Strain Changes due to Atmospheric Loading Observed at an 800m Borehole on Awaji Island

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  • MUKAI Atsushi
    Faculty of Law, Nara Sangyo University
  • FUJIMORI Kunio
    Division of Earth and Planetary Science, Graduate School of Science, Kyoto University

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  • 淡路島800mボアホールで観測された大気荷重に伴なうひずみ変化
  • アワジシマ 800m ボアホール デ カンソク サレタ タイキ カジュウ ニ トモナウ ヒズミ ヘンカ

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Abstract

The characteristics of strain changes due to atmospheric loading were investigated using strain data obtained from the bottom of an 800m borehole in the northwest of Awaji Island, Western Japan. The atmospheric loading effect on strain has been calculated to a precision of 10-8 or more by applying a linear function on the atmospheric pressure. Admittances of the atmospheric effect were determined to be -0.5--2.0×10-9/hPa and correspond to a Young's modulus value in the range 28-36GPa in the vicinity of the instrument. This value is slightly lower than that of general granite (30-60GPa). It is considered that cracks in the surrounding rocks lead to a reduced value of the modulus. The ratio of maximum to minimum principal strain due to atmospheric loading was calculated to be 1.4-1.7 and the direction of the maximum principal strain was nearly northeast-southwest i. e. parallel to the Nojima fault. The anisotropy of the atmospheric loading effect can be explained by the existence of cracks aligned with the fault. The Young's modulus values calculated from the atmospheric loading effect have decreased since 1997. This phenomenon could be caused by decreases in crustal stress around the borehole and reduced moisture content due to the draining of groundwater.

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