Understanding Etnean flank instability is hampered by uncertainties over its western boundary. Accordingly, we combine soil radon emission, interferometric synthetic aperture radar (InSAR), and electronic distance measurement (EDM) data to study the Ragalna fault system (RFS) on the SW flank of the volcano. Valuable synergy developed between our differing techniques, producing consistent results and serving as a model for other studies of partly obscured active faults. The RFS, limited in its surface expression, is revealed as a complex interlinked structure similar to 14 km long that extends from the edifice base toward the area of summit rifting, possibly linking northeastward to the Pernicana fault system (PFS) to define the unstable sector. Short-term deformation rates on the RFS from InSAR data reach similar to 7 mm yr(-1) in the satellite line of sight on the upslope segment and similar to 5 mm yr(-1) on the prominent central segment. Combining this with EDM data confirms the central segment of the RFS as a dextral transtensive structure, with strike-slip and dip-slip components of similar to 3.4 and similar to 3.7 mm yr(-1), respectively. We measured thoron (Rn-220, half-life 56 s) as well as radon, and probably because of its limited diffusion range, this appears to be a more sensitive but previously unexploited isotope for pinpointing active near-surface faults. Contrasting activity of the PFS and RFS reinforces proposals that the instability they bound is divided into at least three subsectors by intervening faults, while, in section, fault-associated basal detachments also form a nested pattern. Complex temporal and spatial movement interactions are expected between these structural components of the unstable sector.
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