Auckland was built on top of an active — in geologic terms — volcanic field: The Auckland Volcanic Field (AVF). The field has 50+ volcanoes dispersed over the area.
As a result, GeoNet has instrumented the area with a regional network of seismometers.
Seismologists monitor the AVF with this network, while DEVORA (Determining Volcanic Risk in Auckland) 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 allow us to study the subsurface using surface waves and body waves properties.
The surface wave approach uses 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. To do this properly for the AVF, we first need to determine the orientation of each seismic station.
The body waves approach takes earthquake data from distant events to be used in a tomographic study. To gauge the resolution that we can expect, a synthetic checker-board test is done. The subsequent study then perform the tomographic inversion using data from 100+ teleseismic (far away) events.