Stepwise Variations of Strain Observed by Two Strainmeters Installed in Nearby Boreholes in a Fracture Zone

Abstract

   Stepwise variations of strain (hereafter called the strain step) were observed by a couple of borehole strainmeters installed 30 meters apart in a fracture zone of the southern Fossa Magna. Strain changes for a year after the installation carried at December, 1986—January, 1987 were apparent expansion indicating the cooling of grout and the relaxation of stress concentrated around the strainmeter. For two years after the installation the number of the strain steps decreased gradually. No clear relationships are seen between the occurrence of strain steps and the environmental conditions such as the rainfall, the atmospheric pressure, the groundwater temperature, and the groundwater level. The strain steps that occurred in the two boreholes are independent, which means that the spatial extent of the phenomenon is less than the distance between the strainmeters. The amplitude-frequency relationship for the strain steps is analogous to those of fracture processes such as earthquakes and acoustic emission in rocks. The decrease in the number of strain steps with time is represented by a formula similar to Ohmori's for the aftershock sequences of earthquakes. A laboratory experiment showed that the frequency of strain step occurrence depends on the stress levels in the surrounding media. Consequently it is inferred that the strain steps observed by our strainmeters are caused by local fracturing processes around the boreholes, the number of which indicates relaxation of the initial stress induced by the installation of the strainmeters.