Seismic imaging of mid-crustal heterogeneity beneath geothermal systems, central Taupō Volcanic Zone, New Zealand

Abstract

The Taupō Volcanic Zone (TVZ) in New Zealand is a region of highly productive Quaternary volcanism and high hydrothermal heat flux. We investigate the mid-crustal seismic velocity structure of a region within the central, rhyolitic part of the TVZ encompassing high-temperature geothermal systems (e.g. Wairakei, Rotokawa). Using double-difference tomographic inversion of local earthquake data we derive 3-D models of P-wave velocity (Vp) and Vp/Vs for the subsurface. Both high ($>$ 6.0 km/s) and low ($<$ 5.5 km/s) Vp heterogeneities are seen in the mid-crust between 5 and 11 km depth. Regions with high Vp are interpreted to indicate the presence of solidified, more mafic, material within an otherwise quartzo-feldspathic crust, while regions with low Vp values are inferred to represent bodies of crystal-rich magma with a low melt fraction. Using the new 3-D velocity model we then relocated $\sim$9100 earthquakes recorded between 2009 and 2022. The relocated seismicity is strongly clustered, including in the vicinity of some of the geothermal systems (e.g. Rotokawa) where fluid is currently being extracted for electric-power production. Mid-crustal seismicity is also observed west of the Wairakei geothermal field, as well as along the south-eastern margin of the Ngakuru graben and on the western margin of the Whakamaru caldera. The depth distribution of the highest-quality hypocentres shows that 90% of the seismicity at Rotokawa geothermal field occurs at depths shallower than 5.1 km, consistent with a shallow brittle–ductile transition and the presence of a cooling pluton beneath Rotokawa seen in magnetotelluric data.