A Seismological Investigation of the Kawerau Geothermal Field

Abstract

We use a recently installed seismometer network to investigate the seismicity of the Kawerau Geothermal Field. Our dataset consists of 401 shallow earthquakes reported by GeoNet between 1 January 2008 and 1 July 2013 with an average magnitude (ML) of 1.85. P and S phase arrivals and rst motions were manually picked and the events relocated using NonLinLoc and HYPODD. The resulting set of locations showed a domed seismicity distribution centred on the central part of the geothermal eld, with the base of seismicity increasing from around 4 km in the centre to 6􀀀‒7 km around the fringes, with the depth increase most notable to the East. This provides support for the conceptual model of the eld and suggests that higher temperatures may be located directly beneath the eld rather than below Putauaki as previously thought. Focal mechanism solutions were obtained for 14 events and revealed a combination of strike-slip and dip slip faulting. Closely spaced similar mechanisms suggested we see repeated activity on the same fault structures within a 10 month time frame. Inverting these focal mechanisms for stress revealed horizontal maximum and minimum stress vectors with SHmax oriented at 65°, slightly clockwise from the 43° rift axis of the Whakatane Graben. A low stress ratio of 0.2 suggested the extension associated with the rifting of the Whakatane Graben is the dominant stress regime at Kawerau.