Time variant analysis of geomagnetic signals describes the volcanic activity

Volcanomagnetic anomalies have been mostly observed during strong eruptions. Our aim is to improve the geomagnetic data analysis to
evidence the anomalies occurring in a larger time span, especially in the phases preceding the eruptive events. We developed a time variant
statistical approach and applied it to the 20002002 Etna geomagnetic temporal series. It is based on an algorithm that statistically predicts the
geomagnetic field at the station on the volcanic edifice by that recorded at the remote one. In such a way a number of significant changes in the
time series (called statistical innovations), marking the local magnetic field change, were detected. The distribution of such statistical innovations
accurately describes the Etna volcanic evolution: we note a progressive increase of the innovation occurrence as the eruptive cycles were
approaching and only few and weak innovations at times between the various eruptive cycles. The significance of this analysis is further
confirmed by the close agreement among the mean square prediction error, strain release and the volcanic activity behavior. On the contrary, the
geomagnetic field at a single station or its difference at two stations do not have any clear correlation with other measured physical quantities. The
complex pattern of the prediction error was also investigated by a multifractal analysis. We found that the Holder regularity increases with the
intensification of the volcanic activity, implying that innovations tend to be less sporadic and correlated during the major volcanic phases.