A dataset comprised of well‐constrained focal mechanisms for 6147 earthquakes recorded in the northwest Geysers geothermal field during the period of 2005–2012 was utilized to conduct a detailed stress study within and below the geothermal reservoir. The high‐quality focal mechanisms were organized into grid blocks of varying size using a 3D octree gridding algorithm in which discretization was governed by data density. This method allows for separate inversions of contiguous blocks of seismicity at a relatively fine scale. We obtained the three principal stress orientations for every grid block containing at least 25 events by inverting for the best‐fit stress tensor within each grid block. The principal stress orientations were used to determine which of the two nodal planes for each focal mechanism had the highest ratio of resolved shear‐to‐normal stresses and was thus more likely to be the fault plane. We found a normal/strike‐slip faulting regime (SHmaxSv>Shmin) both within and below the reservoir, consistent with the extensional and strike‐slip tectonics in the region surrounding The Geysers. In addition, an average SHmax orientation of N26°E was obtained for the studied crustal volume. These observations suggest that injection and production activities over the past 50+ years do not appear to have significantly affected the local stress field. The presumed fault planes are steeply dipping with northeast–southwest to east–west strike directions suggesting that these are the principal flow directions both within the low matrix permeability graywacke reservoir and in the wholly concealed granitic pluton (locally referred to as the felsite) basement below.

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