Fig. 4: Pre- and post-collapse bathymetric maps of the seafloor surrounding Anak Krakatau.

A Raster surface of the pre-collapse bathymetry collected in 1990⁹, showing the location of seismic reflection profiles collected pre-collapse in 2017. B Raster surface of the post-collapse bathymetry collected in 2019 showing the presence of a large landslide mass extending up to 1.5 km into the caldera basin. Also shown in Fig. 3B are the seismic reflection profiles collected post-collapse in 2019. C 3D rendering of 2019 bathymetry raster with hill-shade showing the large, blocky landslide deposit at the base of slope of Anak Krakatau’s SW flank. Numbered arrows indicate similar features between the datasets: (1) base of SW flank, no landslide debris in 1990 (Fig. 3A), but a blocky landslide deposit in 2019 (Fig. 3B); (2) older block or intrusion adjacent to Sertung Island; (3) older landslide blocks N of Rakata Island showing burial and gullying from erosion; (4) erosional channel cutting the SW corner of the 1883 Krakatau caldera.