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The Korean Earth Science Society (한국지구과학회)
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Bedrock Depth Variations and Their Applications to identify Blind Faults in the Pohang area using the Horizontal-to-Vertical Spectral Ratio (HVSR)

포항지역 HVSR에 의한 기반암 심도와 단층 식별 연구

  • Kang, Su Young (Institute of Geologic Hazard & Industrial Resources, Pusan National University) ;
  • Kim, Kwang-Hee (Department of Geological Science, Pusan National University)
  • 강수영 (부산대학교 지질재해산업자원연구소) ;
  • 김광희 (부산대학교 지질환경과학과)
  • Received : 2022.01.15
  • Accepted : 2022.02.18
  • Published : 2022.02.28
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Abstract

Some deep faults do not reach the ground surface and are seldom recognized. Gokgang Fault area in the east of the Heunghae area of the Pohang basin has been selected to confirm the feasibility of the Horizontal-to-Vertical Spectral Ratio (HVSR) approach to identify blind faults. Densely spaced microtremor data have been acquired along two lines in the study area and processed to obtain resonance frequencies. An empirical relationship between the resonance frequency and the bedrock depth was proposed using borehole data available in the study area. Resonance frequencies along two lines were then converted to bedrock depths. The resulting depth profiles show significant lateral variations in the bedrock depth. As expected, considerable variation in the resonance frequency is observed near the Gokgang fault. The depth profiles also present additional significant variations in the resonance frequencies and the bedrock depths. The feature is presumably related to a blind fault that is previously unknown. Therefore, this case study confirms the feasibility of the HVSR technique to identify faults otherwise not recognized on the surface.

지하의 단층들이 지표면까지 도달하지 못하면 단층의 유무를 인지 못하는 경우가 있다. HVSR 분석법으로 지표면 아래 단층의 존재를 판별할 수 있는지 살펴보기 위해 포항분지 흥해지역 동편의 곡강단층을 통과하는 지역을 선정하여 연구를 진행하였다. 교차하는 두개의 측선에서 조밀하게 관측한 배경잡음으로 HVSR 분석을 통해 공명주파수를 산정하였다. 연구지역 내 시추 자료의 기반암 깊이 정보와 시추 지역의 공명주파수를 이용해 상관식을 도출한 후 두측선에서의 공명주파수를 상관식에 적용해 기반암 심도를 산출하였다. HVSR 분석 결과 두 측선에서 기반암 심도가 급격히 변하는 곳이 있다. 특히 곡강단층 인근에서 공명주파수가 급격히 변화하였고, 기존에 알려진 단층이 없는 지역에서도 공명주파수와 기반암 심도가 급격히 변화하는 곳이 있음을 알 수 있었다. 이 형상은 이전에 알려지지 않은 blind fault와 관련이 있을 것으로 사료된다. 본 연구는 지표면에서 인식하지 못하는 단층을 식별하기 위한 HVSR 방법의 적용 가능성을 확인하였다.

Keywords

Acknowledgement

심사과정에서 논문을 읽고 많은 도움을 주신 두분 심사위원과 편집위원께 감사드립니다. 본 논문은 부산대학교 기본연구지원사업(2년)에 의하여 연구되었습니다.

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