Micro-seismicity, fault structure, and hydrologic compartmentalization within the Coso Geothermal Field, California, from 1996 until present

Geothermal reservoirs derive their capacity for fluid and heat transport in large part from faults and fractures. In conventional reservoirs, preexisting faults and fractures are the main conduits for fluid flow, while in enhanced geothermal systems (EGS), fractures and faults that are generated or enlarged (i.e., through increases in surface area and aperture) by hydraulic stimulation provide the main pathways for fluids and heat. In both types of geothermal systems, seismicity can be used to locate active faults, which can act either as conduits for along-fault fluid flow and/or barriers to cross-fault flow. We relocate 14 years of seismicity in the Coso Geothermal Field (CGF) using differential travel time relocations to improve our knowledge of the subsurface geologic and hydrologic structure. The seismicity at Coso has been recorded on a local network operated by the Navy Geothermal Program, which provides exceptional coverage and quality of data. Using the relocated catalog, we employ a newly developed algorithm for fault identification using the spatial seismicity distribution and a priori constraints on fault zone width derived from local geologic mapping. We avoid having to assume a particular fault-normal seismicity distribution by finding regions of maximum spatial seismicity density. Assuming a maximum spatial density is physically plausible since faults, or more accurately fault zones, generate most of the associated seismicity within a central fault core or damage zone. These techniques are developed for naturally occurring, active faults within the CGF on which seismicity is induced, in part, by changes in production and injection. They can also be applied to EGS if seismicity is induced within newly created fracture systems of comparable width or if this seismicity is generated by stimulating pre-existing, partially sealed faults. The results of the relocations reveal that clouds of seismicity shrink into distinct oblate volumes of seismicity in which we interpret the faults to be located. The faults that are identified reveal a complicated image of the subsurface structure of the CGF and tend to fall within and between hydrologic compartments known to exist within the reservoir. This suggests that some of the faults in the CGF serve as conduits for fluids and heat, while others seal portions of the reservoir and function as hydrologic domain boundaries. We anticipate that our method can be applied in many conventional geothermal systems, either before plant design to guide well placement (if natural seismicity is present and recorded well) or as part of evaluating the hydraulic network stimulated by EGS. This type of information will also be useful in locating and designing EGS, either in seismically active areas lacking prior geothermal development or on the margins of existing geothermal fields. The improved knowledge of subsurface structure provided by this type of modeling can also aid in assessing seismic hazards associated with EGS stimulations or ongoing injection and production when used in concert with fault-stress transfer models.