From Source to City: Using Ambient Noise to Characterise Shaking from the Alpine Fault, New Zealand.

<p><strong>The Alpine Fault is a major seismic hazard in New Zealand. Yet, there is no empirical data on ground shaking caused by a major Alpine Fault earthquake, as no such event has been recorded instrumentally. Regardless, paleoseismic studies provide an earthquake cycle of ∼300 years, with the last rupture occurring in 1717 AD. This research exploits the ambient seismic field to improve our understanding of ground motion after an Alpine Fault earthquake. The study has three key objectives: characterising the ambient seismic noise field in the South Island, calculating robust Green's functions using ambient noise records, and computing Virtual Earthquakes to simulate future Alpine Fault ruptures. The spatiotemporal characteristics of the ambient noise field are analysed through a Maximum Amplitude Rotation (MAR) analysis and the asymmetry of the coherency-based noise functions, revealing insights into the energy sources and seasonal variations of the secondary microseismic peak. The second microseismic peak has strong sources in the south and east of the South Island. The study evaluates cross-correlation, deconvolution, and coherency methods for extracting Green’s functions from the ambient noise field, concluding that coherency is the most reliable due to its amplitude preservation. These functions are then used in the Virtual Earthquake Approach to generate virtual waveforms simulating buried double-couple sources. Corrections for source depth, radiation pattern, and source duration are applied, and the virtual earthquakes are validated against real seismic records. The methodology is further extended to simulate finite rupture scenarios, integrating a homogeneous slip model with virtual waveforms to estimate long-period ground motion for potential Alpine Fault earthquakes. To achieve all this, we installed the SALSA network, a temporary network of 47 seismic stations along the Alpine Fault trace.</strong></p>