Seismotectonic state of reservoirs inferred from magnitude distributions of fluid-induced seismicity

Fluid injections in geothermal reservoirs usually induce small magnitude earthquakes (M < 2). Sometimes, however, earthquakes with larger magnitudes (M similar to 4) occur. Recently, we have shown that under rather general conditions, the probability of an event having a magnitude larger than a given one increases proportionally to the injected fluid mass. The number of earthquakes larger than a given magnitude also depends on the tectonic conditions of an injection site. A convenient parameter for the characterisation of the seismotectonic state of a reservoir location is the seismogenic index Sigma. It combines four, generally unknown, site-specific seismotectonic quantities. Using this index, we comparatively analyse the seismotectonic state of several geothermal as well as non-geothermal reservoir locations. The seismogenic indices of the considered locations are in the range of -10 < Sigma < 0.5. Although the number of reservoirs under examination is limited, we see a clear separation between hydrocarbon and geothermal reservoirs with respect to the seismotectonic state. In addition to a higher seismogenic index, geothermal reservoir locations are characterised by a lower b value. It means that fluid injections in geothermal reservoirs have a higher probability to induce an earthquake with a significant magnitude. Our formulation provides a basis for estimating expected magnitudes of induced earthquakes. This can potentially be used to avoid the occurrence of large magnitude earthquakes by correspondingly planning fluid injections.