Tectonic seismogenic index of geothermal reservoirs
Abstract
Identifying the parameters which define the earthquake magnitude and its frequency is a key point to assess the seismic hazard by fluid injections. Recently, it was found that the injected fluid mass on the one hand, and seismotectonic, site-specific parameters, such as fracture bulk concentration and critical stress state, on the other hand, are controlling factors of the magnitude distribution of fluid-induced events (Shapiro et al, 2010, TLE, doi:10.1190/1.3353727). We introduced the seismogenic index to characterize the seismotectonic state of a reservoir location. This single quantity which combines four generally unknown seismotectonic parameters can be computed from the induced seismicity observed during fluid injections. It was shown that the seismogenic index can be used to calculate occurrence probabilities of given magnitude events (Shapiro et al, 2010, TLE, doi:10.1190/1.3353727). A disadvantage, however, is that the seismogenic index is not an a priori knowledge of an undeveloped reservoir location. Here, we address the question whether tectonic seismic activity within an area where a fluid injection is planned can be used to evaluate its seismotectonic state. Intuitively, we may expect a higher activity of fluid-induced seismicity in a region characterized by a high level of tectonic seismicity. Hence, tectonic seismicity in such an area would provide a possibility to estimate the potential of inducing significant earthquake magnitudes (M>2). For this purpose, we expand and reformulate the theoretical framework which essentially applies to describe the occurrence of fluid-induced seismicity (see e.g. Dinske and Shapiro, 2012, JoS, doi:10.1007/s10950-012-9292-9) to the case of tectonic seismicity. Based on this model, we introduce the tectonic seismogenic index which can be determined prior to an injection if the tectonic seismicity rate and the crustal deformation rate are known in the reservoir region. We apply the derived formalism to reservoir locations where the seismogenic index for fluid-induced seismicity had already been obtained. In this study, we consider the geothermal reservoirs in Basel, Ogachi and Soultz, and the Bureau of Reclamation injection site in Paradox Valley. Our results show that the tectonic seismogenic index can be used as a proxy for the seismogenic index of fluid-induced seismicity. We find that the tectonic seismogenic index is systematically but only slightly lower and we discuss several factors which have an influence on the estimates. We conclude that our formalism can contribute to avoid the occurrence of large-magnitude fluid-induced earthquakes by properly selecting and developing reseroir locations.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.S32A..02D
- Keywords:
-
- 7200 SEISMOLOGY;
- 4300 NATURAL HAZARDS