Failure mechanism and tsunamigenesis of the Anak Krakatau landslide on 22 December 2018

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Tsunamis represent a major natural hazard and are capable of inundating low-lying coastlines, destroying infrastructure and homes, and causing injury and loss of life. Tsunamis are commonly associated with earthquakes beneath the seafloor; however, landslides into or beneath the sea can also generate catastrophic tsunamis. Volcanic islands are particularly susceptible to landslides due to their often steep and unstable submarine slopes. On December 22 2018, the SW slope of the Anak Krakatau volcanic island, in Indonesia, collapsed into the sea and created a tsunami that travelled across the Sunda Strait in less than 30 minutes. The tsunami destroyed over 2,750 buildings and 510 ships in southern Sumtra and northern Java, displaced over 43,000 people, injured 14,059 people, and caused 437 fatalities.Although landslides from other volcanic islands have generated tsunamis, the case at Anak Krakatau is very unique as Anak Krakatau is a very young and active post-collapse cone. Anak Krakatau formed in the centre of the Krakatau caldera following the famous and catastrophic Krakatau caldera-collapse eruption in 1883. In 1926, renewed volcanic activity in the centre of the collapse-caldera created the volcanic island of Anak Krakatau. The landslide and tsunami at Anak Krakatau represents a new hazard scenario as collapses on such a young volcanic edifice have not been reported at this scale and not been reported to generated tsunamis that have resulted in such devastating destruction and loss of life. It is therefore urgent that we study this event to better understand why and how it happened.We propose to survey the submarine slopes of SW Anak Krakatau and map the submarine landslide using multi-beam echosounders to analyse the seafloor surface and seismic reflection methods to investigate beneath the surface. This will provide invaluable insights into the size of the landslide, how the slope failed, and the dynamics of the landslide as it moved downslope. These properties are important towards understanding the tsunami that was generated, and will provide more accurate landslide measurements that can be used to model the landslide-tsunami process. This opportunity is very timely, as no submarine landslides have been studied so soon after the event. Volcanic islands are very dynamic environments, and photographic evidence shows that the embayment above sea level is already being infilled by lavas and volcanic sediments. Therefore, it is urgent to survey the submarine slopes of Anak Krakatau before they are modified by future landslides and sediment flows or are buried by volcanic sediments. To better understand these natural hazards we have previously had to study volcanic island landslides in the geological record. However, study of these past examples often present large uncertainties in some of the properties we are trying to measure because the slopes and landslide deposits have been eroded, reworked and buried over time. In particular, landslide volume can have significant uncertainties, and yet it is arguably the largest contributing factor to generated tsunamis. Here, we have an opportunity to accurately study the properties of this particularly unique volcanic island collapse only months after the event.We also propose to study the distal deposits of sedimentary flows from the Anak Krakatau landslide. These deposits can indicate whether the original landslide failed in a single or multiple stages. This is important to determine because if the landslide failed in a series of stages the volume was not released all at once, and so the tsunami potential would decrease. It is urgent to sample these deposits as the sedimentary record may become ambiguous should subsequent landslides and sediment flows occur and obscure the record of this event.We propose to use the results of this study to better inform tsunami models used to determine mitigation strategies, and pass these results to stakeholders in Indonesia.