• Economic and Environmental Geology
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• v.22 no.2
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• pp.91-102
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• 1989

The gravity measurement has been conducted at 61 stations with an interval of about 500 to 1,000 m along two survey lines of about 47 Km between $Chungju-Jech{\check{o}}n$ and $Salmi-D{\check{o}}cksanmy{\check{o}}n$ in order to study on the subsurface geologic structure and structural relation between $Okch{\check{o}}n$ Group and Great Limestone Group of $Chos{\check{o}}n$ Supergroup. The Bouger gravity anomalies were obtained from the reduction of the field observations, and the distribution patterns of the basement and subsurface geologic structure were interpreted by means of the Fourier-Series and Talwani method for two-dimensional body. The depth of Conrad discontinuity varies from 12.7 Km to 15.7 Km, and vertical displacements along the Osanri and Bonghwajae faults are 1.0 Km and 1.5 Km, respectively between Chungju and $Jech{\check{o}}n$. The depth of Conrad discontinuity varies from 13.8 Km to 15.4 Km, and vertical displacement along the Bonghwajae fault is 0.5 Km between Salmi and $D{\check{o}}cksanmyon$. The basement is widely exposed at several places between Chungju and $Jech{\check{o}}n$. In the unexposed area between Osanri and $W{\check{o}}lgulri$, its depth is from 1.5 Km to 2.1 Km. It is displaced downward along the Osanri and Bonghwajae faults by 0.8 Km and 0.6 Km, respectively, and is displaced upward along the Dangdusan fault by 1.6 Km. On the other hand, the depth of the basement varies abruptly by the Sindangri, Jungwon, Kounri, and Bonghwajae faults between Salmi and $D{\check{o}}cksanmy{\check{o}}n$, and it is from 2.8 Km to 3.2 Km around $Salmimy{\check{o}}n$, from 1.6 Km to 2.5 Km between the Sindangri and Bonghwajae faults, 3.0 Km near Koburangjae, and 2.5 Km at $Doj{\check{o}}nri$. The high Bouguer gravity anomalies are due to the accumulation of $Okch{\check{o}}n$ Group and $Jangs{\check{o}}nri$ Metamorphic Complex whose density is higher than the basement exposed between Sondong and Osanri, and imply the existance of Bonghwajae Metabasite or hornblende gabbro of high density distributed along the Bonghwajae fault in the vicinity of Koburangjae. The low Bouguer gravity anomalies resulted form the fracture zone associated with fault or rock of low density imply the existance of the Osanri, Bonghwajae, Dangdusan faults and $Daed{\check{o}}cksan$ thrust between Chungju and $Jech{\check{o}}n$, the uplift of the basement by the Sindangri, Jungwon, Kounri, and Bonghwajae faults, and extensive distribution of Cretaceous biotite granites between Salmi and $Docksanmy{\check{o}}n$. The thickness of $Okch{\check{o}}n$ metasediments varies from 1.5 Km to 3.2 Km, and that of Great Limestone Group of $Chos{\check{o}}n$ Supergroup from 200 m to 700 m. It is interpreted that $Okch{\check{o}}n$ Group is in contact with Great Limestone Group of $Chos{\check{o}}n$ Supergroup by the fault zones of the Bonghwajae and $Daed{\check{o}}cksan$ faults, and the Bongwhajae fault is a thrust of high angle, by which the east of the basement is displaced downward 0.5 Km between Chungju and lechon, and 1.0 Km between Salmi and $D{\check{o}}cksanmy{\check{o}}n$.

• Economic and Environmental Geology
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• v.16 no.2
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• pp.65-78
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• 1983

This report deals with the sedimentological study of the littoral sand of beaches in the Busan area. The purpose of this report is to know the grain size, mineralogical composition, heavy mineral and clay mineral of the beach sands, and gravity measurements of the Nagdong River Deltas. 1) As a whole, the littoral sand of the beaches are composed of uniformly medium grained, moderately sorted and nearly symmetrical. The barrier sand of the Nagdong Estuary is composed of fine grained, well sorted and nearly symmetrical. 2) The littoral sand of the beaches is transported by saltation and rolling. The barrier sand of the Nagdong Estuary is transported by suspension and saltation. 3) In the littoral sand of the beaches, the ratio of feldspar to quartz is 1 :2.31 and in the barrier sand of the Nagdong Estuary 1:1.40. 4) The content of heavy mineral of samples ranges from 0.54 to 3.87 %. The principal heavy minerals are hornblende, pyroxene, epidote, garnet, leucoxene, zircon, apatite, magnetite, hematite and ilmenite with minor accessories of rutile and olivine. 5) The x-ray diffraction analysis of the clay mineral informs the existence of quartz, feldspar, kaolinite and montmorillonite. The montmorillonite is considered to have been derived from the alteration of acidic volcanic rocks. 6) To determine the depositional structure of the Nagdong Estuary, Gravity measurements were made. Free air anomaly ranges from 14.5 mgal to 33.5 mgal and Bouguer anomaly ranges from 14.3 to 23.5 mgal and both are closely related to the topography. According to the interpreted layer structure, the upper layer composing sand, silt and clay, the intermediate layer composing sand with gravel, the lower layer composing weathered and soft rock, and bed rock composing hornfels or andesite. 7) The depositional environments of the study, the littoral area is dominated by the marine environment and the Nagdong Estuary by the mixed environment.

• Economic and Environmental Geology
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• v.18 no.4
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• pp.321-329
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• 1985

The gravity measurement has been conducted at 113 stations with an interval of about 1km along the national road of about 120km running from Busangdong to Pohang through Waekwan, Daegu, Youngchun and Aankang. The subsurface geology and structure along the survey line is interpreted from Bouguer anomaly by applying Fourier method and Talwani method for two dimensional body. The mean depth of Moho discontinuity is 31.4km, and the depth decreases very slowly from inner continent toward east coast. The depth of Conrad discontinuity increases from 11km at the east coastal area to 17km at the inner continental area, and especially increases rapidly in the area between Waekwan to Busangdong. The depth of basement of Kyoungsang Basin inereases from near Waekwan toward Daegu upto about 4. 8km, and increases rapidly to reach the maximum depth of about 8.5km at 8km east of Daegu. But it starts to decrease from the place of 10km west of Youngchun, and is about 7.2km at Youngchun and about 6km at 6km east of Youngchun. The depth starts to increase smoothly beyond this point, and is 7km at 15km east of Youngchun. From this point, the depth starts to decrease again, and is about 3.8km at Ankang. The depth of basement of Pohang Basin is 500m at Pohang and about 650m at 5km west of Pohang. A massive granite body which is considered to be a part of Palgongsan Granite exposed at the depth of 1. 5km at 9km west of Youngchun. Another massive granite body is situated underneath the Pohang Basin at depth of 1.5 to 2km, and sedimentary rocks of Kyoungsang Group and volcanic rocks are distributed between Pohang Basin and this granite body. Finally, Yangsan Fault is identified at about 2.5km east of Ankang.

• 한국해양학회지
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• v.26 no.1
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• pp.1-12
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• 1991

A multi-disciplinary geophysical study including gravity, magnetic, and seismic reflection profiling was carried out in the area between the Clarion fracture zone and the Clippertone fracture zone o the northeastern equatorial Pacific basin. There are small free-air gravity anomalies of less than 20 mgal over seamounts and the east-west trending abyssal hills. The negative residual gravity anomalies over seamounts may indicate the existence of low density seamount roots compared to surrounding oceanic crust. Non-existence of magnetic lineations and the magnetic anomalies of small smplitude with no polarity change in the east-west direction support that the study area belongs to the Cretaceous magnetic quite zone. Positive magnetic anomalies over seamounts offset 100 km in the east-west direction in the southern part of the study area suggest a possibility of left-lateral movement of those seamounts along unknown fractures. The sedimentary section in the study area can be divided into three units (Unit I, unit IIA, and Unit IIB) n the basis of reflection characteristics. the total thickness of sedimentary section varies from 200 to 400 meters and the sedimentary section is thicker in the southern area of rough topography near the seamount belt than in the northern flat area. Manganese nodules are abundant in the southern part of the study area where the ridges are developed and the Unit I layer is thicker than 100 meters.

• Geophysics and Geophysical Exploration
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• v.27 no.1
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• pp.23-36
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• 2024

In this study, we analyze the structure of the Korea-Japan Joint Development Zone (JDZ) using gravity, magnetic, and seismic data. Gravity and magnetic data analysis confirmed that the Jeju Basin exhibits low anomalies compared to adjacent areas. We applied the total gradient to the Bouguer anomaly to identify basin boundaries, and computed the analytic signal from the total magnetic anomaly data to enhance the edges of the magnetic anomalies. The Taiwan-Sinzi Belt, exhibits high magnetic anomalies and crosses the center of the JDZ in the northeast-southwest direction; we presume that intrusive rocks are sporadic in the JDZ. The 3D inversion results of the gravity and magnetic data show a strong correlation between magnetic susceptibility and density (i.e. a low-density zone in the Jeju Basin and the Ho Basin, and a high magnetic susceptibility distribution in the Taiwan-Sinzi Belt). Comparison of the density and seismic profiles of the Jeju Basin shows that high densities are associated with sill, horst, and basement highs, whereas low densities are associated with basement low and grabens. These results suggest that interpretations based on seismic, gravity and magnetic data can effectively reveal the subsurface structure of the JDZ.

• Economic and Environmental Geology
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• v.18 no.4
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• pp.309-320
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• 1985

The crustal structure of the southern part of the Korean peninsula has been investigated based on the results of processing and anlaysis of gravity data. The processing techniques involve i) seperation of regional and residual anomalies by polynomial fittings, ii) power spectral analyses to determine the mean depth to the crustal base, iii) a filtering operation called "high-cut filtering and resampling," and iv) downward continuation to determine the undulation of the crustal base. The Bouguer anomalies show a lineation in the NE-SW direction which is the same as that of most mountains and tectonic lines of this area. The mean crustal depth is found to be 34km. The depth of the crustal base is varying in the estimated range of 26km to 36km with a thinner crust below the east coast than that of the west coast. The relief of the crustal base is appeared to be correlated with the regional surface topography. The linear regression relations computed between elevations and gravity anomalies indicate that the crust of this area seems to be not in perfect isostatic equilibrium but a little undercompensated state.

• Economic and Environmental Geology
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• v.53 no.5
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• pp.597-608
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• 2020

Even after the Gyeongju earthquake and the Pohang earthquake, hundreds of aftershocks and micro-earthquakes are still occurring in the southeastern part of the Korean Peninsula. These phenomena mean that the stress is constantly working, implying that another huge earthquake may occur in the future. Therefore, the gravity field interpretation method was used to analyze the deep geological structure of the Pohang-Ulsan region in the southeastern Korean Peninsula. First, a gravity survey was performed to collect the insufficient data and to calculate the detailed Bouguer gravity anomaly in the study area. Based on the gravity anomaly data, the location, direction, and maximum depth of deep fault lines were analyzed using the inversion methods "Curvature analysis" and "Euler deconvolution method". As a result, it is interpreted that at least six fault lines(C1~C6) exist in deep depth. The deep fault line C1 is well correlated to the Yeonil Tectonic Line(YTL), suggesting that YTL is extended up to about 4000m deep. The deep fault line C2 consists of several segment faults and well correlated to the fault lines on the surface. Inferred fault lines C3, C4, and C5 have an NW-SE direction, which is parallel to the Ulsan fault. The deep fault line C6 has the direction of NE-SW, and it is interpreted that the eastern boundary fault of Eoil Basin is extended to the deep. Comparing the inferred fault lines with the distribution of micro-earthquakes, the location of the deep fault line C1 is well correlated to the hypocenter of micro-earthquakes. This implies that faults in deep depth are related to the recent earthquakes in the southeastern Korean Peninsula.

• Economic and Environmental Geology
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• v.36 no.4
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• pp.305-312
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• 2003

This study was performed to delineate the subsurface geology, geologic structure, and distribution pattern of the Palgongsan granitic body, and to reveal the relationship between the Kyeongsang basin and Yongnam massif by gravity survey. The study area is located between the latitude of 35$^{\circ}$45'-36$^{\circ}$21'N and longitude of 128$^{\circ}$15'-129$^{\circ}$00'E. Total of 966 gravity data measured by Seoul National University, KlGAM(Korea Institute of Geology, Mining ＆ Materials), Pusan National University and Yonsei University were used. The Bouguer gravity anomaly in the study area ranges from -12.88 to 26.01 mgal with a mean value of 11.27 mgal. A very low anomaly zone is located in the Yongnam massif in west of the study area. The anomaly value increases going from west to east. A low anomaly distribution in Palgongsan granite and Yongnam massif is interpreted as the effect of their lower density than that of Kyeongsang Super Group. Power spectrum analysis is applied to evaluate the average depth of basement the Kyeongsang Basin and Conrad discontinuity from gravity anomaly. The average depths of density discontinuities are calculated 10.45 km and 4.9 km, and these are interpreted as Conrad discontinuity and depth of basement of the Kyeongsang Basin, respectively. The depth of Palgongsan granite is derived by means of 2-dimensional modeling and it decreases gradually toward the east. The gravity anomaly east of the study area decreases abruptly due to Shingryeong fault and Nogosan ring fault. Two deepest and sharp roots of Palgongsan granite are recognized by 2-dimensional modeling of each profiles. The depths of those roots are 5.3 km on a profile AA' and 7 km on a profile BB' which is the maximum depth of Palgongsan granite. Small granitic bodies are also seen to be intruded around the Palgongsan granite. The root of Palgongsan granite is shown by 3-dimensional analysis based on the interpolation of 2-dimensional modeling along each profiles to exist in the southwest vicinity of Palgongsan granite. The total volume of Palgongsan granite is approximately 31.211 $Km^3$.

• Economic and Environmental Geology
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• v.55 no.6
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• pp.633-648
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• 2022

Gravity data were analyzed to identify the cause of clustered seismicity that occurred intensively in Yeoncheon, located in the central part of the Korean Peninsula. Our analysis suggests that the En echelon faults developed in the northwest-southeast direction. In addition, in the eastern part of the Dongducheon Fault, it was interpreted that high-density lower bedrock intermittently lifts close to the surface due to vertical tectonic movement accompanied by a flower structure. The fracture zone of the Dongducheon Fault is estimated that the width is about 200 m, the depth is at least 5 km, and the density is about 15% lower than the adjacent rocks. It is analyzed that the shallow earthquakes that occurred within 5 km depth was concentrated along the low-density En echelon fault fracture zone developed between the high-density rocks intruding close to the surface. Therefore, the earthquakes can be interpreted as the result that the north-south stress caused by the dextral tectonic movement of the Dongducheon Fault activated the En echelon fault in the northwest-southeast direction.

• Proceedings of the KSEG Conference
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• 2005.04a
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• pp.101-106
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• 2005

The three-dimensional Tomography developed by Kim and Bae(2004) was applied to 64,024 P and 64,618 S wave arrival times observed at 238 seismic stations for 4050 local earthquakes in the depth range from 0 to 300 km in and around Hokkaido, Japan. High and low velocity zones for Vp/Vs were clearly imaged in and around Hokkaido. The upper seismic planes of the double seismic zone (DSZ) were found in the subducted Pacific Plate beneath Hokkaido at depth of 40- 80 km, which produced high seismicity around Hokkaido. The findings of high Vp/Vs anomalies beneath the Moho discontinuity supports an evidence of a surface triple-collision hypothesis prepared by Moriya(1994) that the Kuril Arc(Okhotsk Plate or North American Plate) is colliding against the NE Japanese Arc(Amurian Plate or Eurasian Plate), along and beneath the Hidaka Mountain Range, and at the same time the Pacific Plate is subducting into these two plates, making an equilibrium of tectonic forces along the Hikada Mountain Range (HMR) corner and the central tectonic axis(142 ~ 143E) in Hokkaido. The low Vp and Vs were also found in east and west along the central tectonic axis in which the focal mechanism represents the extensional forces. These phenomena are also consistent with low Bouguer gravity anomalies in this region. It is understood why most of great earthquakes occurred outside Hokkaido where the balance of tectonic forces are breaking from the triple junction of three tectonic forces in Hokkaido.