Detecting the effects of rapid tectonically induced subsidence onMayotte Island since 2018 on beach and reef morphology, and implicationsfor coastal vulnerability to marine flooding
Résumé
Subsidence is a widespread phenomenon on the world's coasts. Mayotte, a coral reef-fringed archipelago in the SW Indian Ocean, experienced, in 2018 and 2019, 32 earthquakes with a recorded magnitude > 5 sourced by magmatic extraction from a deep reservoir 40-60 km east of the archipelago. This crisis resulted in island subsidence of up to 0.2 m. Profile measurements of three beaches in 2019-2021, referenced to benchmarks adjusted for subsidence, did not show any notable morphological change when compared to earlier profiles obtained in 2006/2008. This suggests that the rapid but limited subsidence has not significantly affected these systems, excluding eventual ecological repercussions that are not investigated here. Profile changes reflect seasonal variations in monsoon and trade-wind wave energy, and additionally the effect of local mild terrigenous sediment inputs. The sea-level rise caused by subsidence is, however, leading to more frequent spring high-tide flooding of some low back-beach areas and roads on the densely populated northeastern shores of the archipelago, the zone most affected by this tectonic movement. The case of Mayotte is interesting inasmuch as the subsidence caused by a distant submarine volcanic event has not been abrupt as in tectonically active areas nor continuous as in subsidence related, for instance, to glacio-isostatic readjustment, to continuous natural sediment compaction, or to anthropogenic loading. The crisis has been relatively quiescent since July 2021. Continuous monitoring will be needed to see how subsidence will eventually further affect the beach-reef flat systems of Mayotte.