# Mapping Recent Shoreline Changes Spanning the Lateral Collapse of Anak Krakatau Volcano, Indonesia
## 2 The AK 2018-2019 Activity  
The Krakatau volcanic complex (Figure 1), nestled between the islands of Java and Sumatra (Indonesia), is one of the most famous volcanoes worldwide due to the caldera-forming and tsunamigenic eruption of 1883 [21; 22; 23] and the near-continuous eruptions (at least 45 over the last century) [24] that led to the formation in late 1927 of a new subaerial volcanic cone, called AK or \"child of Krakatau\" that lies within the remnants of the collapsed caldera [25; 26]. The volcanic complex is currently an archipelago with the volcanically active AK surrounded by Sertung island to NW, Panjang island to NE and Rakata island to SE (Figure 1). Rakata is a remnant of the island that was volcanically active in 1883, whereas Sertung and Panjang are remnants of a caldera wall that pre-dates this.  
Since its subaerial emergence, eruptive activity at AK has continued periodically [27] and the shoreline has undergone substantial changes as the planform of the island has evolved--from an aerial extent of 1.55 km\({}^{2}\) in 1935 [28], 2.29 km\({}^{2}\) in 1940 [29], 1.52 km\({}^{2}\) in 1950 [30], 2.2 km\({}^{2}\) in 1952 [30], 3.16 km\({}^{2}\) in July 1996 [31] and 3.03 km\({}^{2}\) in December 2018 [32]. These fluctuations reflect the many eruptive episodes [24] during which the island has grown through emission of lava flows and deposition of tephra [27]. Such activity led to periods of coastal erosion both during and between eruptions and possibly also small earlier flank collapses, although no large and destructive tsunamis are known to have been produced in the Krakatau group since 1883, before that of 22 December 2018.  
After 15 months of quiescence AK began a new eruption phase on 21 June 2018 (and currently ongoing), characterized by Strombolian activity that generated low-level ash plumes and ballistic ejecta, alongside lava flows [24]. This activity was typical in style of that which occurred in preceding  
Figure 1: Location of the Anak Krakatau archipelago from true color composite image of Sentinel-2 captured on 15 August 2016.  
decades, although there is evidence that the overall magmatic flux was notably high in this period [32]. On the 22 December 2018, several months after eruptive activity had begun, a lateral collapse event reduced the height of the island from 320 to 120 m [32] and approximately halved the island area [33]. The south-west flank is thought to have collapsed, partially as a result of its over steepening due both to long-term growth on this side of the island, which is built on a submarine scarp of the 1883 caldera and the additional volume erupted between June and December 2018 [32; 33].  
The lateral collapse cut the active vent beneath sea-level, resulting in a transition to phreatomagmic explosive activity. Particularly intense phreatomagmatic activity in the days following the collapse caused rapid-regrowth of the island with a very different morphology, including a largely new coastline whose development we document in this paper.  
In contrast to previous studies focusing on exploiting satellite imagery from cloud-based platforms like Google Earth Engine (GEE) for multi-decadal [34] or worldwide assessment of shorelines [35], here we present and validate a methodology developed in GEE based on the adaptation of Reference [36] and using the freely-available Sentinel-2 multi-spectral satellite data. The methodology is applied for the detailed analysis, both in space and time, of the shoreline evolution of AK and provides constraints on the volcanic activity for the 2018-2019 period. Sentinel-2 is a constellation of twin satellites (Sentinel-2A and Sentinel-2B) developed and operated by the European Space Agency within the Copernicus program--the European Union's EO program. The constellation collects multi-spectral data with 13 bands in the visible, near-infrared and short wave infrared part of the spectrum (Figure 2) with a systematic global coverage of land surfaces at latitudes of 56\({}^{\circ}\) S to 84\({}^{\circ}\) N.