Active Strike-Slip Fault Monitoring Using Marine Geodesy, Offshore Mt Etna, Sicily (Italy)

In the framework of the European Research Council (ERC) funded project – FOCUS, testing laser reflectometry in a fiber optic cable to detect movement across an active submarine fault in real time, an array of eight acoustic beacons has been set up for monitoring motions across the fault and calibrating the observation from the fiber optic cable (see Gutscher et al. abstract in session SM2.1). The two experiments jointly started in October 2020. The selected North-Alfeo Fault is located at the foot of Mount Etna and shows evidence of right-lateral strike-slip motion.

The geodetic array forms a triangular web of 28 baselines, 16 of which cross the fault and 4 of which are parallel to the sections of fiber-optic cable cutting the fault.  Beacon depths range from 1910 to 1806m.  Each baseline, 400 to 1800 m long, is measured 4 times a day in both directions. Additional sensors simultaneously monitor the temperature (at ±0.001˚C) and pressure (at ±0.01dbar), so that sound-speed can be derived, and acoustic ranging (at ±1 microsecond) converted into distances. Inclinometers monitor the stability of the beacons (at ±0.05˚), mounted on 3m-high tripods, which, so far, have remained stable on the seabed.

The collected data (Jan. 2022) show transient and inhomogeneous environmental changes, due to cold bottom-water flows or mixing that last from days to weeks, and hence causing transient changes in the sound-speed and measured acoustic flight-times between beacons. Sound-speed (SSP) varies up to 0.1 m/s, inducing changes up to 25 microsecondes in one-way flight-times (equivalent to a 4-cm displacement at a constant SSP). Unfortunately, such an episode occurred when the optic fiber detected a significant elongation (20 - 40 microstrain) at two fault crossings, between 19 and 21 November 2020. Further processing is underway to extract possible actual displacements from the first 14 months of continuous acoustic ranging.