Fig. 1: Maps of the study area location.

A Map of the Sunda Trench, showing the position of Anak Krakatau at an inflection point in the subduction of the Indo-Australian plate beneath Eurasia. B Sentinel-2 true-colour image (15/05/2018) of Anak Krakatau pre-collapse overlain with the distribution of effusive lavas between June 09/06/2018 and the flank collapse, based upon Sentinel-2 false-colour images⁵ (Supplementary 4). New lavas exclusively accumulated on the SW flank within the limits of the new redefined failure plane. C Sentinel-2 true-colour image (15/05/2018) of Anak Krakatau pre-collapse overlain with distributions of Strombolian ejecta between 09/06/2018 and the flank collapse, based upon Sentinel-2 false-colour images⁵. Strombolian ejecta are restricted to within the limits of the failure plane. D Sentinel-2 true-colour image (15/05/2018) of Anak Krakatau pre-collapse overlain movement of the crater from 2008 until the flank collapse, based upon Landsat-8 and Sentinel-2 true-colour images⁵ (Supplementary 3). The summit began as two vents in 2008, migrated to a northern location by 2011 where it remained until migrating southwards from July 2018. Location of (D) from inset on (B). E Sentinel-2 true-colour image (15/05/2018) of Anak Krakatau pre-collapse overlain with (i) position of faulting on the northern flank noticed from 07/07/2018 until the flank collapse and (ii) potential fumarole activity in this time, based upon Sentinel-2 true-colour images⁵ (Supplementary 3). Numbered faults dated: (1) 07/07/18; (2) 09/07/18; (3) 01/08/18; (4) 18/08/8; (5) 12/10/18; (6) 06/11/18; (7) 16/11/18; and (8) 26/11/18. Faults appeared on the northern flank of Anak Krakatau from 07/07/2018 with fumarole deposits appearing from 27/07/2018 and aligning to the eventual trace of the failure plain. Location of (E) from inset on (C). B–E show the failure plane interpretations from previous published works (e.g. fine-dashed line, Williams et al., 2019; coarse-dashed line, Walter et al., 2019) and this study (solid line).