to the Moho discontinuity. The velocity tomograms show (1) existence of a low-velocity zone centered at 6-7 km depth under the Okchon fold belt, (2) extension of the Yeongdon fault down to greater than 10 km, and (3) existence of high-velocity materials under the Gyeongsan basin whose thickness is less than 4.2 km.

• Economic and Environmental Geology
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• v.33 no.2
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• pp.101-109
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• 2000

The gravity measurement has been carried out to study the deep geologic structure at 331 gravity stations with an interval of 1∼1.5 km along the national road which crosses the southern part of the Korean peninsula from Pohang to Manripo. The Bouguer gravity anomalies were obtained from the observed gravity values, and interpreted by means of upward continuation using FFT (Fast Fourier Transform), Fourier-series method and nonlinear 2-D inversion method to determine the depths of Conrad and Moho discontinuities. The linear regression relations between elevations and gravity anomalies were also obtained to test isostasy in the study area. The depth of Conrad discontinuty is 13km between Pohang and Daegu, 16.5 km between Kimchon and Okchon, 9.7 km between Okchon and Daejeon, and 16.3 km near Manripo. The depth of Moho discontinuty is 32km between Pohang and Daegu, 35 km between Kimchon and Okchon, 28.7 km between Okchon and Daejeon, 40.5 km between Daejeon and Kongju, and 34.5 km between Kongju and Manripo. The result of testing isotasy indicates that the crust of this area seems to be not in perfect isostatic equilibrium but in a little undercompensated sate.

• Journal of the Korean Geophysical Society
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• v.7 no.4
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• pp.283-291
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• 2004

To find out the locus of the Quinling-Dabie-Sulu continental collision’s boundary and to estimate underground structure of the sedimentray basin in the Yellow Sea, three dimensional density modelling is carrid out by using gravity dataset (Free Air Anomaly), which is measured by Tamhae 2, KIGAM in a period between 2000 and 2002. The measured gravity anomaly in the investigations area is mainly responsed by depth and density differences between the sedimentary basin and the basement. The high density model-bodies extend mainly from the southern part of China to the middle-western part of the Korean Peninsula, which might be emplaced along the continental collision’s boundary. The total volume of the very low density model-bodies might be expected at about 20,000 km3 in the model area.

• Journal of the Korean Geophysical Society
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• v.5 no.1
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• pp.1-8
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• 2002

The attenuation study of Lg waves is very important in the southeast Korea because the Yangsan fault, believed to be active faults, lied in the industrialized region of the area. By applying the reversed two-station method for the vertical component of the velocity seismogram, we first estimated the Lg attenuation coefficient in this area: $${＼gamma}=(0.009±0.0005)＼;f^{0.06+0.03}$$ between 0.87 and 10 Hz. The ${Q_{Lg}}^{-1}$ values converted from ${＼gamma}$ prove to be higher than those of S-waves, and show the highest values in the world for the high frequency part around 10 Hz. This high attenuation of Lg may be related to a block of Lg propagation near the East Sea and/or an undulately thinning crust of the studied area.