How to detect slow slip and long-term seafloor deformation? Lessons from two acoustic ranging campaigns on the submerged flank of Mt Etna

The Earth’s ocean floor deforms continuously under the influence of tectonic and non-tectonic processes. In the recent decade, the installation of seafloor geodetic instruments to accurately monitor fault displacement and strain accumulation has greatly improved our understanding of seafloor deformation and our knowledge of associated offshore hazards. In particular, the application of acoustic direct-path ranging networks allows the detection of displacement and strain accumulation of faults in millimeter-level precision.

On-land geodetic networks revealed that the Southeast flank of Mount Etna slides seawards at a rate of ~3 cm/yrs. The highest rates are observed near the coast and the volcano flank extends far into the Ionian Sea. The long-term deformation is superimposed by frequent slow-slip events with up to ~3 cm displacement. Our first acoustic ranging measurements between 2016 and 2018 confirmed offshore active deformation and seafloor displacement by detecting a slow-event of up to ~4 cm with a right-lateral offset. Thus, the application of direct-path ranging transponders has proven to be a promising tool to monitor horizontal and vertical displacement of such strike-slip fault zones. However, the observation of long-term deformation, as observed on onshore faults, is lacking. Therefore, we conducted a second acoustic geodetic deployment at the same site offshore Mount Etna between September 2020 and November 2021 and used a different network design. The new data set shows an indication for slow long-term seafloor deformation, which had not been resolved in the first deployment. By comparing the different configurations of the acoustic direct-path networks we were able to improve data processing to achieve millimeter-level precision. We have learned that longer-distance measurements over a sharp fault favor the detection of slow-slip events, but impede the observation of slow long-term deformation. In order to resolve the latter movement, very short baselines close to the fault trace are ideal. Therefore, a trade-off between long and short-distance measurements might be the key for compressive deformation monitoring. Our results prove that the direct-path acoustic ranging technique is well-suited to detect different styles of fault slip at faults with sharp surface traces.