Influence of pore-pressure on the event-size distribution of induced earthquakes

During an Enhanced Geothermal System (EGS) experiment, fluid is injected at high pressure into crystalline rock, to enhance its permeability and thus create a reservoir from which geothermal heat can be extracted. The fracturing of the basement caused by these high pore-pressures is associated with microseismicity. However, the relationship between the magnitudes of these induced seismic events and the applied fluid injection rates, and thus pore-pressure, is unknown. Here we show how pore-pressure can be linked to the seismic frequency-magnitude distribution, described by its slope, the b-value. We evaluate the dataset of an EGS in Basel, Switzerland and compare the observed event-size distribution with the outcome of a minimalistic model of pore-pressure evolution that relates event-sizes to the differential stress sigma(D). We observe that the decrease of b-values with increasing distance of the injection point is likely caused by a decrease in pore-pressure. This leads to an increase of the probability of a large magnitude event with distance and time. Citation: Bachmann, C. E., S. Wiemer, B. P. Goertz-Allmann, and J. Woessner (2012), Influence of pore-pressure on the event-size distribution of induced earthquakes, Geophys. Res. Lett., 39, L09302, doi:10.1029/2012GL051480.