| Abstract |
A dataset comprised of 4900 well-located and well-constrained earthquake focal plane mechanisms recorded in the Northwest Geysers geothermal field during the period of Jan 2005 – May 2012 was used to determine stress orientations and relative magnitudes at fine scale. The stress inversions were organized into gridblocks of varying size using a three-dimensional oct-tree gridding algorithm in which scale of the gridblocks is governed by three-dimensional data density. This method retains the spatial distribution of hypocenters but allows separate inversions for contiguous blocks of seismicity at the finest scale warranted by the data. A minimum of 25 focal plane mechanisms were used to find the best-fitting the orientation of the stress tensor in each gridblock. The stress orientations in each gridblock were then used to determine which of the two nodal planes of the focal mechanism had the highest ratio of resolved shear to normal stresses and was thus more likely to be the active fault plane. Not surprisingly, the faulting regime was found to be normal/strike-slip, as is present in the Geysers, and was quite uniform, even at fine scale. The maximum horizontal principal stress orientation, SHmax, and the vertical principal stress, Sv, are approximately equal in value with and average SHmax of N23E. A very consistent stress field was found within the three-dimensional grid studied. The apparent fault planes (assumed to be the primary conduits for fluid flow) were found to be steeply dipping and include approximately north to east striking strike-slip faults and NE trending normal faults, all consistent with the regional SS/NF faulting regime. The greatly improved resolution of the earthquake data allows us to analyze the seismicity in great detail and to determine the orientations of faults providing pathways for fluid flow in the reservoir. |