The Evolution of the State of Mt. Etna Volcano in the last ten Years Inferred Through Multidisciplinary Investigations

In the last 30 years the eruptive activity of Mt. Etna has occurred near annually. After the 1991-1993 eruption, the most important lateral eruption in the last three centuries both in terms of duration (473 days) and lava erupted (ca. 235.106 m3), the volcano has not shown its activity through lateral eruptions but, since 1995, through frequent summit eruptive phenomena as vigorous strombolian activity, more than one hundred spectacular lava fountains, and lava flows from the summit craters. The last lateral July-August 2001 eruption occurred after this intense summit activity about ten years later the previous 1991-1993 lateral eruption. In this work we present multidisciplinary investigations conducted during these ten years preceding the 2001 eruption. The different disciplines and monitoring techniques (volcanological observations, seismology, ground deformation, petrology, gravimetry, geochemistry, geomagnetism) allowed us to obtain a knowledge of the state of the volcano and to evaluate its evolution. The positive gravity anomaly, the ground deformation inflation, the seismicity pattern and the SO2 flux indicate that from 1994 an intrusive process interested a crust volume at 2-5 km b.s.l. under the summit crater area. This volume probably represents an intermediate storage zone. From the second half of 1996 to 1997 the recharge process of the volcanic system was particularly intense, as evidenced by the maximum value of the gravity anomaly, the continuous increasing of the areal dilatation, the increasing of the cumulative seismic strain release pattern, the increasing of the SO2 mean flux. The chronology of these phenomena, together with a strong seismic swarm occurred in the western flank on January 1998, suggested magma migration towards the upper part of the plumbing system. This is confirmed by the composition of the erupted products from the summit craters that became progressively more primitive. Also previous eruptions, such as the 1989 and 1991-1993, showed analogous phenomenal pattern culminating with the eruptive phases. From 1998 the intensification of the summit craters activity, which indicated re-alimentation and strong tension accumulation, has been preceded by a decrease of the gravity anomaly and accompanied by a decrease of the SO2 mean flux, and by a partial attenuation of the areal dilatation and of the cumulative seismic strain release trends. Moreover, a slow and continuous increase in the total geomagnetic field observed on the north flank of the volcano and the absence of significant anomalies at the south supported a possible demagnetisation in the summit area. Different observations indicated that the tension accumulation did not appear equilibrated by the energy discharge occurred through the lava flow summit eruptions (February-November 1999; January-June 2001) and through the several tens of strong explosive events which took place at the summit craters during the 1998-2001 before the July 2001 eruption.