Volcanomagnetic effects at Vulcano Island (Aeolian archipelago, Italy)

The installation of two continuously operating proton magnetometers, 2 km a part and sampling simultaneously every 5 min, on Vulcano Island from 1990 N ovember to 1991 December allowed the detection of some transient anomalies thought to result from internal variations in the physical state of the vol cano. The difference of mean values in the magnetic held at the two station s underwent a slow variation of 2 nT during the period of measurements. The mean values in the total intensity at one station were inversely correlate d to the difference between the mean values of the two stations. This sugge sts that the contribution to the signal generated by the transitory magneti c 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 des cription of the magnetic field, a method of analysis was developed to disti nguish between transients of volcanomagnetic origin and transients generate d by strong variations in the external transitory magnetic field. An accura te analysis, which takes into account the correlation between the measureme nts at the two stations, leads to the singling out of three magnetic anomal ies (on 1991 April 2, July 18 and December 23). Comparison of the experimen tal data with the list of earthquakes occurring at Vulcano during 1991 show ed the close temporal correspondence between the magnetic anomalies and ear thquakes. The amplitude (between 2 and 4 nT), spatial extent (approximately 2 km) and polarity of anomalies point to the piezomagnetic effect as the p rimary physical mechanism driving these transient changes. On the other han d, 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 spheric al 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.