Characterising Seismicity at Wairakei-Tauhara Geothermal Field Using a Dense Nodal Array

<p><strong>The high-temperature Wairakei-Tauhara geothermal system is located in the Taupō Volcanic Zone in the North Island of New Zealand, and is used to generate electricity at five power stations. Microseismicity is common in geothermal systems, and precisely locating these earthquake events helps to characterise both the subsurface architecture and fluid processes of these systems. However, it is more challenging to detect and locate low-magnitude earthquakes due to the smaller amplitudes of their waveforms. In this thesis, we use an augmented seismic network to construct an earthquake catalogue for Wairakei-Tauhara, then leverage these data to improve the understanding of the geothermal system. We obtained 81 days of seismic data at Wairakei-Tauhara and the surrounding areas using the Wairakei Earthquake and Tomography Array (WĒTĀ), a high-density array of 141 seismic nodes. With WĒTĀ and the permanent GeoNet network data combined, we detected earthquake phase arrival times with a machine learning-based approach, located earthquakes with non-linear and relative relocation methods, and calculated a local magnitude scale and focal mechanism solutions to characterise seismicity. With absolute and cross-correlation-derived phase arrival times, we located 881 and 613 earthquakes, respectively, spanning a local magnitude range of -0.5 to 3.3. Our catalogue reveals that earthquakes predominantly occur within the Wairakei, Rotokawa, and Ngatamariki geothermal systems, and are clustered around production and injection fields. We used the distribution of hypocentres to identify faults and fracture networks, many of which correlate with pre-existing modelled fault locations. Focal mechanism solutions reveal predominantly normal and strike-slip mechanisms broadly aligning with the regional tectonic regime, and the temporal analysis indicates that much of the seismicity in these geothermal systems occur in swarms. We interpret that fluid-assisted faulting and fracturing, thermal contraction, and stress transfer may be prevalent source mechanisms for seismicity in these systems. We also use the high-density array to analyse ground shaking variations across the study area, and find that areas close to Lake Taupō, northwest of Wairakei, and Rotokawa were more prone to large amplitude ground shaking. We discuss the benefits and challenges of large-N arrays in geothermal settings and offer recommendations for future nodal deployments.</strong></p>