Carbon Loss from Earthquake Induced Landslides in Fiordland

Abstract

Fiordland is a carbon sink, and the influence of landslide events on carbon transport and sequestration in the region needs to be understood. Landslides triggered by two earthquakes in Fiordland were mapped using Google Earth Pro satellite imagery (eye altitude of 1.5–2.5 km, enabling ± 50 m precision). Ground acceleration during the 2003 M_w 7.2 Secretary Island Earthquake resulted in at least 1852 landslides. The larger 2009 M_w 7.8 Dusky Sound Earthquake had lower accelerations, producing only 313. Both events dislodged large swaths of native forest, grassland vegetation and soil, some landing in rivers and fjords.  The landslide maps and magnitude–frequency distributions show close similarity to a published Global Forest Loss dataset derived from 2001–2022 satellite imagery. Assuming forest loss here is predominantly landslide related, it enables the more-precise earthquake-induced landslide mapping to be placed in a longer-term context of other vegetation loss, mostly rainfall-induced landslides, during the past two decades. The total area of forest loss during the 2003 earthquake was anomalous, whereas during 2009 the earthquake losses were similar to areas of forest loss assumed to be rainfall-induced landslides throughout Fiordland each year. Carbon concentrations of landslide vegetation were calculated by defining vegetation types from a published Land Cover Database, and soil organic carbon concentration from a 1 km resolution raster dataset. Total carbon loss from earthquake-induced landslides amounts to 2.05 Mt for the 2003 M_w7.2 and 0.217 Mt for the 2009 M_w 7.8. By way of comparison, New Zealand’s total annual carbon sequestration was 6.3 Mt in 2020, and CO₂ emissions were 9.2 MtC. Multiple-occurrence regional landslide events can account for changes carbon storage and sequestration in areas of dense vegetation, such as Fiordland. Processes of landscape disturbance are significant for carbon accounting and could be included in estimates of national greenhouse gas emissions.