Thickness Analysis of the Alluvium and Yeonil Group in Pohang Area through the Microtremor HVSR Techniques

상시미동 HVSR 기법 적용을 통한 포항지역의 충적층 및 연일층군의 층후 분석

  • Received : 2021.04.27
  • Accepted : 2021.06.23
  • Published : 2021.06.30


The magnitude 5.4 earthquake occurred in 2017 near 7.5 km north of Buk-gu, Pohang. In order to find out the characteristics of microtremor in Buk-gu, Pohang, Gyeongsangbuk-do, where earthquake damage occurred significantly, a total of 39 points were acquired to analyze the horizontal to vertical spectral ratio (HVSR). Microtremor vary from region to region, so the resonant frequency of the region is obtained by examining the microtremor. For Thickness analysis, we analyze the structure and properties of shear wave velocity (Vs) up to the underlying rock to compare resonance frequencies to match the horizontal to vertical spectral ratio (HVSR) analysis technique against nearby boring data. Using F0 = Vs/4H with a resonance frequency of alluvium is 1.3 ± 0.07 Hz and a resonance frequency of Yeonil group is 0.69 ± 0.22 Hz, the alluvium thickness was found to be 26~30 m and the Yeonil group thickness was 170~250 m.

포항지진은 2017년 규모 5.4의 포항시 북구 북쪽 7.5 km 부근에서 발생하였다. 지진피해가 크게 발생한 경상북도 포항 북구 지역의 상시미동의 특성을 알아보기 위하여 총 39 지점에 상시미동 자료를 취득하여 수평 대 수직 스펙트럼 비율(Horizontal to Vertical Spectral Ratios, HVSR) 분석연구를 실시하였다. 상시미동은 지역에 따라 각각 다르므로 상시미동을 조사해 그 지역의 공명주파수를 취득한다. 층후 분석을 위해서는 기반암까지의 전단파 속도(Vs) 구조 및 특성을 분석하여 공명주파수를 인근의 시추자료와 대비하여 수평 대 수직 스펙트럼 비율(Horizontal to Vertical Spectral Ratios, HVSR) 분석기법이 일치하는지 비교하였다. 충적층의 공명주파수 1.3 ± 0.07 Hz, 연일층군의 공명주파수 0.69 ± 0.22 Hz로 F0 = Vs/4H를 이용하여 층후를 구하면 충적층은 26~30 m, 연일층군은 170~250 m로 인근 시추자료와 비교하여 대체로 일치한 것으로 분석하였다.



  1. Ahn, H.S., Park, J.Y., Kim, J.M., Kim, J.C., 2015, Three-dimensional geologic modeling of the Pohang Basin distributed in Haedo-dong, Nam-gu, Pohang-si, Korea, Journal of the Geological Society of Korea, 51(1), 21-36 (in Korean with English abstract).
  2. Field, D.J., Hayes, A., Hess, R.F., 1993, Contour integration by the human visual system: evidence for a local "association field", Vision Research, 33(2), 173-193.
  3. Hong, M.H., Kim, K.Y., 2010, H/V spectral-ratio analysis of microtremors in Jeju Island, Geophysics and Geophysical Exploration, 13(2), 144-152 (in Korean with English abstract).
  4. Jung, H.O., Kim, H.J., Jo, B.G., Park, N.R., 2010, The microtremor HVSRs in the SW Korean Peninsula I: Characteristics of the HVSR peak frequency and amplification, Journal of the Korean Earth Science Society, 31(6), 541-554 (in Korean with English abstract).
  5. Kagami, H., Okada, S., Shiono, K., Oner, M., Dravinski, M., Mal, A.K., 1986, Observation of 1 to 5 second microtremors and their application to earthquake engineering, Part III. A two-dimensional study of site effects in the San Fernando Valley, Bulletin of the Seismological Society of America, 76(6), 1801-1812.
  6. Kanai, K., Tanaka, T., 1954, Measurement of the microtremor, Bulletin of Earthquake Research Institute, 32, 199-209.
  7. Kang, S.Y., Kim, K.H., Kim, D.Y., Jeon, B.U., Lee, J.W., 2020, Effects of meteorological variations and sensor burial depths on HVSR analysis, Journal of the Korean Earth Science Society, 41(6), 658-669 (in Korean with English abstract).
  8. Kim, Y.J., Ko, K.W., Kim, B.M., Park, D.H., Kim, K.S., Han, J.T., Kim, D.S., 2020, Evaluation of liquefaction triggering for the Pohang area based on SPT and CPT tests, Journal of the Korean Geotechnical Society, 36(10), 57-71 (in Korean with English abstract).
  9. Lee, H.K., Kim, R.Y., Kang, T.S., 2017, Seismic response from microtremor of Chogye Basin, Korea, Geophysics and Geophysical Exploration, 20(2), 88-95 (in Korean with English abstract).
  10. Lee, J.H., 1968, Genesis of the native copper deposits in Mesozoic basalt flows in the Yongyang Basin, Korea, The Journal of the Geological Society of Korea, 4(3), 111-166.
  11. Lee, J.W., Um, S.H., Kim, C.S., 1967, Report of exploration for natural gas in the Pohang area, Korea, In: Geological Report on Coal Fields of Korea, No. 8, Technical Report KR-58 No. 8, Geological Survey of Korea (GSK), Seoul, Korea, 1-22.
  12. Lee, J.W., Um, S.H., Kim, C.S., 1968, Summary of report of exploration for natural gas in the Pohang area, Korea, In: Geology and Ore Deposit, No. 4, Technical Report KR-66 No. 4, Geological Survey of Korea (GSK), Seoul, Korea, 23-32.
  13. Lermo, J ., Chavez-Garcia, F.J., 1993, Site effects evaluation using spectral ratios with only one station, Bulletin of the Seismological Society of America, 83(5), 1574-1594.
  14. Nakamura, Y., 1989, A method for dynamic characteristics estimations of subsurface using microtremors on the ground surface, Quarterly Report of Railway Technical Research, 30, 25-33.
  15. Ohmachi, T., Nakamura, Y., Toshinawa, T., 1991, Ground motion characteristics in the San Francisco Bay area detected by microtremor measurements, Proceedings of the 2nd International Conference on Recent Advances in Geotechnical Earth Engineering and Soil Dynamics, Expanded Abstract, LP08, 1643-1648.
  16. Parolai, S., Bormann, P., Milkereit, C., 2002, New relationships between Vs, thickness of sediments, and resonance frequency calculated by the H/V ratio of seismic noise for the Cologne area (Germany), Bulletin of the Seismological Society of America, 92(6), 2521-2527.
  17. Song, Y.H. et al., 2006, Development of deep, low-enthalpy geothermal energy, Technical Report KR-03-01, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon, Korea, 87.
  18. Teves-Costa, P., Matias, L., Oliveira, C.S., Mendez-Victor, L.A., 1996, Shallow crustal models in the Lisbon area from explosion data using body and surface wave analysis, Tectonophysics, 258(1-4), 171-193.
  19. Theodulidis, N.P., Bard, P.Y., 1995, Horizontal to vertical spectral ratio and geological conditions: an analysis of strong motion data from Greece and Taiwan (SMART-1), Soil Dynamics and Earthquake Engineering, 14(3), 177-197.
  20. Um, S.H., Lee, D.W., Park, B.S., 1964, Geological report of the Pohang sheet (1:50,000), Geological Survey of Korea, 21.
  21. Walling, D.E., Schuller, P., Zhang, Y., Iroume, A., 2009, Extending the timescale for using beryllium 7 measurements to document soil redistribution by erosion, Water Resources Research, 45(W02418), 1-13.
  22. Yamanaka, H., Takemura, M., Ishida, H., Niwa, M., 1994, Characteristics of long-period microtremors and their applicability in exploration of deep sedimentary layers, Bulletin of the Seismological Society of America, 84(6), 1831-1841.