Landslides in Fiordland, New Zealand, and implications for carbon accounting

Landslides are significant natural hazards, particularly in mountainous active tectonic landscapes with high rainfall, but their environmental damage also affects carbon sequestration and greenhouse gas emissions. Landslides triggered by two Fiordland earthquakes were mapped using Google Earth Pro satellite imagery (eye altitude of 1.5–2.5 km, enabling ± 50 m precision) then translated into an ArcGIS Pro dataset and classified with multiple data attributes. There were at least 1852 landslides (total area = 37.6 km²) during the 2003 Mw 7.2 Secretary Island Earthquake, but only 313 landslides (total area = 3.7 km²) during the larger 2009 Mw 7.8 Dusky Sound Earthquake. The mapped inventory was used to train and test a logistic regression model, accounting for eight parameters including peak ground acceleration, slope angle and aspect. Resulting landslide susceptibility maps for the entire Fiordland region show the probability of landslide occurrence under peak ground acceleration conditions with a 10% in 50-year probability of exceedance. Manually mapped earthquake-induced landslides and equivalents derived from a remotely sensed Global Forest Change dataset showed comparable landslide magnitude–frequency relationships, enabling the punctuated earthquake-related events to be placed in a wider temporal context of continuous decadal-scale rainfall-induced landslides. While rapidly devastating large areas of Fiordland, earthquake-induced landslides only account for 18% of forest loss in the region over the 22-year period from 2001–2022. Much sediment falls directly into fjords, or is transported from landslides by rivers into fjords, then to the open ocean following a multiple-occurrence regional landslide event. Two sets of Landsat 5 imagery were used to map changes in fjord turbidity following the 2003 event. Sediment dynamics within the fjords depend on fjord geometry, proximity to landslides and rivers, and distance from the open ocean. Total carbon loss from earthquake-induced landslides, calculated from the forest and soil within landslide affected polygon areas, amounted to 2.05 Mt for the 2003 M_w 7.2 earthquake and 0.217 Mt for the 2009 M_w 7.8 earthquake. The earthquakes and associated landslides are a major hazard in Fiordland but also significant within the context of New Zealand’s total annual carbon budget.