Journal of the Korean earth science society (한국지구과학회지)

The Korean Earth Science Society (한국지구과학회)
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Characterization of Fault Kinematics based on Paleoseismic Data in the Malbang area in the Central Part of the Ulsan Fault Zone

고지진학적 자료를 이용한 울산단층대 중부 말방지역에서의 단층운동 특성 해석

  • Park, Kiwoong (Department of Earth & Environmental Sciences, Pukyong National University) ;
  • Prasanajit, Naik Sambit (Active Fault & Earthquake Mitigation Institute, Pukyong National University) ;
  • Gwon, Ohsang (Department of Earth & Environmental Sciences, Pukyong National University) ;
  • Shin, Hyeon-Cho (Active Fault & Earthquake Mitigation Institute, Pukyong National University) ;
  • Kim, Young-Seog (Department of Earth & Environmental Sciences, Pukyong National University)
  • 박기웅 (부경대학교 지구환경과학과) ;
  • 정숙진 (부경대학교 활성단층 및 지진재해저감연구소) ;
  • 권오상 (부경대학교 지구환경과학과) ;
  • 신현조 (부경대학교 활성단층 및 지진재해저감연구소) ;
  • 김영석 (부경대학교 지구환경과학과)
  • Received : 2022.02.09
  • Accepted : 2022.02.28
  • Published : 2022.02.28
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Abstract

According to the records of historical and instrumental earthquakes, the southeastern part of the Korean Peninsula is considered the highest seismic activity area. Owing to recent reports of numerous Quaternary faults along the Yangsan and Ulsan fault zones, paleoseismological studies are being actively conducted in these areas. The study area is located in the central part of the Ulsan fault zone, where the largest number of active faults have been reported. Based on lineament and geomorphic analysis using LiDAR images and aerial photographs, fault-related landforms showing topographic relief were observed and a trench survey was conducted. The trench length 20 m, width 5 m, depth 5 m is located approximately 300 m away to the northeast from the previously reported Malbang fault. From the trench section, we interpreted the geometric and kinematic characteristics of the fault based on the deformed features of the Quaternary sedimentary layers. The attitude of the reverse fault, N26°W/33°NE, is similar to those of the reported faults distributed along the Ulsan fault zone. Although a single apparent displacement of approximately 40 cm has been observed, the true displacement could not be calculated due to the absence of the slickenline on the fault plane. Based on the geochronological results of the cryogenic structure proposed in a previous study, the most recent faulting event has been estimated as being earlier than the late Wurm glaciation. We interpreted the thrust fault system of the study area as an imbrication structure based on the previous studies and the fault geometry obtained in this additional trench. Although several previous investigations including many trench surveys have been conducted, they found limited success in obtaining the information on fault parameters, which could be due to complex characteristics of the reverse fault system. Additional paleoseismic studies will contribute to solving the mentioned problems and the comprehensive fault evolution.

역사지진과 계기지진 기록에 따르면 한반도 남동부는 우리나라에서 지진활성도가 가장 높게 평가되는 곳으로, 최근에 양산단층대와 울산단층대를 따라 제4기 단층이 다수 보고되어 고지진학적 연구가 활발하게 이루어지고 있다. 특히 울산단층대의 중부지역에 해당하는 경북 경주시 외동읍 말방리 일원은 울산단층대 내에서 가장 많은 활성단층이 보고된 지역이다. 따라서 이 지역에 대한 고지진학적 특성을 이해하기 위하여 먼저 LiDAR 영상 및 항공사진을 이용한 지형 및 선형구조 분석을 실시하여 단층에 의한 기복으로 추정되는 지형인자를 확인하고, 야외답사와 물리탐사를 통해 단층을 추적하여 기 보고된 말방단층 지점에서 약 300 m 북서쪽에 위치한 곳에서 길이 20 m, 너비 5 m, 깊이 5 m의 굴착조사를 실시하였다. 굴착단면을 통해 분석된 제4기 퇴적층의 특징을 바탕으로 단층의 기하학적·운동학적 특성을 해석하여 고지진학적 특성을 규명하고자 하였다. 이번 굴착단면에서 확인된 역단층의 기하를 보이는 단층의 자세는 N26°W/33°NE로 울산단층대를 따라 분포하는 기 보고된 단층들과 유사하다. 약 40 cm의 단일 겉보기 변위가 인지되었으나 단층조선의 부재로 실변위는 산출할 수 없었다. 선행연구에서 제안된 극저온구조층의 연대결과 값을 토대로 단층의 최후기 운동시기는 후기 뷔름빙기 이전으로 추정하였다. 기 보고된 연구결과와 본 굴착단면에서 획득한 단층기하를 종합하여 이 지역에 발달하는 단층계를 인편상구조로 해석하였고, 단층특성을 반영한 모델을 제시하였다. 말방리 일원에서 수 회의 굴착조사를 비롯한 다수의 선행연구가 수행되었음에도 불구하고 구체적인 단층변수에 대한 정보가 미진하고 각 지점들 간의 상관성이 명확하게 규명되지 않은 것은 역단층의 복잡한 운동학적 특성에 기인한 것으로 판단된다. 추후 고지진학적 연구가 추가적으로 수행된다면 상기의 문제점들을 해결하여 종합적인 단층의 형태와 운동사가 규명될 수 있을 것으로 판단된다.

Keywords

Acknowledgement

이 연구는 산업통상자원부(MOTIE)와 한국에너지기술평가원(KETEP)의 지원을 통해 수행된 연구과제입니다(No.20201510100020). 본 눈문을 투고하기까지 세심한 검토와 건설적인 조언을 해주신 두 분의 심사위원님과 편집위원장님께 진심으로 감사드립니다. 연구를 도와주신 부경대학교 지질구조재해연구실 연구원분들에게 감사드립니다.

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