• Geophysics and Geophysical Exploration
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• v.21 no.1
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• pp.33-40
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• 2018

We investigate the crustal and uppermost mantle SV- and SH-wave velocity structure and radial anisotropy beneath East Asia including Korea, China and Japan. Rayleigh waves and Love waves were extracted from the seismic data recorded at broadband seismic stations in East Asia. Using the MFT (Multiple Filter Technique), we obtained group velocity dispersion curves of Rayleigh and Love waves with a period range of 3 to 200 s. We obtained 62466 Rayleigh-waves dispersion-curve measurements in vertical components and 54141 Love-waves dispersion-curve measurements in transverse components, respectively. The inverted models using these data sets provide SV- and SH-wave velocity structure of crust and uppermost mantle down to 100 km depth. In both cases of the S-wave velocity structures, strong high-velocity anomalies are observed down to 30 km depth beneath the East Sea, and deeper than 30 km depth, strong low-velocity anomalies are found beneath the Tibetan plateau. In the case of the SH-wave velocity structure, strong low-velocity anomalies are observed beneath the East Sea deeper than 30 km depth, leading to negative anisotropy. On the other hand, positive anisotropy is usually observed beneath the Tibetan plateau.

• Economic and Environmental Geology
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• v.39 no.4 s.179
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• pp.369-384
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• 2006

Korean Peninsula, located on the southeastern part of Eurasian plate, belongs to the intraplate region. The characteristics of intraplate earthquake show the low and rare seismicity and the sparse and irregular distribution of epicenters comparing to interplate earthquake. To evaluate the exact seismic activity in intraplate region, long-term seismic data including historical earthquake data should be archived. Fortunately the long-term historical earthquake records about 2,000 years are available in Korea Peninsula. By the analysis of this historical and instrumental earthquake data, seismic activity was very high in 16-18 centuries and is more active at the Yellow sea area than East sea area. Comparing to the high seismic activity of the north-eastern China in 16-18 centuries, it is inferred that seismic activity in two regions shows close relationship. Also general trend of epicenter distribution shows the SE-NW direction. In Korea Peninsula, the first seismic station was installed at Incheon in 1905 and 5 additional seismic stations were installed till 1943. There was no seismic station from 1945 to 1962, but a World Wide Standardized Seismograph was installed at Seoul in 1963. In 1990, Korean Meteorological Adminstration(KMA) had established centralized modem seismic network in real-time, consisted of 12 stations. After that time, many institutes tried to expand their own seismic networks in Korea Peninsula. Now KMA operates 35 velocity-type seismic stations and 75 accelerometers and Korea Institute of Geoscience and Mineral Resources operates 32 and 16 stations, respectively. Korea Institute of Nuclear Safety and Korea Electric Power Research Institute operate 4 and 13 stations, consisted of velocity-type and accelerometer. In and around the Korean Peninsula, 27 intraplate earthquake mechanisms since 1936 were analyzed to understand the regional stress orientation and tectonics. These earthquakes are largest ones in this century and may represent the characteristics of earthquake in this region. Focal mechanism of these earthquakes show predominant strike-slip faulting with small amount of thrust components. The average P-axis is almost horizontal ENE-WSW. In north-eastern China, strike-slip faulting is dominant and nearly horizontal average P-axis in ENE-WSW is very similar with the Korean Peninsula. On the other hand, in the eastern part of East Sea, thrust faulting is dominant and average P-axis is horizontal with ESE-WNW. This indicate that not only the subducting Pacific Plate in east but also the indenting Indian Plate controls earthquake mechanism in the far east of the Eurasian Plate. Crustal velocity model is very important to determine the hypocenters of the local earthquakes. But the crust model in and around Korean Peninsula is not clear till now, because the sufficient seismic data could not accumulated. To solve this problem, reflection and refraction seismic survey and seismic wave analysis method were simultaneously applied to two long cross-section traversing the southern Korean Peninsula since 2002. This survey should be continuously conducted.

• Journal of the Korean Geophysical Society
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• v.5 no.2
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• pp.87-97
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• 2002

P wave travel-time delays have been analyzed and average travel-time anomalies due to lateral variation of the crustal structure have been calculated at the broad-band earthquake observatories of Korea Meteorology Administration (KMA) using the teleseismic deata collected during the period from 2000 to 2001. Maximum variation in the relative travel-time residuals is almost 1.5 seconds Azimuthal variation in the travel-time residuals is observed to indicate the existence of lateral P velocity heterogeneity beneath the stations with velocity contrasts of -4~4%. The estimated average travel-time delays are ranging from 0.05 to 0.3 seconds at the stations including Seoul, Chuncheon, Kanneung, Uljin, and Ulleung Island showing slow velocity contrasts of 0~4% P through the crust. Faster velocity contrasts of 0~4% have been observed at the stations including Seosan, Taejeon, Daegu, Seoguipo, and Busan showing average travel-time delays ranging from -0.05 to -0.3 seconds.

• Ocean and Polar Research
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• v.24 no.3
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• pp.179-187
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• 2002

The Ayu Trough, located in the southern end of the Philippine Sea, represents a divergent boundary between the Philippine Sea and the Caroline Plates. A detailed geophysical survey was carried out in the Ayu Trough by R/V Onnuri. Topographically, the Ayu Trough resembles an slow spreading ridge. The trough can be divided into three sections: the south $(0^{\circ}-1^{\circ}30'N),\;middle\;(1^{\circ}30'-4^{\circ}N)$, and north $(4^{\circ}-6^{\circ}30'N)$. The seafloor in the middle section is characterized by features asymmetric with respect to the axis. These features were probably produced by NW-SE and NNW-SSE extensions and seem to support the argument that the opening of the Ayu Trough occurred in an oblique fashion. Farther south, a long transform fault but with a short offset defines the boundary between middle and southern sections. The axial depth increases a stepwise to the south of $1^{\circ}30'N$. A clear difference can be seen between the southern and middle sections with the latter exhibiting much higher mantle Bouguer anomaly values in the axial region. The anomaly indicates that the axial crust perhaps experienced a much higher degree of extension in the middle than in the southern section. The analyses of magnetic field data reveal that the region beyond 100km exhibits considerable variations, whereas the magnetic anomalies within 100km from the trough axis are very much subdued. This observation suggests that the opening of the Ayu Trough involved an initial stage of rifting of existing volcanic arcs, followed by production of new seafloor.

• Economic and Environmental Geology
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• v.38 no.3 s.172
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• pp.299-304
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• 2005

Geoid undulation and gravity anomaly were calculated from GRACE satellite data on the eastern Asia including Korean peninsula. Geoid undulation varies from -60m in the China to 60m toward the Pacific Ocean across the Korean Peninsula. Calculated gravity anomalies are in the range of -60 and 60 mgal except the subduction zone showing -100 mgal. High positive values are observed at Mt. Baekdu, Kaema highland and Taebaek mountains, and low values at Ulleung, Japan and Yamato basins in the East sea. We removed regional components below the spherical harmonic degree of 10 from gravity anomaly to get the residual anomaly for crust components. Residual gravity anomaly shows high anomalies at the northern mountainous area and Kyungsang basin in the Korean Peninsula. And low anomalies appears at the western Korea bay basin, Kunsan basin, Cheju basin, and Ulleung basin in the marine. Anomalies separated by the spherical harmonic degree as well as the residual anomalies are useful for the study of large crustal structure about geologic scale and depth distribution and for the survey of natural resources.

• The Journal of Engineering Geology
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• v.5 no.2
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• pp.181-192
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• 1995

The discrimination studies between earthquakes and underground nuclear explosions have been carried out by various seismologists(Nuttli and Kim, 1976; Dahiman and Israelson, 1977; Masse, 1981). The discrimination between local microearthquakes and artificial underground explosions(epicentral distance not greater than 400Km), however, has not been actively studied so far in the light of seismological aspects. Futhermore this kind of research has never been performed in Korea even if it is of great importance for IAEA (International Atomic Energy Agency) to clearly analyze the military nuclear power of North Korea at present. This research has been carried out by using some of the artificial underground explosions(about 100 events) have occurred in North Korea for the last six years. The azimuths, apparent incidence angles, epicentral distances and locations are determined using a single station of 3 - component data. The detection, location and identification are performed through the polarization and the bandpass filtering. This technique can be also applied to study the inhomogeneous crustal structure finding the converted waves.

• 한국지구물리탐사학회:학술대회논문집
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• 2007.12a
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• pp.1-14
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• 2007

This paper briefly reviews the history of the Korean geophysics and analyze the current status of geophysical researches. And the future prospects of geophysics are discussed based on social demands for the science and technology in Korea. About thirty universities offer geophysics courses in their academic curricula. Although the number of Ph.D. graduates in geophysics had been small until the year of 1990, but is rapidly increasing. In recent years about $7{\sim}8$ Ph.D's are produced every year. The major geophysical methods used in Ph.D. theses are seismic, electrical and electromagnetic methods, and earthquake waves and research themes are computational geophysics, which involve data processing, modelling, inversion and tomography, geological structures, and paleomagnetic studies in the order of numbers. The Solid Earth Geophysics is generally distinguished in two categories such as "Global Geophysics" and "Exploration Geophysics". However, they are intimately connected, and overlap in many sectors, especially in large scale research projects. The global geophysics has a more academic and general scientific meaning, and several research groups in Korean universities are carrying out the earthquake seismology and paleomagnetic studies. On the other hand the exploration geophysics focuses on practical application of geophysical concepts, and the public research institutes conduct large projects for exploration of energy and mineral resources and to cope with environmental and natural disaster problems. The geophysical studies for local geology and regional crustal structure utilize various survey methods and usually cover both academic and exploration purposes. The computational geophysics constitutes the indispensable theoretical backgrounds for all geophysical sectors. Many young Korean geophysicists, who have strong background in mathematics and physics, devote to the computational geophysics and several groups have made the internationally highest level achievements. But, Korean geophysicists have to expand their research interests to include more global-scale, high-tech researches and collaborative works with various other science groups.

• 한국방재학회:학술대회논문집
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• 2008.02a
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• pp.249-252
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• 2008

The solutions are expressed in terms of a double integral transformation over wavenumber and frequency. The complete solution is considered in such a full wave theory approach. This method can handle a larger number of plane layers. Therefore, the result of FK method is very similar to real data. Using the models that were modified in velocity and Q value with depth by iterative process from a model (Kang and Park, 2006) and considered as one of the best models in Korean Peninsula, the synthetic data are simulated for explosions and earthquakes of North Korea. This study notes that the wave shape of the synthetic data is very dependent on Q value, velocities, and thickness of sedimentary layers. Comparing between the real and the synthetic, fitting well in arrival time of first arrival and wave shape causes us to arrive at an indication that the model is very close representation of upper crustal structure and simulations are well done in amplitude fitting and in identification of phases of local and regional waves.

• Journal of Astronomy and Space Sciences
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• v.40 no.4
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• pp.199-215
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• 2023

The Korea Pathfinder Lunar Orbiter (KPLO), the first South Korea lunar exploration probe, successfully arrived at the Moon on December, 2022 (UTC), following a 4.5-month ballistic lunar transfer (BLT) trajectory. Since the launch (4 August, 2022), the KPLO magnetometer (KMAG) has carried out various observations during the trans-lunar cruise phase and a 100 km altitude lunar polar orbit. KMAG consists of three fluxgate magnetometers capable of measuring magnetic fields within a ± 1,000 nT range with a resolution of 0.2 nT. The sampling rate is 10 Hz. During the originally planned lifetime of one year, KMAG has been operating successfully while performing observations of lunar crustal magnetic fields, magnetic fields induced in the lunar interior, and various solar wind events. The calibration and offset processes were performed during the TLC phase. In addition, reliabilities of the KMAG lunar magnetic field observations have been verified by comparing them with the surface vector mapping (SVM) data. If the KPLO's mission orbit during the extended mission phase is close enough to the lunar surface, KMAG will contribute to updating the lunar surface magnetic field map and will provide insights into the lunar interior structure and lunar space environment.

• Economic and Environmental Geology
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• v.6 no.2
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• pp.81-114
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• 1973

The purpose of the present study is to clarify the stratigraphy and geologic structure of the Great Limestone Series by means of study on fossil conodonts and detail investigation of geologic structure. In recent years very few geologists in Korea argue without confident evidences against the age and stratigraphy of the Great Limestone Series which have been rather well established previously in most parts of the regions although it is ambiguous and has not been studied in other areas. Five type localities in the Kangweon basin where the Great Limestone Series is well cropped out were chosen for the study. Total 26 genus and 66 species of conodont were identified from 290 samples collected and treated. From the study on conodonts the age of each formations of the Great Limestone Series has been determined as follows: The Great Limestone Series of Duwibong type Duwibong limestone: Caradocian (mid-Ord.) Jikunsan shale: Landeilian (mid-Ord.) Maggol limestone: Llanvirn-Llandeilian (mid-Ord.) Dumugol: Arenigian (Ord.) Hwajeol: Upper Cambrian The Great Limestone Series of Yeongweol type Mungok (Samtaesan) : Ordovician Machari: upper Cambrian The Great Limestone Series of Jeongseon type Erstwhile Jeongseon limestone: mid-Ord. The erstwhile Jongseon Limestone formation in Jeongseon district is separated into Hwajeol, Dongjeom, Dumudong, and Maggol formations which were cropped out repeatedly by folding and faulting, but Maggol is predominant in areal distribution. Yemi Limestone Breccia bed is not a single bed but distributed in several horizons so that it bears no stratigraphic significance. The limestone bed above Yemi Limestone Breccia, which was believed by some geologists to be much younger than Ordovician, is identified to be Maggol and its age is determined to be mid-Ordovician. Sambangsan formation in Yeongweol district was believed to be Cambrian age and lower horizon than Machari formation by Kobayashi, but C. M. Son believed that it might belong to later than Ordovician and lies above the Great Limestone Series of Yeongweol type. It was identified to be upper Cambrian and lies beneath the Machari formation and above the Daeki formation, the lower most horizon of the Great Limestone Series. The age of Yeongweol type Choseon system is contemporaneous with that of Duwibong type Choseon system. The difference in lithofacies is not due to lateral facies change, but due to the difference in its depositional environment. The Yeongweol type Choseon system is believed to be deposited in the small Yeongweol basin which was separated from the main Kangweon sedimentary basin. Judging from these facts it is definitely concluded that there exists no Gotlandian formation in the regions studied. Structurally the Kangweon basin comprises five basins and two uplifted areas. These structures were originated by at least two crustal movements, that is, Songrim disturbance of Triassic and Daebo orogeny of Jurasic age.