| Abstract: | Forced changes in the water head within a granite-penetrating borehole were found to induce anomalously large free-surface strains and tilts in the vicinity of the hole. This deformation is shown to be due to fluid-pressure-induced changes in the aperture of a compliant hydraulically-conductive fracture at a depth of 100 m and a quantitative analysis of the deformation data is performed to recover fracture characteristics. The importance of the effect for secular earth strain measurements is discussed in the light of the ubiquitous nature of both water table fluctuations and fractures in the shallow crust. |
