Submarine landslide megablocks show half of Anak Krakatau island failed on December 22nd, 2018

As demonstrated at Anak Krakatau on December 22nd, 2018, tsunamis generated by volcanicflank collapse are incompletely understood and can be devastating. Here, we present thefirsthigh-resolution characterisation of both subaerial and submarine components of the collapse.Combined Synthetic Aperture Radar data and aerial photographs reveal an extensive sub-aerial failure that bounds pre-event deformation and volcanic products. To the southwest ofthe volcano, bathymetric and seismic reflection data reveal a blocky landslide deposit (0.214± 0.036 km3) emplaced over 1.5 km into the adjacent basin. Ourfindings are consistent withen-masselateral collapse with a volume≥0.175 km3, resolving several ambiguities in previousreconstructions. Post-collapse eruptions produced an additional ~0.3 km3of tephra, buryingthe scar and landslide deposit. The event provides a model for lateral collapse scenarios atother arc-volcanic islands showing that rapid island growth can lead to large-scale failure andthat even faster rebuilding can obscure pre-existing collapse.