WHEN A TROPICAL STORM GOES TO BATTLE WITH AN ASEISMIC RIDGE: LANDSLIDE EROSION CAUSED BY CYCLONE HAROLD, APRIL 2020, ESPIRITU SANTO, VANUATU

Cyclones can trigger floods and landslides and cause geomorphically significant erosion when making landfall over high oceanic islands. Event-driven sediment and organic carbon transport can exceed background rates by several orders of magnitude in topographically conducive settings. The South Pacific nation of Vanuatu is a compelling location for evaluating the coupling and feedback between atmospheric, geomorphic, and tectonics processes. The forearc island of Espiritu Santo is situated above the New Hebrides Subduction Zone that accommodates subduction of the Australian Plate beneath the New Hebrides Microplate. Subduction of the aseismic d’Entrecasteaux ridge system beneath central Espiritu Santo results in increased surface and rock uplift rates. The subduction of this aseismic ridge is expressed topographically, with the island’s highest peaks (e.g., Mount Tabwemasana; 1879 m) in line with the incoming ridge system. Here streams show a marked increase in the normalized channel steepness index (ksn), and maps of the transformed stream distance variable (χ) indicate that the trench-parallel drainage divide is likely migrating eastward.

We integrate pre-and-post event satellite imagery, vegetation change metrics (NDVI), and the HazMapper Google Earth Engine application to create an inventory of landslides triggered by the landfall of Cyclone Harold (Cat-4) in April 2020 on Espiritu Santo. The inventory covers two catchments, one on either side of the island’s topographic divide and located within the region experiencing differential uplift from ridge subduction. The inventory is used to evaluate the land-to-ocean transfer of sediment and terrestrial organic carbon resulting from this storm. Within the two catchments, 3,777 landslides were mapped with a combined surface area of 3.95 km² and an estimated volume of 3.85 ± 1.85 Mm³ (>500,000 dump-truck loads!), equivalent to rock erosion of 32 mm in the west and 13 mm in the east basin when normalized by basin area. The asymmetry in calculated erosion rates is consistent with faster uplift and erosion of the west-facing side of the island. Compared to topographic-relief-derived estimates, Cyclone Harold resulted in 100 to 180 years worth of annual erosion from the east and west basins, respectively. Importantly, we estimate that the cyclone-triggered landslides from the two study basins will contribute 229,000 ± 57,000 Mg of terrestrial (forest biomass and soil-bound) carbon to the coastal ocean.