# Mapping Recent Shoreline Changes Spanning the Lateral Collapse of Anak Krakatau Volcano, Indonesia
## 4 Results and Discussion
Our drone data, acquired in August 2019, indicate that the lake level has remained constant and at the same level as the surrounding sea, despite the rapid evaporation from the steaming lake surface due to input of magmatic heat via the subsurface hydrothermal system. This implies an inflow of sea water to balance evaporation via an efficient hydraulic connection through intergranular and fracture permeability in the tephras and rocks forming the island and its below-sea-level substructure. Therefore, the change in the surface area of the lake through time after its formation implies a change in the shape of its bed or alternatively larger-scale subsidence and uplift of the entire island. The latter seems unlikely because it would produce marked changes in the position of the sheltered north-east coastline of the island, as this has a relatively gentle slope and perhaps also of the largely bare lava coast on the north-west side depending on the shoreline gradient in that area. We therefore consider it most likely that the change in the size and shape of the lake reflects (i) initial infilling as loose tephra is eroded from the crater walls (hence the continuous exposure of older deposits that has developed) and deposited on the lake shore; (ii) subsequent depression or subsidence of the lake bed alone. This could in principle be due to a significant amount of excavation by further explosions, which have indeed occurred but have been mild; but most of the tephra from these small explosions would be deposited in and around the lake and therefore be prone to be transported back to the shoreline to continue infilling of the lake basin. Another possibility, that tephra beneath the lake has contracted due to compaction, is potentially problematic because this would tend to reduce its permeability, although since the porosity-permeability relationship is not known this cannot be excluded at present because the permeability may still be high enough to allow inflow of seawater to balance the evaporation from the lake surface. A final explanation is that as the activity has declined through 2019, the conduit at greater depths beneath the lake has contracted and so there has been localized subsidence in the area of the lake only, rather than affecting the entire island, which allows the lake area to increase without also causing coastline retreat. Spatially, during the May 2018-November 2019 period the shoreline change has followed a distinct compartmentalization pattern, with the shoreline prograding/advancing up to 300 m offshore in the northeastern and southeastern sectors and retreating inland up to 230 m on the western flank (Figure 6). The westward advancement of the shoreline is particularly evident in the May 2018-November 2018 period (Figure 6a), mostly due to the deposition of volcanic materials on this sector that, despite being concentrated in the July 2018-September 2018 period (Figure 6b), does contribute to increase the size of the island only from August 2018. The eastward advancement pattern seems to have dominated between January 2019 until July 2019 (Figure 6c) followed by a slight but consistent decrease in area from that time (Figure 6d). In particular the shoreline retreat on the SW sector begins as early as April 2019. Overall, shorelines in the northern and southern sectors in the pre-collapse period are irregular, as a result of the lobate form and steep fronts of lava flow margins that have extended the coastlines in this part of the island since the 1960s. This irregular coastline contrasts with the eastern sector, which exhibits a gentler slope angle, since it has been constructed by the relatively uniform deposition of tephra built up from phreatomagmatic activity between 1927 and the 1960s and subsequently mantled by Strombolian pyroclastic deposits. In the post-collapse period, regrowth via phreatomagmatic ash deposition has resulted in a smoother coastline around the S and W, as well as the eastern sectors, with lava deltas surviving from the pre-collapse period in the north.