25 years recording of the long-base fluid tiltmeter installed at 2.850 m a.s.l. observatory on Etna volcano

Etna is a very active volcano with frequent eruptions and it is well controlled through different monitoring techniques. Continuous control of ground deformation is also provided by tilt measurements especially through a network of short-base borehole sensors. Since 1996 this network has been integrated with a long-base fluid tiltmeter installed in two 80 m long tunnels present at the high altitude (2,850 m a.s.l.) volcanological observatory of Pizzi Deneri (PDN) located on the summit area of the volcano. The instrumentation was created with an innovative configuration composed of mercury, free to move along the entire length of 80 m in response to the ground tilt, and laser sensors to measure the changes of the mercury level at the ends of the length where this fluid is positioned. In this study for the first time we present the entire set of 25 years of data (1997-2022) recorded by this instrumentation. During this long time interval, the Etna volcano was characterized by numerous main eruptions due to dyke intrusions. The tilt variations recorded in the short term during the rapid intrusive phases are presented and discussed. These signals contributed to the modeling of eruptive processes, and in particular in the case of the 2002-2003 eruption they contributed to the real-time understanding of the ongoing eruptive phenomenon, supporting the correct hazard assessment. In the medium-long term (years to decades) the signal maintained a trend that cumulated approximately 200 microrads on both components during the entire observation. PDN long term is primarily affected by the dynamics of the marked sliding of the entire eastern sector of the volcano.