Temporal Velocity Variations Accompanying Geodetically Detected Deformation at Taupō Supervolcano
Authors:   , Schuler Jessica 
Published: June 2023

<p><b>Ambient noise interferometry is a tool for measuring relative seismic velocity on and around volcanoes. Changes in seismic velocity have been shown to be sensitive to magmatic movement and subsurface pressure increases, therefore it has potential as a tool for volcano monitoring. However, velocity changes can also be sensitive to processes not linked to volcanism, such as weather and tectonic events. </b></p>
<p>Taupō volcano, New Zealand, has produced two of Earth’s largest eruptions in the past 70,000 years, making it one of New Zealand’s most hazardous volcanoes. However, Taupō’s periods of volcanic unrest are not well understood, and many techniques for monitoring the volcano are impeded by the overlying Lake Taupō. Monitoring and understanding the modern behaviour and structure of the Taupō volcanic system is key to helping inform hazard mitigation efforts and emergency response planning in the event of an eruption. This study aims to assess if temporal velocity changes are a feasible monitoring method at Taupō, and to establish what processes unrelated to the volcano can also generate significant changes in seismic velocity. We used ambient noise to measure subsurface seismic velocity variations beneath Taupō over the past seven years, focusing particularly on periods of geodetically detected deformation. Through this we determined whether measurable seismic velocity variations were correlated with volcanic unrest and deformation. </p>
<p>We identified an increase of velocity correlating with inflation beneath Taupō for both the 2019 unrest and beginning of the 2022 unrest, and once significant seismicity began velocity subsequently decreased. We propose that this reflects a volumetric compressional strain imposed upon the subsurface by an inflating source, forcing cracks to close until the pressurisation became too great, forming new cracks. In addition to volcanic sources, tectonic earthquakes were also found to induce observable velocity changes. We consider strong ground shaking a likely candidate for velocity decreases associated with distal earthquakes, while local earthquakes are instead associated with static stress changes inducing both velocity increases and decreases. Finally, a seasonal velocity trend was observed, whereby velocity is higher in the summertime when the lake level is at its highest. We propose that this is due to water diffusion raising pore pressures and reducing the area of grain contact. </p>
<p>The findings of this thesis highlight the potential benefits of using ambient noise monitoring to track changes in seismic velocity at Taupō. This approach can complement other monitoring methods and enable timely warnings to be issued to the public in the event of changes in volcanic activity. However, as we have found that non-volcanic signals can produce significant velocity changes, it is necessary to also be able to reliably detect and characterise non-volcanic sources as well as volcanic sources. Due to the sparse broadband network surrounding Taupō, the GeoNet seismic network would need to undergo improvements to make this feasible. Moreover, incorporating additional techniques like single-station velocity changes could be advantageous.</p>