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Econ. Environ. Geol. 2022; 55(6): 633-648

Published online December 31, 2022

https://doi.org/10.9719/EEG.2022.55.6.633

© THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY

Analysis of the Causes of Clustered Scismicity Registered in Yeoncheon, the Middle Part of the Korean Peninsula through Gravity Field Interpretation and Modeling

Sungchan Choi1, Sung-Wook Kim1,*, Eun-Kyeong Choi1, Younghong Shin2, Tae-Kyung Hong3

1Geo-information Institute, GI Co. Ltd., Busan 47598, Republic of Korea
2Active Tectonics Research Center, Korea Institute of Geoscience and Mineral Resources, Daejeon 34132, Republic of Korea
3Department of Earth System Sciences, Yonsei University, Seoul 03722, Republic of Korea

Correspondence to : *Corresponding author : suwokim@chol.com

Received: November 15, 2022; Revised: December 19, 2022; Accepted: December 19, 2022

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

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.

Keywords clustered seismicity, Bouguer anomalies, curvature analysis, Dongducheon Fault, En echelon fault

중력이상 수치해석을 통한 연천지역 군발지진 원인분석

최승찬1 · 김성욱1,* · 최은경1 · 신영홍2 · 홍태경3

1(주)지아이 지반정보연구소
2한국지질자원연구원 활성지구조연구센터
3연세대학교 지구시스템과학과

요 약

경기도 연천 지역에서 집중적으로 발생한 군발지진의 원인을 파악하기 위해서 중력 자료를 분석하였다. 그 결과 연천 지역에는 북서-남동 방향의안행상(En echelon) 구조가발달하는것으로나타났다. 또한, 동두천단층동쪽지역은꽃다발구조(flower structure)를 수반한 수직 지구조 운동으로 인해서 고밀도의 기반암 물질들이 부분적으로 지표 가까이 상승한 것으로 해석되었다. 동두천단층의 파쇄대 폭은 약 200 m, 깊이는 최소 5 km이며, 밀도는 인접 지층보다 평균 약 15% 낮은 것으로 평가된다. 지하 5 km 이내에서 발생한 천발지진은 지표 가까이 관입한 고밀도 암석에 발달된 저밀도의 안행상 단층 파쇄대를 따라 집중되어 발생한 것으로 분석되는데, 이는 동두천 단층의 우수향 운동 때문에 남북 방향으로 작용한 응력이 안행상 단층을 활성화한 결과로 해석된다.

주요어 군발지진, 부게이상, 곡률 분석, 동두천단층, 안행상단층

Article

Research Paper

Econ. Environ. Geol. 2022; 55(6): 633-648

Published online December 31, 2022 https://doi.org/10.9719/EEG.2022.55.6.633

Copyright © THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY.

Analysis of the Causes of Clustered Scismicity Registered in Yeoncheon, the Middle Part of the Korean Peninsula through Gravity Field Interpretation and Modeling

Sungchan Choi1, Sung-Wook Kim1,*, Eun-Kyeong Choi1, Younghong Shin2, Tae-Kyung Hong3

1Geo-information Institute, GI Co. Ltd., Busan 47598, Republic of Korea
2Active Tectonics Research Center, Korea Institute of Geoscience and Mineral Resources, Daejeon 34132, Republic of Korea
3Department of Earth System Sciences, Yonsei University, Seoul 03722, Republic of Korea

Correspondence to:*Corresponding author : suwokim@chol.com

Received: November 15, 2022; Revised: December 19, 2022; Accepted: December 19, 2022

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

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.

Keywords clustered seismicity, Bouguer anomalies, curvature analysis, Dongducheon Fault, En echelon fault

중력이상 수치해석을 통한 연천지역 군발지진 원인분석

최승찬1 · 김성욱1,* · 최은경1 · 신영홍2 · 홍태경3

1(주)지아이 지반정보연구소
2한국지질자원연구원 활성지구조연구센터
3연세대학교 지구시스템과학과

Received: November 15, 2022; Revised: December 19, 2022; Accepted: December 19, 2022

요 약

경기도 연천 지역에서 집중적으로 발생한 군발지진의 원인을 파악하기 위해서 중력 자료를 분석하였다. 그 결과 연천 지역에는 북서-남동 방향의안행상(En echelon) 구조가발달하는것으로나타났다. 또한, 동두천단층동쪽지역은꽃다발구조(flower structure)를 수반한 수직 지구조 운동으로 인해서 고밀도의 기반암 물질들이 부분적으로 지표 가까이 상승한 것으로 해석되었다. 동두천단층의 파쇄대 폭은 약 200 m, 깊이는 최소 5 km이며, 밀도는 인접 지층보다 평균 약 15% 낮은 것으로 평가된다. 지하 5 km 이내에서 발생한 천발지진은 지표 가까이 관입한 고밀도 암석에 발달된 저밀도의 안행상 단층 파쇄대를 따라 집중되어 발생한 것으로 분석되는데, 이는 동두천 단층의 우수향 운동 때문에 남북 방향으로 작용한 응력이 안행상 단층을 활성화한 결과로 해석된다.

주요어 군발지진, 부게이상, 곡률 분석, 동두천단층, 안행상단층

    Fig 1.

    Figure 1.Location and geology of study area. (A) Study area, located in the north-western part of the Gyeonggi-Massif, the central part of Korean peninsula. (B) Geological map, which shows that the western and eastern part of the research area are mainly covered by Paleozoic Misan Fm. and Cretaceous volcanics, respectively. (C) Location of earthquake epicenters and a seismic profile by Malehmir et al. (2022), which across the Dongducheon fault (DF). CFS represents Chugaryung Fault System and ① Dongducheon fault.
    Economic and Environmental Geology 2022; 55: 633-648https://doi.org/10.9719/EEG.2022.55.6.633

    Fig 2.

    Figure 2.Rock densities. (A) Densities of igneous, sedimentary, and volcanic rocks from Park et al. (2009) (see also Table 1). (B) Vp-ρ (black line) and Vs-ρ (grey line) relationship that was used for calculating densities from the seismic velocities (Hermann, 2005, Cho et al., 2007).
    Economic and Environmental Geology 2022; 55: 633-648https://doi.org/10.9719/EEG.2022.55.6.633

    Fig 3.

    Figure 3.Construction of subsurface density structure model along the seismic profile. (A) Seismic reflection profile (Malehmir et al., 2022; see Fig. 1C for its location), which shows prominent reflectivities at depths of about 1, 3, 5, 7 and 8 km (R1 – R5). Comparing these reflectivities and the geologic map (Fig. 1B), we speculated rock types at each depth. (B) P-wave velocity distribution, which shows the core of the Dongducheon fault (DF) corresponds to the low velocity zone with a width of about 200 m. (C) Density distribution, which was calculated from the P-wave velocity (Fig. 2B). (D) the shear wave velocity model (Kil et al., 2021) along the seismic profile. The western region from the DF is characterized by high shear wave velocity to a depth of 3 km, while the eastern region shows relatively low velocity. Densities for each 1 km thick layer are calculated from the shear-wave velocity (Fig. 2B). (E) Comparison of all available geophysical information, which will be used as constraints for the gravity forward modeling.
    Economic and Environmental Geology 2022; 55: 633-648https://doi.org/10.9719/EEG.2022.55.6.633

    Fig 4.

    Figure 4.The complete Bouguer anomaly map of the study area. Higher gravity anomalies are observed in the west region of the Dongducheon fault (DF), while lower gravity anomalies are observed on the east side. Microseismicities are concentrated mainly in the low gravity region between high gravity anomalies (HG1 and HG2).
    Economic and Environmental Geology 2022; 55: 633-648https://doi.org/10.9719/EEG.2022.55.6.633

    Fig 5.

    Figure 5.Location of gravity measurements along seismic profile. The existence of high gravity anomaly region (HG1) is reliable because the two gravity data sets obtained at different times agree well with each other.
    Economic and Environmental Geology 2022; 55: 633-648https://doi.org/10.9719/EEG.2022.55.6.633

    Fig 6.

    Figure 6.Reconstructed fault system inferred from the dip-curvature analysis of gravity. (A) Schematic illustration of dip-curvature analysis. (B) Dip-curvature map, which shows anticipated fault lines with a NW-SE direction (EE1 ~ EE6 and WSF) across the Dongducheon fault (DF) and a N-S direction (SF1 ~ SF3). (C) Reconstructed fault system, where the NW-SE directing- and N-S extending fault lines are assumed to be the En echelon lines and strike-slip duplex structure, which are formed by dextral strike slip movement of the DF.
    Economic and Environmental Geology 2022; 55: 633-648https://doi.org/10.9719/EEG.2022.55.6.633

    Fig 7.

    Figure 7.Comparison of our fault system model with the classical model of the strike-slip duplex structure model. (A) Distribution of microseismicities and anticipated fault lines. (B) A classical tectonic model of the strike-slip duplex structure (Twiss and Moores, 1992). (C) Application of the classical model to our study area. It is observed that the microseismicities are distributed along the fault lines inferred by gravity analysis. Distribution of microsesmicities and consistency with the classical strike-slip duplex model support that our model of En echelon lines and strike-slip duplex structure across and along the Dongducheon Fault (DF) are reliable.
    Economic and Environmental Geology 2022; 55: 633-648https://doi.org/10.9719/EEG.2022.55.6.633

    Fig 8.

    Figure 8.Gravity forward model along the seismic profile. A 2D gravity forward model is suggested by using all available constraints from gravity field (A, B), geologic map (A), rock type with densities (C), and spatial distribution of seismic reflectivities (Malehmir et al., 2022). The final model indicates that the prominent gravity lows (DF, SF1~SF3 in B) are mainly affected by the fault cores, which are characterized by low densities about 15% less than surroundings.
    Economic and Environmental Geology 2022; 55: 633-648https://doi.org/10.9719/EEG.2022.55.6.633

    Fig 9.

    Figure 9.Gravity model along the Dongducheon Fault line. A 2D gravity forward model is suggested by using all available constraints from gravity field (A, B) and geologic map (A), and rock type with densities (C). The final model indicates that the prominent gravity highs (HG1 and HG2 in A and B) are mainly caused by the high density material (HD1 and HD2 in C), which may be uplifted from the basement of the Gyeonggi Massiv (GM). The vertical stress distribution (D) at depth of 4.5 km shows a prominent vertical stress change from 3 MPa to –3 MPa in the area between EE1 and EE2 that can be explained by the density difference between En echelon lines.
    Economic and Environmental Geology 2022; 55: 633-648https://doi.org/10.9719/EEG.2022.55.6.633

    Table 1 . Densities of various rock types sampled from the middle part of the Korean peninsula (Park et al., 2009).

    Rock TypeSampleDensity (g/㎤)
    RangeMeanSD
    Andesite (Kban)1382.43 ~ 3.002.700.08
    Basalt (Qba)32.62 ~ 2.832.720.10
    Biotite Granite (Jbgr)3152.26 ~ 2.832.640.05
    Breccia (Ksb)92.42 ~ 2.702.530.08
    Granitic Gneiss (PRgrgn)3912.48 ~ 3.102.680.07
    Granite Prophyry (Kgp)212.52 ~ 2.702.590.04
    Rhyolite (Krh)112.45 ~ 2.732.560.07
    Sand Stone (DM)2032.44 ~ 3.092.670.07
    Schist (PRsch)222.64 ~ 3.182.770.15
    Metasyenite (PRgms)22.69 ~ 3.142.920.30
    Tuff (Kdt)1492.44 ~ 2.862.620.08

    KSEEG
    Apr 30, 2024 Vol.57 No.2, pp. 107~280

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