Low frequency tremor Monitoring of ground deformation Background monitoring Geodetic measurements are used to monitor changes in the shape of the ground-surface caused by the movement of magma towards the surface. Lake levelling is conducted in one of the lakes on Raoul Island, and at lakes Tarawera, and Taupo. At White Island a precise levelling network was established in 1966 on the floor of Main Crater to measure changes in crater height. During regular visits to Ruapehu's Crater Lake samples of water are collected. Changes in groundwater, lake levels, rates of stream flow and the temperatures of such waters often give evidence of unrest within a volcano.
Crater lakes in particular are valuable indicators of the status of volcanic systems.
Other monitoring includes weekly measurements of the levels and temperature of Blue and Green lakes at Raoul Island, along with regular ground temperature measurements and water sampling. Within the Okataina Volcanic Centre, heat flow from the large crater lakes at Waimangu has been continually monitored since 1970 by recording the temperature, level and rate of discharge of the lakes.