Volcanomagnetic effects at Vulcano Island (Aeolian archipelago, Italy)

The installation of two continuously operating proton magnetometers, 2 km apart and sampling simultaneously every 5 min, on Vulcano Island from 1990 November to 1991 December allowed the detection of some transient anomalies thought to result from internal variations in the physical state of the volcano. The difference of mean values in the magnetic field at the two stations underwent a slow variation of 2 nT during the period of measurements. The mean values in the total intensity at one station were inversely correlated to the difference between the mean values of the two stations. This suggests that the contribution to the signal generated by the transitory magnetic field may be broken down into two parts: one of external origin, which is identical for both stations, and another of internal origin, proportional to the former, but dependent on the site. Following on from this simple description of the magnetic field, a method of analysis was developed to distinguish between transients of volcanomagnetic origin and transients generated by strong variations in the external transitory magnetic field. An accurate analysis, which takes into account the correlation between the measurements at the two stations, leads to the singling out of three magnetic anomalies (on 1991 April 2, July 18 and December 23). Comparison of the experimental data with the list of earthquakes occurring at Vulcano during 1991 showed the close temporal correspondence between the magnetic anomalies and earthquakes. The amplitude (between 2 and 4 nT), spatial extent (approximately 2 km) and polarity of anomalies point to the piezomagnetic effect as the primary physical mechanism driving these transient changes. On the other hand, the anomaly with respect to the regional field, observed over one year, is explicable in terms of a thermomagnetic effect detected at both stations. The magnetic static anomaly produced by a completely demagnetized spherical source, of 250 m radius located under the La Fossa crater at a depth of 1300 m, is in agreement with the slow changes observed at Vulcano.