Characteristics of Seismicity at Ngatamariki and Rotokawa Geothermal Fields, North Island, New Zealand

Fluid-induced seismicity (FIS) is a useful indicator of thermal and pressure changes within a geothermal reservoir. Given the difficulty of making measurements in the space between wells, FIS provides one of the only direct observations of fluid-related reservoir processes. Understanding exactly why and how these microearthquakes occur helps us unravel the interaction between the movement of fluid and heat at depth and could allow for a better understanding of the creation and destruction of permeability in the reservoir. We begin with an earthquake catalog of roughly 6200 events which occurred between May 2012 and November 2015 at the Mercury geothermal fields at Rotokawa and Ngatamariki. We expand this catalog using matched filter detection to include 350,000 microearthquakes. Earthquakes at the fields are densely clustered in areas of active fluid injection and the rate of seismicity is highly correlated with injection rate. We present high-precision locations as well as frequency-magnitude distributions and source mechanisms for significant events and discuss their relation to injection and production rates at the fields. In particular, we focus on specific periods of interest including the stimulation of well NM08 in June 2012, the Ngatamariki plant startup in April 2013, the switch of injection at Rotokawa to well RK22 in 2013 and a prolonged period of permeability change at well RK24.