고분해능 시추공 변형률계 활용을 통한 지진 연구 가능성 = A Feasibility Study of Earthquake Monitoring Using a High-resolution Borehole Strainmeter

국문 초록 (Abstract)

고분해능 시추공 변형률계로부터 측정된 변형 신호를 이용하여 지진에 의한 응력 변화 해석의 가능성에 대해 고찰하였다. 응 력 변화 분석을 위해 2010년 4월 4일 발생한 El Mayor-Cucapah 지진(Mw=7.2)에 의한 변형 신호가 기록된 자료를 이용하여 주압축응력 방향과 크기를 산정하였다. 분석 결과는 지진 전후의 응력 변화 양상에 대한 차이점과 지진 발생과 동시에 특징적 인 응력 변화 양상을 보였다. 지진 발생 전후 응력 방향 변화는 지진 해로 규명되는 최대주응력 방향에서의 응력 증가와 연관 성이 있는 것으로 해석된다. 지진 발생 시 응력 크기 변화는 주방향에서 10-3-10-2MPa 수준의 응력 급상승, 하강으로 나타났 다. 산정된 응력 크기 변화량에 대한 검증을 위해 지진과 관련된 매개변수를 이용하여 응력 전파를 모사할 수 있는 쿨롱 응력 전파 모델링을 실시하였다. 시추공 측정 결과로부터 계산된 전단응력 변화량은 (0.3-0.8) × 10-2MPa의 증가가 있었으며 이는 쿨 롱 응력 전파 모델을 통해 얻어진 전단응력 증가량 (0.1-0.6) × 10-2MPa과 상당히 유사함을 확인하였다. 따라서 시추공 변형률 계 자료가 설치 지역에서 작용하는 주응력의 영향과 지진에 의해 변화하는 응력 변화를 충분히 반영하고 있음을 시사하며 시 추공 변형률계 자료로부터 얻어지는 유의미한 응력 정보는 지진 관련 연구에서 활용될 수 있을 것으로 기대한다.

다국어 초록 (Multilingual Abstract)

This work investigates whether stress changes induced by an earthquake can be estimated using the deformation measured by high-resolution borehole strainmeters. We estimate the changes in the orientation and magnitude of the principal compression stresses using borehole strainmeter data recorded before and after the M7.2 El Mayor-Cucapah earthquake on April 4, 2010. Clear differences in the stress orientations and magnitudes are apparent before and after the event. The change in stress orientation appears related to subtle increases of stress in the tectonic maximum principal orientation, which is in agreement with the earthquake focal mechanism solution. The sudden stress drop at the onset of the earthquake was 10-3-10-2 MPa in the principal orientations. The Coulomb stress transfer model, which can estimate stress transfer, predicts a shear stress increase of (0.1-0.6) × 10-2 MPa at the strainmeter site, which is in line with the measured data (0.3-0.8) × 10-2 MPa. Overall, our results suggest that borehole strainmeter data reflect the subtle stress changes associated with earthquake occurrence, and that such data can be utilized for earthquake-related research.

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