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

海啸是一种重大的自然灾害，能够淹没低洼的海岸线，摧毁基础设施和家园，造成人员伤亡。海啸通常与海底以下的地震有关;然而，滑坡进入海中或海下也可能产生灾难性的海啸。火山岛特别容易发生滑坡，因为它们的海底斜坡往往陡峭而不稳定。2018年12月22日，印度尼西亚Anak Krakatau火山岛的西南斜坡坍塌入海，并在不到30分钟的时间内引发了穿越巽他海峡的海啸。海啸摧毁了萨姆特拉南部和北方爪哇的2，750多座建筑物和510艘船只，造成43，000多人流离失所，14，059人受伤，437人死亡。虽然其他火山岛的山体滑坡也引发了海啸，但Anak Krakatau的情况非常独特，因为Anak Krakatau是一个非常年轻和活跃的崩塌后圆锥体。Anak Krakatau在1883年喀拉喀托火山口的中心形成，这是着名的灾难性的喀拉喀托火山口崩溃爆发。1926年，火山口中心重新开始的火山活动创造了Anak Krakatau火山岛。Anak Krakatau的滑坡和海啸是一种新的灾害情景，因为在这样一个年轻的火山建筑物上发生如此大规模的坍塌，没有报告过引发海啸，造成如此毁灭性的破坏和生命损失。因此，我们迫切需要研究这一事件，以更好地了解它是如何发生的。我们建议调查西南Anak Krakatau的海底斜坡，并使用多波束回声测深仪分析海底表面和地震反射方法调查海底滑坡。这将提供宝贵的见解滑坡的大小，斜坡如何失败，以及滑坡的动态，因为它移动下坡。这些特性对于理解海啸的产生非常重要，并将提供更准确的滑坡测量结果，可用于模拟滑坡-海啸过程。这个机会非常及时，因为没有海底滑坡在事件发生后这么快就进行了研究。火山岛是非常动态的环境，照片证据表明，海平面以上的海湾已经被熔岩和火山沉积物填充。因此，迫切需要在Anak Krakatau的海底斜坡被未来的滑坡和沉积物流改变或被火山沉积物掩埋之前对其进行调查。为了更好地了解这些自然灾害，我们以前不得不研究地质记录中的火山岛滑坡。然而，对这些过去的例子的研究往往在我们试图测量的一些属性中存在很大的不确定性，因为随着时间的推移，斜坡和滑坡沉积物已经被侵蚀，改造和掩埋。特别是，滑坡量可能具有很大的不确定性，但它可以说是产生海啸的最大因素。在这里，我们有机会准确地研究这个特别独特的火山岛在事件发生几个月后崩溃的属性。我们还建议研究Anak Krakatau滑坡的沉积流的远端沉积物。这些沉积物可以指示原始滑坡是在单个阶段还是多个阶段失败的。这一点很重要，因为如果滑坡在一系列阶段中失败，那么体积不会立即释放，因此海啸的可能性会降低。迫切需要对这些沉积物进行采样，因为如果随后发生山体滑坡和沉积物流动，沉积物记录可能会变得模糊不清，从而掩盖这一事件的记录。我们建议使用本研究的结果来更好地为用于确定减灾策略的海啸模型提供信息，并将这些结果传递给印度尼西亚的利益相关者。

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.