地震に伴う湧水量および間隙水圧変化から推定された破砕帯の水理学的特徴

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  • Hydraulic Characteristics of the Fracture Zone Estimated by Variations in Groundwater Discharge and Pore Pressure due to the Earthquake
  • ジシン ニ トモナウ ユウスイリョウ オヨビ カンゲキ スイアツ ヘンカ カラ スイテイ サレタ ハサイタイ ノ スイリガクテキ トクチョウ

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説明

<p>The 2011 off the Pacific coast of Tohoku Earthquake caused step-like increases of groundwater discharge and pore pressure at Rokko-Takao station, that were followed by great decreases for one month after the earthquake. We attempted to estimate the variations in hydraulic characteristics, such as permeability, storage coefficient, and groundwater source pressure by applying the one-dimensional groundwater migration model to the observed groundwater discharge and pore pressure. When we estimated the hydraulic characteristics by using the time array data of groundwater discharge and pore pressure, the permeability and groundwater source pressure showed increases of about 20 % and 80 %, respectively, just after the earthquake. However, the groundwater source pressure decreased to about 40 % in one month. The storage coefficient estimated by using the atmospheric effects on groundwater discharge and pore pressure showed an increase of about 200% after the earthquake. These results suggest that seismic motion induced the outflow of mud accumulated in the cracks and the increase in permeability of the surrounding crust. The increase in permeability is expected to cause excessive outflow of groundwater and a great decrease in groundwater source pressure. The shortage of groundwater in the cracks agrees with the increase in storage coefficient. However, the permeability estimated by using atmospheric effects showed an immediate decrease just after the earthquake, which did not agree with the estimated permeability based on the time array data. This discrepancy might have been caused by the difference in extent of the area represented by the estimated permeability. The atmospheric effects showed significant signals in periods shorter than several days and are expected to be dependent on the hydraulic characteristics in the narrow area near the station. It is considered that the mud outflow owing to seismic motion might have been concentrated near the station where groundwater discharge occurs, which in turn might have caused the decrease in permeability in that area.</p>

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