Geophysical imaging of the Paeroa Fault for improved earthquake modelling

The Paeroa Fault is one of the most significant active faults in the Taupō Rift and poses a substantial seismic hazard to central New Zealand. Current national hazard assessments simplify the fault’s subsurface geometry due to limited data, particularly regarding its dip angle and the connectivity of multiple surface fault strands. This uncertainty directly affects estimates of the fault’s maximum earthquake size and therefore influences building standards, infrastructure planning, and emergency management. This project aimed to reduce this uncertainty by testing whether a Dense Nodal Seismic Array (DNSA), combined with ambient noise tomography, could image the shallow subsurface structure of the Paeroa Fault. A network of 147 nodal seismometers was deployed across an 8 by 10 km area at the northern end of the fault for approximately one month, recording continuous seismic data. Ambient noise generated by natural and anthropogenic sources was analysed to derive shear-wave velocity models of the upper ~1 km of the crust. The seismic data quality was high, with clear and symmetric wave propagation indicating favourable conditions for ambient noise analysis.

Velocity models derived from the data show a broadly layered structure with increasing velocity at depth, along with several localised velocity anomalies. However, these anomalies do not clearly align with the mapped surface trace or expected orientation of the Paeroa Fault. As a result, the preliminary velocity model does not yet provide strong constraints on fault dip or subsurface fault geometry. Despite this, the project successfully demonstrates that DNSA deployments and ambient noise tomography can be applied effectively in the Taupō Rift, a geologically complex volcanic and geothermal environment. The study provided valuable experience in deploying nodal seismic networks, and established data processing workflows that can be applied in future projects to speed up data analysis and interpretation.  Overall, while the immediate goal of constraining the Paeroa Fault’s geometry has not yet been achieved, the project provides valuable proof of concept and lays the groundwork for future studies. These methods show strong potential for improving subsurface imaging of faults, and volcanic and geothermal systems in New Zealand, with important long-term implications for seismic hazard assessment and risk reduction.