Thermo-magnetic measurements as a possible tool in the prediction of volcanic activity in the volcanoes of the Rabaul Caldera, Papua New Guinea

Volcano-magnetic effects provide a possible way of monitoring temperature or stress changes within a volcano. A systematic study of several volcanoes in and close to the Rabaul caldera in east New Britain, Papua New Guinea, has been made to identify those in which thermal changes would be expected to cause changes in magnetic field. Two out of nine were chosen as suitable for a programme of magnetic monitoring which commenced in February 1974 and is continued at approximately six monthly intervals. The main criterion used for selection is that the volcano must have a magnetic structure causing a strong anomaly in magnetic field with its source located where its temperature would respond to changing conditions in the volcano. In the first three years of monitoring, no thermomagnetic effects have been observed, but the ongoing programme may be a useful tool in the prediction of volcanic eruption. Estimation of the magnitude of possible field changes depends on determining a satisfactory model of the source of the magnetic anomaly and how the remanent magnetization of the source depends on temperature. The former is a standard problem in geomagnetism and may be solved by computer modelling methods. Measurements of the temperature dependence of thermo-remanent magnetization (TRM) for rocks collected in the Rabaul caldera show the latter to be a much more difficult problem to solve in detail. This is because the temperature range in which TRM changes most rapidly with temperature varies greatly from specimen to specimen, and depends both on the magnetic minerals present and the cooling history of the rock. Rocks from within the anomaly sources were not accessible so doubt remains as to how similar their magnetic properties are to those of rocks collected from the surface. Within this limitation, it is shown that for at least one of the volcanoes studied, changes of total magnetic field of several hundred nanoteslas (1 nT = 1γ) are possible prior to a future eruption.