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，摧毁基础设施和房屋，造成人员伤亡。海啸通常与海底地震有关；然而，海中或海底的山体滑坡也会引发灾难性的海啸。火山岛由于海底斜坡陡峭而不稳定，特别容易发生山体滑坡。2018年12月22日，印度尼西亚喀拉喀托火山岛的西南坡崩塌入海，引发海啸，海啸在不到30分钟的时间内席卷巽他海峡。在苏门答腊岛南部和爪哇岛北部，海啸摧毁了2750多座建筑物和510艘船只，造成4.3万多人流离失所，14059人受伤，437人死亡。虽然其他火山岛的山体滑坡也会引发海啸，但喀拉喀托火山的情况非常独特，因为喀拉喀托火山是一个非常年轻和活跃的后崩塌锥体。喀拉喀托火山是在1883年著名的灾难性喀拉喀托火山崩塌爆发后在喀拉喀托火山口中心形成的。1926年，在崩塌的破火山口中心重新爆发的火山活动形成了喀拉喀托火山岛。喀拉喀托火山的山体滑坡和海啸代表了一种新的危险情况，因为这样一座年轻的火山大厦的崩塌从未发生过如此大规模的报道，也没有报道引发了造成如此毁灭性破坏和生命损失的海啸。因此，我们迫切需要研究这一事件，以便更好地了解它发生的原因和方式。我们建议用多波束测深仪分析海底表面，用地震反射法调查地表以下，对西南阿纳克喀拉喀托的海底斜坡进行调查，并绘制海底滑坡图。这将为滑坡的规模、滑坡是如何塌陷的以及滑坡向下坡移动时的动力学提供宝贵的见解。这些特性对于理解海啸产生的原因非常重要，并将提供更准确的滑坡测量数据，用于模拟滑坡-海啸过程。这个机会非常及时，因为没有在事件发生后这么快就研究过海底滑坡。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满。因此，在阿纳克喀拉喀托的海底斜坡被未来的滑坡和泥沙流改变或被火山沉积物掩埋之前，对其进行调查是迫切需要的。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通常在我们试图测量的一些属性中存在很大的不确定性，因为斜坡和滑坡沉积物随着时间的推移已经被侵蚀、改造和掩埋。特别是，滑坡量可能有很大的不确定性，但它可以说是产生海啸的最大因素。在这里，我们有机会在事件发生几个月后准确地研究这个特别独特的火山岛的性质。我们还建议对喀拉喀托滑坡的远端沉积流进行研究。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发生破坏。确定这一点很重要，因为如果滑坡在一系列阶段中失败，那么体积就不会一次全部释放，因此海啸的可能性就会降低。对这些沉积物进行采样是迫切的，因为如果随后发生滑坡和沉积物流动，沉积记录可能会变得模糊，从而模糊了这一事件的记录。我们建议利用这项研究的结果更好地为用于确定减灾战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攸关方。

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.