Continuous Tilt Monitoring: Lesson Learned from 20 Years Experience at Mt. Etna

In this paper we describe the experiences and results gained in the use of tiltmeters at Mt. Etna during the last twenty years. The tilt data represent a fundamental contribution to understanding of volcanic phenomena. Moreover, the tilt played an important role in analytic modeling of the sources linked to the most important recent lateral eruptions. The tilt data also provided evidence of co-seismic variations for higher energy and shallower seismic events. We also discuss the marked variations that often occur over the entire volcano edifice without being associated with seismicity or eruptive activity. This work also analyses and discusses experiments conducted to verify the limitations of shallow borehole sensors. The main causes of noise are the thermo-elastic effects on the ground and the spurious effects on the sensor due to temperature variations. To this end, results of recent experiments to verify the theoretical filtering of thermal noise at depth and to examine the real thermal coefficients of the sensors through a patented instrument are discussed. Finally, results obtained from signal recordings, applied theoretical studies, experiments performed and technological advances have recently suggested new horizons for tilt monitoring through the development of innovative instrumentation. We describe the application of new technology for the detection of tilt by a long base fluid tiltmeter, installed in 1997, which is able to record very stable, high precision signals with very low noise, thus allowing more detailed investigation of the different phases of volcanic activity.