Permeability in Fracture Zone nearby the Nojima Fault Estimated Using Strain Changes due to Water Injection Experiments

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  • 注水試験に伴うひずみ変化から推定される野島断層近傍の破砕帯の透水性
  • チュウスイ シケン ニ トモナウ ヒズミ ヘンカ カラ スイテイ サレル ノジマ ダンソウ キンボウ ノ ハサイタイ ノ トウスイセイ

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Abstract

Water injection experiments were performed in 1997 and 2000 at the 1800m-deep borehole near the fracture zone of the 1995 Hyogoken-Nanbu earthquake. During these experiments, contraction of strain changes was observed with three-component strainmeters at a bottom of 800m-deep borehole, 70m southwest of the 1800m-deep borehole. We estimated hydraulic properties of the fracture zone nearby the Nojima fault using the observational data of strain in order to investigate a healing of the fault at the postseismic stage. Strain changes due to water injection depend on hydraulic parameters, such as hydraulic diffusivity, hydraulic conductivity and channel of pore fluid flow. Calculated strain changes due to water injection agreed with the observational data, when we assumed that pore fluid flowed on a vertical plane perpendicular to the Nojima fault. The orientation of the minimum principal stress is parallel to the fault in the fracture zone. It is considered that pore fluid was diffused through the fractures perpendicular to the fault, because pore pressure could easily open fractures in the orientation of the minimum principal stress. Hydraulic diffusivities in 1997 and 2000 were determined to be 1.1±0.1 and 0.5±0.1m2/s, respectively. The reduced permeability suggests that the fractures nearby the fault have been healing after the 1995 Hyogo-ken Nanbu earthquake. Hydraulic conductivity was determined to be 1.4-4.3×10-6m/s. This value is about three orders of magnitude larger than the hydraulic conductivities of general rocks estimated in other studies. It is considered that the large hydraulic conductivity was attributed to the following facts; (1) water was injected in the shallow crust at the depth of 540m, (2) the water injection experiments were performed in the fracture zone nearby the fault, (3) a healing of fractures was not completed.

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