Magnetic Field Monitoring at Mt. Etna During the Last 20 Years

Abstract

Over the past few decades we have been intensively monitoring the magnetic field on Mt. Etna. The largest anomaly, about 10 nT, was observed in the geo-magnetic time series recorded in 1981 and associated with the March 17-23 eruption. It was interpreted as the joint effect of piezomagnetism and thermal demagnetization engendered by an intrusive dike. A convincing case of thermo-magnetic effects was observed during the 1989 fissure eruption, when repeated measurements at intervals of 3 months for two years revealed the slow buildup of a 130 nT anomaly. The anomaly vanished laterally within 0.2 km from the sur-face expression of the fissure system. The nature and structure of the anomaly is consistent with the location and time of cooling of a shallow dike. Between Sep-tember and December 1995 geomagnetic changes, greater than 8 nT, associated with the renewal of the NE crater's activity were detected. The center of the magnetic anomaly source, which was thought to be the region heated by hightemperature fluids and gases originating from fresh magma, was estimated, by the spatial distribution of the variation rate, to be at a depth of 500 m near the 1991-93 eruptive vents. Finally, significant changes, ranging from 2 to 7 nT, in the local magnetic field closely related to the main phases of the 2001 eruption were observed. Piezomagnetic models were used to calculate the expected geo-magnetic changes for each volcanic process. Model parameters were based on estimated fault geometry using seismic and ground deformation data from each event.