Electrical measurements as stress-strain monitors

Many of the measurements of phyiscal properties being made in earthquake prediction studies are based on the premise that these properties are influenced by stresses and strains, especially so near the failure point. Electrical properties of rocks are controlled by the fluid in the pores and cracks in the rocks. Because these regions are most influenced by stresses, one should expect electrical measurements to be sensitive measures of changing stresses and strains. Nevertheless, the strain changes we are dealing with are very small, and, consequently, we need very sensitive instruments to detect them.