Quantification of the different processes that contribute to the spatio-temporal stress changes at geothermal sites is critical for the understanding of induced seismicity. Poroelastic coupling and thermal-mechanical processes during hot water extraction and cold water injection are probably the key processes that contribute to the stress changes in The Geysers geothermal field. In particular, with the onset of massive injection of wastewater into the reservoir the number of induced events with magnitudes M > 4 increased significantly. This raised the question which stress changing process is the key control of the induced seismicity rate. In addition to man-made stress changes the tectonic loading from the Pacific plate motion relative to the North America plate also alters the stress state as well as the co-seismic stress change of the induced events itself. In order to assess the relative importance of the various stress changing processes, we built a 3D thermo-hydro-geomechanical numerical model of The Geysers geothermal field. The model accounts for the far-field tectonic loading as well as for the co-seismic stress changes of the M > 4 events. To solve the resulting fully coupled partial differential equations we use the commercial finite element code Abaqus. After the implementation of an initial stress state that is calibrated against data of the orientations of maximum horizontal stress and stress regime from earthquake focal mechanism solutions, we apply kinematic far-field boundary conditions derived from continuous GPS stations. First, preliminary results reveal that tectonic loading contributes within the reservoir little compared to the pore-pressure diffusion on time scales of several years. Furthermore, the relative importance of stress changes due to pore pressure diffusion strongly depends on the spatial and temporal scales that are analyzed as well as on the uncertainties of the rock properties in particular the permeability of the rock matrix.

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