Microearthquakes at the Rotokawa Geothermal Field, New Zealand

Abstract

Two temporary microearthquake surveys have been run in Rotokawa to first determine the level of injection induced microearthquake activity and then map the distribution of these microearthquakes to assist with injection management. A 10 station array operated by GNS Science during simultaneous well flowtests and injection tracer tests detected over 400 local microearthquake events, of which 225 were located. Although no temporal relationship was apparent between injection and seismicity, double difference earthquake locations show a spatial relationship between the seismicity and inferred faults near and beneath a producer, indicating that fluid pressure changes and/or injection-induced cooling triggered seismicity on the deeper fault structure. A tenuous continuation of the microearthquakes into the production field is consistent with low level tracer returns to a more distant production well. The surveys have aided field management by identifying a deep fault structure from microearthquake seismicity that will become a production drilling target.