Seismic response to deep injection changes at Rotokawa geothermal field, New Zealand
While simple spatial locations of seismicity can provide valuable insights about reservoir structure and fluid movement, frequency-magnitude distributions, often of secondary interest to operators, may contain unique information about the distribution of pressure. In this work, we present a four-year catalog of seismicity for the Rotokawa geothermal field in the central Taupō Volcanic Zone, New Zealand starting two years after the commissioning of the 140 MW Nga Awa Purua power station. Using waveform-correlation based signal detection we double the size of the previous earthquake catalog and update the location and orientation of two important reservoir faults, while potentially identifying a new structure. We find the rate of seismicity to be insensitive to major changes in injection strategy during the study period, including the injectivity decline and shift of injection away from the dominant injector, RK24. We also map the spatial distribution of the earthquake frequency-magnitude distribution, or b-value. Although complex, the spatial variation of b can potentially be used to monitor the relative distribution of pressure in the reservoir, where areas of high b correspond to areas of high pore-fluid pressure and a broad distribution of fractures. While not routinely conducted, we believe our results show promise for using earthquake b-value as an additional tool for reservoir monitoring and management.