New Perspectives on Long-term Volcanic Phases at Mt. Etna from 25 Years Continuous Borehole Tilt and Vertical GPS Displacements Data

Abstract:

In recent decades, instrumental technology has enabled recording different kinds of data that can detect volcano deformation. Continuous tilt is usually able to detect deformations that are expected to be large (tens to hundreds of microradians) and fast (hours to weeks) for short-term eruptive events. At Mt. Etna, the continuous measurements from shallow borehole tiltmeters have been successfully used to infer the mechanisms of magma accumulation and intrusive processes that foreran and accompanied the several eruptions occurring during the last thirty years on this very active volcano. On the contrary, there are fewer studies on continuous tilt measurements. Indeed, besides problems related to possible instrument failure causing interruptions in the data series, it is generally assumed that a tiltmeter is less stable for a long-term observation than for a short-term and its record could contain real ground deformation, environmental noise, local site effects and instrumental drift. For the first time, we present here 25 years (1990 – 2014) of continuous borehole tilt recorded at Etna volcano at different stations and vertical displacement periodically measured by GPS. To verify the robustness of the two patterns, we compare the borehole tilt series with the dry-tilt deduced from the GPS measurements. We show these long-term series that comprise several main flank eruptions, which we believe are unique in the landscape of instrumental monitoring of geodynamic active areas. We then discuss the coherence of the deformation patterns obtained by the two techniques that provide a new perspective on the long-term behavior of a very active volcano.