New Zealand geothermal power plants as critical facilities: an active fault avoidance study in the Wairakei Geothermal Field, New Zealand

Abstract

Active faults in rifts commonly provide high crustal permeability and control geothermal fluid pathways. However, active faults can also pose surface deformation hazards to geothermal power plants and associated infrastructure. The New Zealand Ministry for the Environment (MfE) guidelines recommend avoidance of active faults for construction of new buildings based on building importance and the rate of fault activity. Power plants, which are classed as 'high Building Importance Category', are permitted on faults with a rupture recurrence interval greater than 10,000 years. We present a site feasibility study for the Te Mihi Power Plant (Wairakei Geothermal Field), used to determine if there is recent major active faulting at the proposed site. The initial Power Plant site was proposed in an area exhibiting complex surface patterns of active faults with two closely spaced (few metres to hundreds of metres), intersecting, normal fault sets. Detailed aerial photo review and field mapping was undertaken to improve the accuracy of previously mapped fault traces, and to potentially identify previously undocumented faults. Fault scarps were assigned different geomorphic expression (from "clear" to "inferred"). In the study area, recurrence intervals of active faulting can be difficult to estimate because fault scarps are frequently blanketed by tephra from late Quaternary eruptions of the nearby Taupo caldera, and detailed paleoseismic studies are absent. Because of the potential burial of geomorphic fault scarps, GPR surveys and paleoseismic trenching were carried out to investigate the apparent lack of active faulting at the plant footprint, and to better understand fault activity rates close to the newly proposed Power Plant site. Displacements of several post-25 ka tephra marker horizons, and fault planes, were analysed in the trench to assess the presence or absence of recent fault activity, and to calibrate the reflectors observed in the GPR images. The trench study also allowed accurate estimation of fault slip rate and recurrence interval. This study has provided a first calibration for a correlation between geomorphic expression of faults and fault activity in this area. The study revealed that: a) some of the subtle features initially suspected as fault scarps were indeed active faults; b) deep paleoseismic excavations are needed in sites located in close proximity to frequently active volcanoes (due to thick cover beds) even when assessing faults with clear geomorphic expression; and c) GPR is useful when assessing activity of faults with large offsets (in this area, usually faults with recurrence interval less than 5,000 years), but the resolution of GPR might not allow evaluation of minor faulting which should then be assessed at the site during construction. This investigation allowed the Te Mihi Power Plant to be re-sited in an area outside the identified construction-avoidance envelope that conformed to the recommendations of the MfE guidelines.