Auckland may be the only metropolis built on top of an active — in geologic terms — volcanic field: The Auckland Volcanic Field (AVF). This was recognized a long time ago, and GeoNet has instrumented the area with a regional network of seismometers, regional network name AK.
GeoNet’s seismologists carefully monitor the AVF with this network, while DEVORA (Determining Volcanic Risk in Auckland) is a project that studies the geologic (including seismic) hazard and risk associated with the volcanic field. Seismic activity in the area is actually relatively low, but recent advances in seismology make it possible to use the noise of the city and the surrounding oceans to extract information between seismic stations as if one of the seismic stations were the source (the “earthquake”) and the other the recorder. Here is an example of the correlation of 5 days of noise between stations WTAZ and MBAZ. The positive times are seismic surface waves going from WTAZ to MBAZ, whereas the negative times would capture seismic waves going in the opposite direction. The reason you see a distinct wave on the positive side of this plot, but not on the negative side is that the predominant noise (from these 5 days) is from the West. It also appears that the earlier part of the wave is lower in frequency than the higher parts. This is called dispersion and stems from the fact that longer-wavelength surface waves travel through deeper parts of the Earth, which are generally faster. This information can be used to extract the seismic velocity structure in the ground between these two stations.
MSNoise.org provides a suite of tools in Python to calculate the wave fields of these “virtual earthquakes” between each combination of station pairs of the AK network. We repeat this procedure each day, and look for small differences in the calculated waveforms to inform ourselves of possible changes in the subsurface. For example, on Mount Merapi, Indonesia, changes in the waveforms correlate with changes in precipitation saturating the near-surface. However, in principle magmatic changes could be detected this way. Areas of active research include how to do the calculations so that we can exclude causes for changes in the waveforms not related to the Earth. For example, the distribution of noise sources may vary from day to day, affecting the calculated “virtual earthquakes.”