Stress drop variations of induced earthquakes at the Basel geothermal site

We determine stress drops from P-wave spectra of about 1000 earthquakes induced by hydraulic stimulation in crystalline rock for a deep heat mining project in Basel, Switzerland. We observe an increase in stress drop by about a factor of five with radial distance from 10 m to 300 m, which suggests that stress drop correlates with pore pressure perturbations due to the injection. We test this hypothesis by calculating the injection-related pore pressure perturbation based on a simple linear pore pressure diffusion model and find a good correlation of the expected pore pressure perturbation with the estimated stress drops. Citation: Goertz-Allmann, B. P., A. Goertz, and S. Wiemer (2011), Stress drop variations of induced earthquakes at the Basel geothermal site, Geophys. Res. Lett., 38, L09308, doi: 10.1029/2011GL047498.