Seismicity, sub-surface structure and partnership with Tangata Whenua of Taupō Volcano
Authors:   , Mestel Eleanor 
Published: October 2023

<p><strong>Taupō volcano lies beneath the waters of Lake Taupō within the rohe (region) of Ngāti Tūwharetoa in the centre of North Island, Aotearoa New Zealand. It is a frequently active rhyolitic caldera volcano that was the site of Earth’s most recent supereruption (Ōruanui, ∼25.5 ka), as well as one of the most violent eruptions globally of the last 5000 years (Taupō, 232±10 CE). Taupō has erupted 28 times since Ōruanui, and displays elevated unrest activity (seismicity and surface deformation) on roughly decadal timescales, most recently in 2019 and 2022–23. This elevated activity resulted in the Volcanic Alert Level for Taupō being raised to Level 1 for the first time on 2022-09-22. Any resumption of eruptive activity at the volcano poses a major source of hazard, and the magma reservoir and its interactions with the regional tectonics that lead to unrest and possible eruption are not well understood.</strong></p><p>In working to understand the current state of Taupō volcano, we deployed the temporary ECLIPSE seismometer network (October 2019 to May 2022) to complement the permanent national GeoNet network. A core part of the planning, deployment, and management of the ECLIPSE network involved partnering with local Indigenous Māori Iwi and Hapū communities and with emergency management. We reflected upon this atypical co-production approach to geophysical network deployment, that has improved outcomes both for communities and researchers, identifying a central theme of creating and holding space for researchers and communities to engage. We built the co-production approach into the project from the start by involving a broad team including representatives of local Iwi Ngāti Tūwharetoa and Te Arawa as supported key researchers. We worked to respect communities’ time, protocols, and decisions; and to exchange knowledge about the research and results with landowners, community leaders, schools, and young people. Time spent kanohi ki te kanohi (face-to-face) built relationships and trust within and outside the research team with the potential to last beyond the scope of the research project.</p><p>To investigate the processes controlling seismicity at Taupō, we characterised the earthquakes near Taupō between October 2019 to September 2022 in detail, using automatic picking and association; locations; relative relocations; and focal mechanisms. We developed a Taupō-specific one-dimensional velocity model, and inverted for local magnitude scales (horizontal and vertical components). The seismicity outside the northern part of the lake was tectonically-controlled with minor aqueous fluid involvement, with the exception of the swarms beneath the geothermal power plants that we interpret as being induced by anthropogenic activity. Stresses from rifting processes also caused slip on pre-existing structures to the east of the lake. The seismicity in the northern part of the lake was directly related to the magmatic system. It revealed a change in the magmatic system after the end of the 2019 unrest, with seismicity occurring in the centre of the lake for the first time in a decade as the shallow magma system reacted to the 2019 intrusion. In May 2022, there was a seven-fold increase in the seismicity rate as well as uplift beneath the lake, attributed to an intrusion. Seismicity throughout the catalogue defined an arcuate shape with depth 6±1 km, representing the interaction between the magma system and a ring-fault structure. Ongoing seismicity related to the magma system between the two uplift episodes in 2019 and 2022 indicate that this activity can be considered as one four-year unrest episode. We used ambient noise interferometry to investigate the magma reservoir beneath Taupō. We calculated empirical Green’s functions for broadband station pairs by correlating pre-processed seismic ambient noise; used automated frequency-time analysis to extract surface wave dispersion curves for Rayleigh (ZZ, RR components) and Love (TT components) waves; and calculated mean dispersion curves for station-pair paths that passed beneath different areas. We identified a −20% to −30% surface wave velocity anomaly beneath the northeastern part of the lake for periods 5.5–10 s (∼2–10 km depth), compared to the paths that pass outside the Taupō Rift (2.6 km s−1) that we interpret as the active magma system. This anomaly does not extend into Western Bay or the Taupō Fault Belt. First order estimates of melt percentage (1–23%) from these velocity anomalies are broadly consistent with previous estimates. On average, Love waves are slower than Rayleigh waves beneath the northern part of the lake, indicating possible negative radial anisotropy and vertical internal structure of the magma system. We propose that this signal is the result of the long-term control of the rift on magma pathways. Finally, we discuss the implications of these results on our understanding of the volcano, suggest avenues for future research, and offer some recommendations on monitoring Taupō for future unrest and possible eruption.</p>