Large ground deformation on Mount Etna volcano

Abstract

AN 11-km traverse of benchmarks for precise levelling across the summit of Mount Etna was established and levelled in July 1975. The traverse runs from a reference benchmark near the Piccolo Rifugio (about 2515 m a.s.l.) in the south, across the summit cone (highest station about 3210 m a.s.l.) and down to the rim of the Cratere Ellittico Caldera near Pizza Deneri (about 2780 m a.s.l.) (Fig. 1). The traverse is re-occupied about twice a year to monitor vertical ground movements associated with volcanic activity, the method being similar to that used on Kilauea Volcano, Hawaii^1–3. A Zeiss Ni2 self-levelling level with micrometer and 3-m invar staff are used and precise levelling techniques adopted where possible. (Previous precise levelling on Etna⁴ was restricted to dry tilt stations at three localities.) Closures on ½-km traverses are better than first order accuracy. Relevelling has so far been carried out in May 1976 and September 1976, and takes about 6 d for the complete traverse. We report here the results of this repeated precise levelling, which show that an area at least 2 km across subsided > 1 m between July 1975 and September 1976. This could indicate high-level removal of magma, or incipient caldera collapse.