# Mapping Recent Shoreline Changes Spanning the Lateral Collapse of Anak Krakatau Volcano, Indonesia
## 1 Introduction  
Approximately 1500 volcanoes are considered active worldwide [1]. However, the high costs and difficulties of maintaining instrumentation in volcanic environments results in fewerthan half of the potentially active volcanoes being regularly monitored with ground-based sensors and fewer still are considered well-monitored even within countries with relatively well-resourced volcanological observatories [2; 3; 4]. Island volcanoes present particular problems in this regard, both because of difficulties with accessibility and because on small islands the instrumentation has to be placed close to volcanic vents and so is especially vulnerable to damage from even mild background-level activity. Furthermore, island volcanoes present particular hazards because lateral collapses, including large sector collapses, at coastal and island volcanoes can occur with little or no precursory activity to provide unambiguous warning of impending collapses and have the potential to generate regionally destructive tsunamis [5; 6]. Numerous volcanic crises have proven that the lack of monitoring capabilities can have dramatic consequences when violent events occur at remote volcanoes with little or no monitoring in place. Notable historical examples include Lamington, Papua New Guinea, 1951 [7; 8] and El Chichon, Mexico, 1982 [9; 10] but the potential for such unexpected and destructive events still persists. Recent years have presented a number of examples of the challenges of managing rapid-onset volcanic events (e.g., Nabro, Eritrea, 2011 [11]; Calbuco, Chile 2015 [12] and Fuego, Guatem 2018 [13]); the most recent example of was the lateral collapse at Anak Krakatau (AK) in Indonesia on 22 December 2018, which occurred during an eruptive episode that had begun in June 2018 [14]. The lateral collapse generated a tsunami which killed 437 people and displaced more than 16,000 according to the ASEAN (Association of Southeast Asian Nations) Coordinating Centre for Humanitarian Assistance on Disaster Management (AHA) [15]. Satellite remote sensing can provide crucial and unique observations to identify pre-eruptive, syn-eruptive and post-eruptive signals when ground- or airborne-based acquisitions are not available [16] because these areas are difficult to reach due to the danger of working on active volcanoes, as emphasized in particular by the 1993 incident at Galeras, Colombia [17; 18]. Indeed, volcanologists and civil protection agencies need continuous data over a long period to evaluate the level of hazard volcanoes pose. Instruments on board the latest Earth Observation (EO) spacecraft have the ability to supply this much-needed time series information and to provide the data required to accurately map hazardous areas and exposed elements. The aim of this work is to analyze shoreline changes associated with the AK activity for the period May 2018-November 2019, which includes the lateral collapse event. However, as discussed below there is a gap in the data during the period within which the collapse itself occurred, as well as episodes of intense eruptive activity causing rapid post-collapse regrowth--these were obscured from optical sensors by cloud and by the post-collapse eruption plumes for a period of almost two months (68 days). Nevertheless, in this study we show how satellite remote sensing provides data on shoreline change patterns at AK both before and after this data gap, which can be used to understand how the post-collapse regrowth of the island may be increasing the potential for future tsunamigenic lateral collapses at AK. In this paper, we consider the shoreline as the physical interface of land and water according to [PERSON] et al. (1980) definition [19]. Coastal areas and shorelines are the ever-changing boundary between the land and the sea, whose evolution is the product of mutual adjustments in topography, marine processes and, for volcanic islands, also volcanic activity [20].