地殻変動連続観測による断層破砕帯およびその近傍のレオロジー特性

Continuous observations of crustal deformation have been carried out at Rokko-Tsurukabuto observation station. The station is located at Tsurukabuto incline of the Rokko tunnel of San'yo Shinkansen in Kobe City, southwestern Japan. Otsuki fault, one of active faults in the Rokko mountain area, crosses the observation tunnel for the station. The rheological characteristics of substances in and around the fracture zone of this fault were examined by using results of the continuous strain observations and of in situ stress measurements at Rokko-Suwayama site near the station. The results lead to the following three conclusions.(1) The following maximum shear strain rates were obtained in 1981-1984:2.6 × 10^-6/year outside of the fracture zone, and14.5 × 10^-6/year in the fracture zone.The greater part of such large strain rates are considered due not to elastic deformation but to viscous/plastic flow. According to in situ stress measurements made in 1983, the horizontal maximum shear stress in the Rokko mountain area was 0.60 MPa at 100 m below the ground surface. If we assume that rocks in and around the fracture zone are a Maxwell substance for the long-term steady stress and that the observed shear strain rates have been caused by the constant shear stress mentioned above, then viscosities ηM in and around the fracture zone are obtained as follows:ηM =7 × 10¹⁸ Pa⋅s outside of the fracture zone, and=1 × 10¹⁸ Pa⋅s in the fracture zone, (2) Large strain amplitudes for exponential aftereffects associated with an earthquake of magnitude 5.6 occurring at Yamasaki fault (d=60 km) were observed at the station. They were, however, too large to consider that dislocation at the source was elastically propagated. The aftereffects are then supposed to be caused by a local stress redistri bution around the observation tunnel associated with the occurrence of the earthquake. If we assume that the rocks are a Kelvin-Voigt substance for such an instantaneous stress change, using the time constant of the aftereffects and the elastic moduli around the observation tunnel, we can obtain the following values for the viscosity ηK:ηK=7 × 10¹⁴ Pa ⋅ s outside of the fracture zone, =2 × 10¹⁴ Pa ⋅ s in the fracture zone (strongly crushed part), =4 × 10¹⁴ Pa ⋅ s in the fracture zone (the other parts).(3) If we employ Burgers model that consist of the Maxwell and the Kelvin-Voigt elements with each value of the viscosity obtained above for each site in the tunnel, a greater part of the strain changes due to the variation of atmospheric pressure can be explained as the response of the model to a periodic external stress.