Two scales of monitoring are required for active volcanoes. The first is background monitoring between crises using a limited number of fixed instruments or sampling points. The key here is to strike a balance between cost and the need for accurate and reliable data. The second scale is the monitoring during a crisis. This can require a much greater deployment of monitoring equipment and personnel, but only for a finite time.
Earthquakes commonly provide the first indication of volcanic unrest.
There are five volcano seismic networks in operation around New Zealand's volcanoes. Three are operated by GNS in the Taupo Volcanic Zone. The volcano-seismic networks at Taranaki and Auckland are operated by their respective regional councils. Raoul Island, New Zealand's most northern and one of the most active volcanoes, also has a seismograph on it. Volcanoes produce two types of seismic signature, the first are called 'volcanic earthquakes', the other is 'volcanic tremor'.