Fig. 2: Evidence of past flank failures at Anak Krakatau and the influence of landsliding on the submarine slopes.

A Pre-collapse topographic contour map (10 m contours) of Anak Krakatau in 1941 (based upon Seibold & Seibold, 1996). The 1941 pre-collapse topography is overlain by the projected 1949 failure plane based upon the cliff top of the post-collapse island topography (Fig. 9B) and 1950 aerial photographs (Fig. 9C). B Topographic contour map (10 m contours) of Anak Krakatau in 1950 (based upon Decker and Hadikusumo, 1960). C Aerial photograph of Anak Krakatau taken in 1950 following a May 1949 collapse of the SW flank (Image credit: Militaire Luchtvaart and Royal Netherlands Air Force (1950) provided courtesy of pilot Ben Huiskamp); D Aerial still-image taken from drone footage of Anak Kraktau on 26/05/2019 after the 2018 flank collapse (Image credit: Indonesian Nature Film Society). The figure shows the similar failure geometry between collapses in 2018 and 1949, and the similar post-collapse recoveries. E Slope profiles of the SW flank of Anak Krakatau comparing our proposed shallow failure scenario with new 2019 post-collapse bathymetry, pre-collapse 1990 bathymetry and older slope profiles. These profiles show the recession in the slope at a water depth of −100 to −120 m resulting from the 2018 flank collapse. Failure aligns to the position of the position of the past Anak Krakatau tuff ring crater and the scarp of the 1883 caldera margin; suggesting the pre-existing caldera wall may provide a structure to influence flank failures.