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Econ. Environ. Geol. 2021; 54(1): 91-103

Published online February 28, 2021

https://doi.org/10.9719/EEG.2021.54.1.91

© THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY

Gravity Field Interpretation and Underground Structure Modelling as a Method of Setting Horizontal and Vertical Zoning of a Active Fault Core

Sungchan Choi1, Sung-Wook Kim1,*, Eun-Kyeong Choi1, Young-Cheol Lee2, Sangmin Ha3

1Geo-information Institute, GI Co. Ltd., Busan 47598, Korea
2Research Institute of Geologic Hazard and Industrial Resources, Pusan National University, Busan 46241, Korea
3Department of Geological Sciences, Pusan National University, Busan 46241, Korea

Received: November 25, 2020; Revised: February 1, 2021; Accepted: February 5, 2021

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided original work is properly cited.

Abstract

In order to estimate the vertical and horizontal structural in the Yangsan fault core line (Naengsuri area, Pohang), we carried out gravity field measurements and interpretation procedures such as Euler deconvolution method and curvature analysis in addition to the forward modelling technique (i.e. IGMAS+). We found a prominent gravity difference of more than 1.5 mGal across the fault core. This indicates a distinct density difference between the western and eastern crustal area across the Yangsan fault line. Comparing this gravity field interpretation with other existent geologic and geophysical survey data (e.g. LiDAR, trenching, electric resistivity measurements), It is concluded that (1) the prominent gravity difference is caused by the density difference of about 0.1 g/cm3 between the Bulguksa Granite in the west and the Cretaceous Sandstone in the east side, (2) the fault core is elongated vertically into a depth of about 2,000 meters and extended horizontally 3,000 meters to the NNE direction from Naengsuri area. Our results present that the gravity field method is a very effective tool to estimate a three -dimensional image of the active fault core.

Keywords gravity field interpretation, Euler deconvolution method, Yangsan fault, curvature analysis

활성단층의 3차원적인 규모를 결정하기 위한 중력장 데이터의 해석 및 지각구조 모델링: 양산단층에서의 예

최승찬1 · 김성욱1,* · 최은경1 · 이영철2 · 하상민3

1지아이 지반정보연구소 2부산대학교 지질재해·산업자원연구소 3부산대학교 지질환경과학과

요 약

경상북도 포항시 냉수리 지역에 위치하는 양산단층의 수평 및 수직적 규모를 파악하기 위해서 단층을 동-서로 가로지르는 측선을 따라 중력장을 측정하였다. 그 결과, 단층 경계부에서 1.5 mGal의 뚜렷한 중력의 변화를 확인하였다. 이는 양산단층 서쪽과 동쪽의 지각 밀도가 서로 다르다는 것을 나타내며, 기존의 지질 및 지구물리 조사 결과와 비교해 보았을 때, 냉수리를 통과하는 양산단층은 서쪽의 불국사 화강암층과 동쪽의 경상계 퇴적암이 혼합된 파쇄대라는 것을 의미한다. 오일러 디콘볼루션(Euler deconvolution) 및 곡률 분석(Curvature analysis) 방식을 이용하여 파쇄대의 3차원적 규모를 확인한 결과, 깊이는 약 2,000 m이며, 남남서-북북동 방향으로 최소 약 3,000 m 정도 이어지는 것으로 파악되었다. IGMAS+ 소프트웨어를 이용한 지각구조 모델링 결과는 파쇄대 서쪽과 동쪽 지각의 밀도 차이가 약 0.1 g/cm3 이라는 것을 보여주었다. 이상의 연구 결과는 중력장 데이터의 해석과 모델링이 지표면에 나타난 활성단층의 규모를 심부까지 파악 할 수 있는 매우 효과적인 방법이라는 것을 보여준다. 또한 지금까지 지표면을 중심으로 2차원적으로 수행했던 활성단층지도 제작사업의 영역을 3차원으로 확대할 수 있는 이상적인 수단이 된다.

주요어 중력장 해석, 오일러 디콘볼루션, 양산단층, 곡률분석

Article

Research Paper

Econ. Environ. Geol. 2021; 54(1): 91-103

Published online February 28, 2021 https://doi.org/10.9719/EEG.2021.54.1.91

Copyright © THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY.

Gravity Field Interpretation and Underground Structure Modelling as a Method of Setting Horizontal and Vertical Zoning of a Active Fault Core

Sungchan Choi1, Sung-Wook Kim1,*, Eun-Kyeong Choi1, Young-Cheol Lee2, Sangmin Ha3

1Geo-information Institute, GI Co. Ltd., Busan 47598, Korea
2Research Institute of Geologic Hazard and Industrial Resources, Pusan National University, Busan 46241, Korea
3Department of Geological Sciences, Pusan National University, Busan 46241, Korea

Received: November 25, 2020; Revised: February 1, 2021; Accepted: February 5, 2021

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided original work is properly cited.

Abstract

In order to estimate the vertical and horizontal structural in the Yangsan fault core line (Naengsuri area, Pohang), we carried out gravity field measurements and interpretation procedures such as Euler deconvolution method and curvature analysis in addition to the forward modelling technique (i.e. IGMAS+). We found a prominent gravity difference of more than 1.5 mGal across the fault core. This indicates a distinct density difference between the western and eastern crustal area across the Yangsan fault line. Comparing this gravity field interpretation with other existent geologic and geophysical survey data (e.g. LiDAR, trenching, electric resistivity measurements), It is concluded that (1) the prominent gravity difference is caused by the density difference of about 0.1 g/cm3 between the Bulguksa Granite in the west and the Cretaceous Sandstone in the east side, (2) the fault core is elongated vertically into a depth of about 2,000 meters and extended horizontally 3,000 meters to the NNE direction from Naengsuri area. Our results present that the gravity field method is a very effective tool to estimate a three -dimensional image of the active fault core.

Keywords gravity field interpretation, Euler deconvolution method, Yangsan fault, curvature analysis

활성단층의 3차원적인 규모를 결정하기 위한 중력장 데이터의 해석 및 지각구조 모델링: 양산단층에서의 예

최승찬1 · 김성욱1,* · 최은경1 · 이영철2 · 하상민3

1지아이 지반정보연구소 2부산대학교 지질재해·산업자원연구소 3부산대학교 지질환경과학과

Received: November 25, 2020; Revised: February 1, 2021; Accepted: February 5, 2021

요 약

경상북도 포항시 냉수리 지역에 위치하는 양산단층의 수평 및 수직적 규모를 파악하기 위해서 단층을 동-서로 가로지르는 측선을 따라 중력장을 측정하였다. 그 결과, 단층 경계부에서 1.5 mGal의 뚜렷한 중력의 변화를 확인하였다. 이는 양산단층 서쪽과 동쪽의 지각 밀도가 서로 다르다는 것을 나타내며, 기존의 지질 및 지구물리 조사 결과와 비교해 보았을 때, 냉수리를 통과하는 양산단층은 서쪽의 불국사 화강암층과 동쪽의 경상계 퇴적암이 혼합된 파쇄대라는 것을 의미한다. 오일러 디콘볼루션(Euler deconvolution) 및 곡률 분석(Curvature analysis) 방식을 이용하여 파쇄대의 3차원적 규모를 확인한 결과, 깊이는 약 2,000 m이며, 남남서-북북동 방향으로 최소 약 3,000 m 정도 이어지는 것으로 파악되었다. IGMAS+ 소프트웨어를 이용한 지각구조 모델링 결과는 파쇄대 서쪽과 동쪽 지각의 밀도 차이가 약 0.1 g/cm3 이라는 것을 보여주었다. 이상의 연구 결과는 중력장 데이터의 해석과 모델링이 지표면에 나타난 활성단층의 규모를 심부까지 파악 할 수 있는 매우 효과적인 방법이라는 것을 보여준다. 또한 지금까지 지표면을 중심으로 2차원적으로 수행했던 활성단층지도 제작사업의 영역을 3차원으로 확대할 수 있는 이상적인 수단이 된다.

주요어 중력장 해석, 오일러 디콘볼루션, 양산단층, 곡률분석

    Fig 1.

    Figure 1.(A) The study area (black solid rectangle) locates in the northern part of the Yangsan fault line (white dotted line). 2016M5.8_GY: Gyeongju earthquake epicenter, 2017M5.4_PO: Pohang earthquake epicenter. (B) Geologic map indicates that the Yangsan fault line in the study area is the boundary between the Cretaceous biotite Granite and the Cretaceous grey Sandstone. (C) presents measured gravity stations (dots) and electrical resistivity survey line (DD-08) in the Naengsuri. 2D electrical resistivity structure presented in (D) will be used as one of the important constraints to model density structure.
    Economic and Environmental Geology 2021; 54: 91-103https://doi.org/10.9719/EEG.2021.54.1.91

    Fig 2.

    Figure 2.Simplified schema of Euler deconvolution method (A), Curvature analysis (B), and 3D modeling algorithm to understand theoretical background of gravity inverse interpretations and forward modelling (C). SI: Structure Index, RHO: Density
    Economic and Environmental Geology 2021; 54: 91-103https://doi.org/10.9719/EEG.2021.54.1.91

    Fig 3.

    Figure 3.(A) Gravity measurement line parallel to the electrical survey line (ER_DD-08). (B) The complete Bouguer anomaly along the WE profile with a mean value of approximately 28.5 mGal and a range from 27.7 to 29.5 mGal. The lower Bouguer gravity anomalies are generally observed in the western part of the Yangsan fault line, while the area to the east is characterized by higher anomalies. A prominent anomaly change from 27.5 to 29.0 mGal is well coincided with the low resistivity area (LRZ in A).
    Economic and Environmental Geology 2021; 54: 91-103https://doi.org/10.9719/EEG.2021.54.1.91

    Fig 4.

    Figure 4.(A) Bouguer anomalies across the Yangsan fault line are shown (refer to Fig. 1C and Fig. 3A for the location of the profile). (B) Application of the Euler deconvolution technique by using the complete Bouguer anomaly field (A) to calculated x and z-derivative. (C) Distribution of source points. The Yangsan fault core is characterized by the prominent disturbances of xand z-derivatives, source depths are about 120 meters, definitely deeper than the mean depth (50 meters).
    Economic and Environmental Geology 2021; 54: 91-103https://doi.org/10.9719/EEG.2021.54.1.91

    Fig 5.

    Figure 5.A shows that the Bouguer anomaly map over the study area (Sinkwangmyen, Pohang) has a mean value of approximately 30 mGal with a range from 20 to 40 mGal. The higher Bouguer gravity anomalies in the NE area from Sinkwangmyen, while the SW is characterized by lower anomalies. (B) The mapped Dip-curvature shows a prominent density contrast along the Yangsan fault line. Several density discontinuities along the Yangsan fault line (marked with S1-S5) are anticipated to be the fault segmentations.
    Economic and Environmental Geology 2021; 54: 91-103https://doi.org/10.9719/EEG.2021.54.1.91

    Fig 6.

    Figure 6.(A) Results of the application of the Euler deconvolution technique to the Bouguer anomalies in Fig. 5A get information on the 3D distribution of source points. (B) The Bouguer anomalies along the WE profile varies from about 27.0 mGal to about 29.2 mGal. (D) The distribution of the mean source depth showing the fault core extends to a depth of 2 km. Surface details in (C).
    Economic and Environmental Geology 2021; 54: 91-103https://doi.org/10.9719/EEG.2021.54.1.91

    Fig 7.

    Figure 7.(A) Index map is the same as Fig. 6(A). (B) the Bouguer anomalies along the SSW-NNE profile ranging from about 28.5 mGal to about 29.2 mGal. (C) The mean source depth (black dotted line) shows that the depth of the Yangsan fault core is ranging from 2000 to 800 meters, which can be explained by different depths of fault segments.
    Economic and Environmental Geology 2021; 54: 91-103https://doi.org/10.9719/EEG.2021.54.1.91

    Fig 8.

    Figure 8.The final gravity model along the E-W profile in Naengsuri. The subsurface structure (C) is modeled with iterative modifications by using all available constraints from the interpretation of the gravity field (A), results of electrical resistivity survey (B), and the source depths analysis (B).
    Economic and Environmental Geology 2021; 54: 91-103https://doi.org/10.9719/EEG.2021.54.1.91

    Fig 9.

    Figure 9.(A) Geological map with locations of E-W and SSW-NNE profiles. Final gravity models (C and E) along the E-W and SSW-NNE profiles (locations refer to Fig. A) are shown, which are constrained by the interpretation of the gravity field (B and D) and calculated source depths (Yellow rectangles in Fig. C and E).
    Economic and Environmental Geology 2021; 54: 91-103https://doi.org/10.9719/EEG.2021.54.1.91
    KSEEG
    Feb 29, 2024 Vol.57 No.1, pp. 1~91

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