The Auckland Volcanic Field (AVF) is uniquely different from the well-known volcanoes in New Zealand in that each eruption occurs from a new location and the volcanoes do not erupt twice; Rangitoto is the only known exception, as it has erupted at least twice. Globally very few eruptions have occurred historically from volcanic fields so we do not have a lot of knowledge of what happens before an eruption. Traditionally, monitoring an active volcano is based around recording seismic signals, ground deformation and gas or water chemistry. When we do not know where the next vent might be we are left with just one method to use; earthquake recording.
The DEVORA research project has been looking at many aspects of volcanic activity in Auckland, including possible eruption precursors. This has involved looking for historical examples of similar eruptions elsewhere in the world. Was there volcanic unrest? How did it manifest? One of the common unrest indicators is earthquake activity. Another clue can be found in the rocks that the volcano has erupted. Many signals are preserved in the crystals and they tell us about the journey the rocks have been on. From this work, we know the Auckland magmas start out from great depth (80-100 km). We also know that they rise relatively fast and don’t appear to stall on the way. What is not clear is what signals these processes will make. Will they make many or just a few earthquakes? How big will the earthquakes be? What type of earthquake will they be? Will there be volcanic tremor?
Based on these challenges GeoNet has had to devise the monitoring in Auckland around a seismic network that covers all the known locations of volcanoes. The Auckland Volcanic Field spans about 27 km north-south and 19 km east-west and underlies a major city. There is no geothermal system and we will not see any ground deformation until shortly before the eruption starts. The greatest issue is cultural noise (the city) and this stops us seeing small earthquakes. This is overcome by using borehole sensors. Auckland Regional Council (ARC) started building a near-real time seismic network in 1993 with NZ Geological Survey and this was operational from the mid 1990’s. The network was connected to the GeoNet project in 2003. At that time, there were 5 seismic sensors in the Auckland area, four of which were in boreholes.
Starting in 2006 GeoNet upgraded the network by adding 3-component sensors and then by adding more borehole sensors and increasing the area covered. Today the seismic sensors in Auckland are fully integrated into GeoNet. We operate seven 3-component borehole sensors, 1 single component borehole site, three 3-component sites and 4 strong motion sites. Since 1994 we have recorded and located 372 earthquakes in the greater Auckland area, about 16 a year. None of these appear to be related to volcanic processes.