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

海啸是一种重大自然灾害，能够淹没地势低洼的海岸线，摧毁基础设施和房屋，造成人员伤亡。海啸通常与海底以下的地震有关；然而，向海底或海底以下的山体滑坡也可能产生灾难性的海啸。由于火山岛的海底斜坡往往陡峭且不稳定，因此特别容易发生山体滑坡。2018年12月22日，印度尼西亚Anak Krakatau火山岛西南坡坍塌入海，引发海啸，不到30分钟就席卷了舜达海峡。海啸摧毁了南部萨姆特拉和爪哇北部的2750多座建筑和510艘船只，超过43,000人流离失所，14,059人受伤，437人死亡。尽管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倒塌后圆锥体。Anak Krakatau形成于1883年著名的灾难性的Krakatau火山口崩塌喷发之后，形成于Krakatau火山口的中心。1926年，新一轮的火山活动在崩塌的火山口中心形成了阿纳克克拉卡托火山岛。Anak Krakatau的山体滑坡和海啸是一种新的危险情况，因为这样一座年轻的火山建筑上的坍塌没有如此规模的报告，也没有报告导致如此毁灭性的破坏和生命损失的海啸。因此，我们迫切需要研究这一事件，以更好地了解它发生的原因和方式。我们建议调查Anak Krakatau西南部的海底斜坡，并使用多波束回声探测仪分析海底表面和地震反射方法调查表面下的海底滑坡。这将为了解滑坡的规模、斜坡如何破坏以及滑坡向下移动时的动力学提供宝贵的见解。这些特性对于理解产生的海啸很重要，并将提供更准确的滑坡测量，可用于模拟滑坡-海啸过程。这次机会非常及时，因为在事件发生后这么快就没有研究过海底山体滑坡。火山岛是非常动态的环境，摄影证据表明，海平面以上的海湾已经被熔岩和火山沉积物填满。因此，当务之急是在Anak Krakatau的海底斜坡被未来的滑坡和泥石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，我们以前曾研究过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，斜坡和滑坡堆积物已经被侵蚀、重新加工和掩埋。特别是，山体滑坡的数量可能有很大的不确定性，但可以说，它是造成海啸的最大因素。在这里，我们有机会在事件发生几个月后准确地研究这一特别独特的火山岛坍塌的性质。我们还建议研究Anak Krakatau滑坡的沉积流的远端沉积。这些沉积物可以指示原始滑坡是在单个阶段还是在多个阶段失事。确定这一点很重要，因为如果山体滑坡在一系列阶段失败，体积不会一下子释放出来，因此海啸的可能性将会降低。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泥石流，沉积记录可能会变得模糊，并掩盖这一事件的记录。我们建议利用这项研究的结果，更好地为用于确定缓解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.