Exploring the Nature and Influence of Intermediate-Depth Seismicity Beneath the Taupō Volcanic Zone

The central Taupō Volcanic Zone (TVZ) hosts the most productive rhyolitic volcanic system in the world. A link has been proposed between the composition of deep subduction fluids and the extreme productivity of the caldera volcanoes in the central TVZ (Taupō and Ōkataina), largely based on geochemical studies. It is hypothesised that fluids freed from dehydration reactions in the subducting slab facilitate intermediate-depth earthquakes through dehydration-related embrittlement. These fluids can migrate into the mantle wedge and contribute to partial melting of the primitive magma source, which influences the flux of magmas to the central TVZ calderas. In this thesis we aim to investigate the potential link between these earthquakes and the subduction fluid signature that is present in the central TVZ caldera volcanoes. Well-constrained earthquake locations and their faulting parameters can help us better understand this relationship. The new earthquake catalogue produced in this thesis contains 397 earthquakes between depths of 50–305 km. These were located with 40,266 manual P- and S-picks, a 3D velocity model, and relocated using relative relocation techniques. These locations are further refined by observations of converted and reflected seismic phases, unique to earthquakes nucleating in various regions of the subducting slab.

We observe a double seismic zone (DSZ) beneath the central TVZ. This occurs in one section of the slab, at depths of 145–190 km and the seismic planes are separated by an aseismic plane <8 km wide. The stress state of the subducting slab, as determined by 47 focal mechanisms, has an upper seismic plane in down-dip extension and a lower seismic plane in down-dip compression. We also identify earthquakes occurring in the mantle wedge whose computed focal mechanisms indicate that they typically occur on near vertical faults.

The pattern of intermediate-depth seismicity we observe is consistent with earthquakes being facilitated by dehydration reactions in the oceanic slab crust and mantle. The abundant fluids released from these reactions have distinct geochemical signatures, that have been previously identified in the eruptive products of the caldera volcanoes in the central TVZ. We therefore provide evidence of a link between intermediate-depth seismicity and magmatism in the central TVZ. Additionally, the uniqueness of the Hikurangi slab, which has an unusually high fracture permeability, its oblique subduction beneath the North Island and its thermo-mechanic structure, is likely a significant contributor to the anomalous rhyolitic volcanism observed in the central TVZ.