Microseismicity at Rotokawa Geothermal Field, 2008 to 2012

Abstract

Induced microearthquake (MEQ) activity has been observed at Rotokawa Geothermal Field since deep reinjection began in 2005. Following temporary observations in 2005 and 2006, continuous MEQ recording began in 2008 and continues. A dense network and careful site selection have provided data of sufficient quality to provide constraints for reservoir modelling (detailed in a companion paper by Sewell et al., 2013) though this has required manual refinement of automatic phase picks and relocation using double-difference approaches. Since October 2008, MEQs have concentrated in a diamondshaped region between the production zone and current reinjection zone at depths of 1.5 to 3 km, but have moved depending on where fluid has been reinjected. Before October 2008, along-strike fault-controlled permeability influenced MEQ locations, but with reinjection in the south-east of the reservoir an across-strike fault barrier to reinjected fluids is thought to be important. The rate of microseismicity has varied closely with the rate of deep fluid reinjection, especially related to extra reinjection required for the Nga Awa Purua power station. The relative importance of reinjection pressure and injectate temperature in inducing MEQs is unclear. There have been about 50 MEQs of magnitude more than 2, but only three of magnitude more than 3. Less than 10 events have been reported felt and all have been too small to cause public concern.