Journal of the Geological Society of Korea (지질학회지)

The Geological Society of Korea (대한지질학회)
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Characteristics of sand volcanoes caused by 2017 Pohang Earthquake-induced liquefaction and their paleoseismological approach

2017 포항지진 액상화에 의한 모래화산의 발달 특성 및 고지진학적 접근

  • Lee, Hoil (Geological Research Division, Korea Institute of Geoscience and Mineral Resources) ;
  • Kim, Jin Cheul (Geologic Environment Research Division, Korea Institute of Geoscience and Mineral Resources) ;
  • Ko, Kyoungtae (Strategic Technology Research Division, Korea Institute of Geoscience and Mineral Resources) ;
  • Ghim, Yong Sik (Geological Research Division, Korea Institute of Geoscience and Mineral Resources) ;
  • Kim, Jisung (Geologic Environment Research Division, Korea Institute of Geoscience and Mineral Resources) ;
  • Lee, Seung Ryeol (Geological Research Division, Korea Institute of Geoscience and Mineral Resources)
  • 이호일 (한국지질자원연구원 국토지질연구본부) ;
  • 김진철 (한국지질자원연구원 지질환경연구본부) ;
  • 고경태 (한국지질자원연구원 전략기술연구본부) ;
  • 김용식 (한국지질자원연구원 국토지질연구본부) ;
  • 김지성 (한국지질자원연구원 지질환경연구본부) ;
  • 이승렬 (한국지질자원연구원 국토지질연구본부)
  • Received : 2018.03.21
  • Accepted : 2018.06.19
  • Published : 2018.06.30
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Abstract

A large number of sand volcanoes were observed in the vicinity of the epicenter due to the $M_L$ 5.4 earthquake occurred on November 15, 2017 in Heunghae area, Pohang City. In this study, trench investigations were carried out on unconsolidated sediments to trace the vertical migration of the sand volcanoes and the liquefaction zone under the surface. The examined sections are markedly dominated by muddy sediments in the upper part, sandy sediments in the lower part, which have been deposited in a floodplain. The liquefied sand was injected along the irregular cracks of the silty clay layer at the upper part of the trench. The sand volcanoes show a similar grain size distribution to the sediments at the altitudes of 2.4-3.0 m showing the mode of $200-300{\mu}m$. Considering the development of the soft-sediment deformation structure in this section, the sand volcano is considered to have been originated at the altitude of 2.6-3.0 m. The lenticular coarse-grained sand layer showing a remarkable difference in particle size at the altitude of 2.5 m is likely to have been injected from lower liquefied sand by paleo-earthquake event in this area. Therefore, it is considered that M>5 earthquake occurred during AD 1360-1640 based on the radiocarbon dating from the upper and lower layers of the lenticular coarse-grained sand layer. This result could be basic data to trace paleo-earthquake in the southeastern part of Korea.

2017년 11월 15일 경상북도 포항시 흥해읍 일원에 발생한 ML 5.4의 지진으로 진앙지와 인접한 지역에 광범위한 모래화산이 다수 관측되었다. 이번 연구에서는 액상화 현상에 의해 지표로 분출된 모래의 이동과정 및 지하의 액상화 구간을 추적하기 위하여 미고결 퇴적층을 대상으로 트렌치 조사를 실시하였다. 연구지역의 미고결 퇴적층은 하부에 모래질이 우세한 반면, 상부에는 점토질이 우세한 특징을 보이며 범람원 환경으로 해석된다. 이번 지진으로 인해 액상화된 모래는 실트질 점토층의 불규칙한 균열면을 따라 주입되어 지표로 분출된 것으로 확인되었다. 입도분석 결과, 분출된 모래화산은 $200{\sim}300{\mu}m$의 최빈값을 보여주는 미고결 퇴적층의 해발 고도 2.4~3.0 m 구간과 유사한 입도 분포를 보여준다. 이 구간에서 확인된 연질퇴적변형구조를 고려하면 고도 2.6~3.0 m 구간에서 모래화산이 기원된 것으로 판단된다. 한편, 고도 2.53 m 부근에서 현저한 입도 차이를 보이는 렌즈상의 조립질 모래층은 과거 지진에 의해 액상화된 모래가 주입되었을 것으로 추정된다. 따라서 렌즈상의 조립질 모래층 상하위층에서 각각 산출된 탄소연대와 액상화가 발생할 수 있는 최소 지진 규모를 바탕으로 AD 1360~1640년 시기에 규모 5이상의 지진이 발생했던 것으로 추정된다. 이러한 결과는 포항지역을 포함하는 한반도 동남부 일대 고지진 연구에 기초적인 자료가 될 수 있을 것으로 사료된다.

Keywords

Acknowledgement

Grant : 국토 대단층계(양산단층 중부지역) 위험요소 평가연구

Supported by : 한국지질자원연구원

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