Large explosive eruptions can bury landscapes beneath thick layers of tephra. Rivers subsequently overloaded with excess pyroclastic sediments have some of the highest reported specific sediment yields. Much less is known about how hillslopes respond to tephra loads. Here, we report a pulsed and distinctly delayed increase in landslide activity following the eruptions of the Chaitén (2008) and Puyehue–Cordón Caulle (2011) volcanoes in southern Chile. Remote-sensing data reveal that landslides clustered in densely forested hillslopes mostly two to six years after being covered by tephra. This lagged instability is consistent with a gradual loss of shear strength of decaying tree roots in areas of high tephra loads. Surrounding areas with comparable topography, forest cover, rainfall and lithology maintained landslide rates roughly ten times lower. The landslides eroded the landscape by up to 4.8 mm on average within 30 km of both volcanoes, mobilizing up to 1.6 MtC at rates of about 265 tC km–2 yr–1. We suggest that these yields may reinforce the elevated river loads of sediment and organic carbon in the decade after the eruptions. We recommend that studies of post-eruptive mass fluxes and hazards include lagged landslide responses of tephra-covered forested hillslopes, to avoid substantial underestimates.
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The data that support the findings of this study are available from the corresponding author upon request. We used the publicly available Landsat and Sentinel-2 satellite imagery (https://earthexplorer.usgs.gov/ and https://sentinel.esa.int/) and Space Shuttle Radar Topgraphy Mission (https://earthexplorer.usgs.gov/) data to map the landslides. Our rainfall analysis draws on Chilean station data (http://www.cr2.cl/datos-de-precipitacion/). The Global Forest Inventory is available at http://earthenginepartners.appspot.com/science-2013-global-forest/download_v1.5.html.
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This work was partly funded by the projects CONICYT-BMBF PCCI20130045 and BMBF 01DN13060, courtesy of the German Federal Ministry of Education and Research. We thank A. Iroumé and E. Parra for help with logistics, and appreciate the support of E. Gonzalez and the rangers at Pumalín National Park, as well as the Dirección General Aeronautica Civil for permitting UAV flights. We thank J. J. Major for comments on the manuscript. We ran all computations using the statistical environment R (www.r-project.org).
Supplementary Figures, Tables and Discussion