Geomorphological evidence for volcano-tectonic deformation along the unstable western flank of Cumbre Vieja Volcano (La Palma)

In 2021, La Palma's southern volcanic complex Cumbre Vieja erupted for its longest period in historic times. Although the geological record shows no evidence for a collapse of Cumbre Vieja, ground deformation studies and field observations suggest that its western flank is moving seawards, following the direction of previous collapses of the island. To better estimate the hazard of a potential flank collapse of Cumbre Vieja, it is important to identify the lateral extent and depth of the mobile sector. Here, we analyse the volcano-tectonic deformation along Cumbre Vieja's western flank, based on geomorphological analysis of combined topographic and new ship-born bathymetric data as well as the analysis of shallow seismicity records associated with the 2021 eruption. In our interpretation, the shoreline-crossing Puerto Naos Ridge results from tectonic uplift accompanying transpressional deformation along the northern boundary of Cumbre Vieja's moving flank, therefore decoupling a stable sector in the north from the mobile sector farther south. The proposed moving sector is consistent in scale with previous ground deformation studies and documented flank collapses of structurally similar volcanoes. We present a workflow for semi-automatically detecting boundaries of unstable volcanic flanks based on morphological changes captured in digital elevation data. The method correctly delineated the known boundaries of the unstable flanks of Mt. Etna and Kilauea volcanoes. The ability to constrain potential boundaries of unstable volcanic flanks should inform the planning of future geophysical and geodetic campaigns aiming to identify precursory signals of potential flank failures.