Constraints on the tempo and magnitude of explosive volcanism: facilitating long-term ash fall hazard assessments

火山爆发对火山喷口附近地区造成毁灭性影响，火山碎屑密度流可造成重大生命损失，但向大气中注入大量火山灰并随后将其扩散到数百至数千公里，构成重大和深远的危险。火山灰坠落是一种严重和广泛的火山灾害；导致屋顶坍塌、健康(呼吸系统)和农业问题，以及基本基础设施的大范围中断。正如2010年埃亚菲亚德拉火山喷发(冰岛)所证明的那样，即使是在中等爆炸性喷发期间排放的火山灰也会使空中交通中断。因此，广泛的火山灰扩散造成了巨大的经济和社会代价。令人不安的是，全球有8亿人生活在活火山周围100公里以内，但对全球喷发数据库的统计研究表明，过去火山喷发的记录严重不足。例如，这项分析将表明，在过去20万年的地质记录中，高达66%的VEI 5级火山喷发，其规模相当于1980年的圣海伦斯火山喷发。我们对某一特定火山喷发的规模和频率的了解通常偏向于最近的活动，因为关于较早喷发的记录往往不完整，往往是由于侵蚀和/或较新喷发的掩埋。然而，保存较好的较短期记录并不一定反映火山活动的全部范围，或活动节奏的变化。这是长期火山危险评估的主要障碍，并妨碍我们以下能力：一)确定随时间变化的喷发率；二)评估震级-频率关系；三)预测危险的火山灰扩散的复发间隔。这项研究通过重建日本南部和中部火山爆发的长期综合记录，打破了这一僵局。这项研究利用了火山灰层未得到充分利用的记录，这些火山灰层保存在远离火山的海洋和湖泊沉积岩芯的密集网络中。这些连续的沉积物序列提供了史无前例的火山灰沉降库(保存为可见的和微观的沉积物)，不容易受到破坏性的近源火山过程的影响。使用最先进的化学“指纹”技术，有可能查明远端火山灰层的火山源，同时通过岩心网络追踪这些火山灰落下事件，提供了计算模型和绘制过去火山灰扩散图的机会，并计算喷发强度。结合尖端测年技术(40Ar/39Ar/14C)来确定火山灰沉积物的年代，使我们能够揭示单个火山过去爆炸性喷发的时间和节奏，并重要地确定从这些火山广泛扩散的危险火山灰的复发间隔。我们在日本南部和中部的研究一直在成功地解决未报告的喷发问题，并填补了许多火山喷发记录中的空白。在我们下一阶段的研究中，我们将利用国际海洋发现计划(IODP)和日本海洋地球科学技术厅(JAMSTEC)新获得的海洋岩芯，扩大我们方法的应用，以解决日本东北部和千岛弧区的火山问题。此外，利用我们制作全面喷发记录的能力，我们将探索火山与气候的相互作用。将继续与直接负责对个别火山进行火山危险评估的人员和在实地工作的政策制定者合作，审查该项目产生的远端火山记录。

Explosive volcanic eruptions have devastating impacts in near-vent areas where pyroclastic density currents can cause significant loss of life, yet the injection of large volumes of ash into the atmosphere and its subsequent dispersal over hundreds to thousands of kilometres, pose significant and far-reaching hazards. Ash fall is a severe and wide-ranging volcanic hazard; causing roof collapse, health (respiratory) and agricultural issues and wide-scale interruptions to essential infrastructure. Even ash emitted during moderately explosive eruptions can ground air traffic as was demonstrated by the 2010 Eyjafjallajökull eruption (Iceland). As such widespread volcanic ash dispersals present huge economic and societal costs. Disturbingly, 800 million people live within 100 km of active volcanoes globally, yet statistical studies of global eruption databases indicate significant under-recording of past volcanic eruptions deeper in time. For instance, this analysis would indicate up to 66% of VEI 5 eruptions, equivalent in scale to the 1980 Mount St Helens eruption, are missing within the geological record spanning the last 200,000 years. Our understanding of the magnitude and frequency of eruptions at a particular volcano is typically skewed to recent activities, because records of older eruptions are fragmentary often owing to erosion and/or burial by more recent eruptions. The better-preserved, shorter-term records, however, do not necessarily reflect the full range of volcanic activity, or variations in the tempo of activity. This is a major obstacle for long-term volcanic hazard assessments and hampers our ability to: i) determine changing eruption-rates through time, ii) evaluate magnitude-frequency relationships and iii) project the recurrence intervals of hazardous ash dispersals. This research has overcome this impasse by reconstruct comprehensive long-term records of explosive volcanism for volcanoes in southern and central Japan. This research exploits the under-utilised record of volcanic ash layers preserved in dense networks of marine and lake sediment cores away from the volcano. These continuous sediment sequences present unprecedented repositories of ash fall (preserved as visible and microscopic deposits), which are not susceptible to destructive near-source volcanic processes. Using state-of-the-art chemical 'fingerprinting' techniques, it is possible to pinpoint the volcanic source of the distal ash layers, whilst tracing these ash fall events across a network of cores provides the opportunity to computationally model and map past ash dispersals, and calculate eruption magnitudes. Integrating cutting-edge dating techniques (40Ar/39Ar/14C) to date the ash deposits, enables us to reveal the timing and tempo of past explosive eruptions at an individual volcano, and importantly determine the recurrence intervals of widespread hazardous volcanic ash dispersals from these volcanoes. Our research in south and central Japan has been successfully tackling eruption un-reporting and plugged the gaps in the eruption records of numerous volcanoes. In the next phase of our research we will expand the application of our methods to address the volcanoes of NE Japan and the Kurile Arc, utilising newly available marine cores from International Ocean Discovery Programme (IODP) and Japan Agency For Marine-Earth Sciences and Technology (JAMSTEC). In addition, using our ability to produce comprehensive eruption records, we will explore volcano-climate interactions. The distal volcanological records generated in this project will continue to be examined in partnership with those directly responsible for volcanic hazard assessments at individual volcanoes, and policy-makers working in the field.