Abstract
Coastal and ocean island volcanoes are renowned for having unstable flanks. This can lead to flank deformation on a variety of temporal and spatial scales ranging from slow creep to catastrophic sector collapse. A large section of these unstable flanks is often below sea level, where information on the volcano-tectonic structure and ground deformation is limited. Consequently, kinematic models that attempt to explain measured ground deformation onshore associated with flank instability are poorly constrained in the offshore area. Here, we attempt to determine the locations and the morpho-tectonic structures of the boundaries of the submerged unstable southeastern flank of Mount Etna (Italy). The integration of new marine data (bathymetry, microbathymetry, offshore seismicity, reflection seismic lines) and published marine data (bathymetry, seafloor geodesy, reflection seismic lines) allows identifying the lineament north of Catania Canyon as the southern lateral boundary with a high level of confidence. The northern and the distal (seaward) boundaries are less clear because no microbathymetric or seafloor geodetic data are available. Hypotheses for their locations are presented. Geophysical imaging suggests that the offshore Timpe Fault System is a shallow second-order structure that likely results from extensional deformation within the moving flank. Evidence for active uplift and compression upslope of the amphitheater-shaped depression from seismic data along with subsidence of the onshore Giarre Wedge block observed in ground deformation data leads us to propose that this block is a rotational slump, which moves on top of the large-scale instability. The new shoreline-crossing structural assessment may now inform and improve kinematic models.
Highlights
Hundreds of beams at each side, MBES attached to the hull of a ship covers a strip of the seafloor up to six times the water depth wide
The new MBES data off Etna, which were collected in 2020 have a vertical resolution of
During SO277, MBES data were acquired with hull-mounted Kongsberg EM122 and EM710 systems
Summary
Most volcanoes form in tectonically active areas on top of deformed continental or oceanic basement. To understand flank collapse hazard at volcanoes whose edifices are, to a large extent, covered by water, the offshore geometry and kinematics of the unstable sector needs to be rigorously understood This knowledge can lend constraints to kinematic models that seek to explain observed ground deformation but lack information on the offshore extent, architecture, and geometry of the moving flank. We present new bathymetric and microbathymetric maps from the continental bulge and the transition to the Ionian basin, as well as new offshore seismicity data from an Ocean Bottom Seismometer network installed from April 2016 to February 2017, and as yet unpublished seismic reflection lines These new data, in combination with marine geological, geodetic, and geophysical data published in recent years allow us to determine the location of the southern boundary, as well as to discuss the yet widely unexplored northern and seaward boundaries of the unstable flank. We further identify the main uncertainties and key questions that remain unanswered concerning the geometry of Mount Etna’s unstable sector, and propose pathways to overcome these
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