Constraints on the tempo and magnitude of explosive arc 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；造成屋顶坍塌、健康(呼吸系统)和农业问题，以及基本基础设施(如电力、食品/水供应；公路和铁路关闭)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100公里范围内，但对详细的喷发数据库(例如日本)的统计研究显示，过去火山喷发的记录严重不足。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一个主要障碍，并阻碍了我们的能力：i)确定随时间变化的喷发率，ii)评估震级-频率关系，以及iii)预测危险的火山灰扩散的重复间隔。这项研究将打破这一僵局，重建多产弧形火山的爆发火山活动的全面长期记录。它将利用保存在密集的海洋沉积物岩心网络中的火山灰层未得到充分利用的记录。这些连续的序列代表了前所未有的火山灰沉降库(保存为可见的和微观的沉积物)，不容易受到破坏性的近源过程的影响。使用最先进的地球化学‘指纹’技术，有可能查明海洋火山灰层的火山来源，而在岩芯网络中追踪这些火山灰落下事件提供了一个独特的机会，以计算模型和绘制火山灰扩散图，并计算喷发强度。尖端的Ar-Ar测年技术直接测定火山灰沉积的日期，将揭示单个火山过去爆炸性喷发的速度，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。鉴于有证据表明，日本的喷发记录少报了火山爆发活动，这项研究将利用日本群岛周围丰富的海洋沉积物记录，包括国际海洋发现计划(IODP)和日本地质调查局(AIST)的记录。然后，这项研究将寻求扩展到其他生产性火山弧环境，特别是那些由于没有充分记录爆发火山活动(例如环太平洋火山弧)而容易受到伤害的环境。这些新的近海火山记录将与直接负责个别火山的危险/风险评估的人员和在这一领域工作的政策制定者合作进行审查。

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 (e.g., electricity, food/water supplies; roads and rail closures). 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 detailed eruption databases (e.g., Japan) reveal significant under-recording of past volcanic eruptions deeper in time. 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 will overcome this impasse and reconstruct comprehensive long-term records of explosive volcanism for productive arc volcanoes. It will exploit the under-utilised record of ash layers preserved in dense networks of marine sediment cores. These continuous sequences represent unprecedented repositories of ash fall (preserved as visible and microscopic deposits), which are not susceptible to destructive near-source processes. Using state-of-the-art geochemical 'fingerprinting' techniques, it is possible to pinpoint the volcanic source of the marine ash layers, whilst tracing these ash fall events across a network of cores provides a unique opportunity to computationally model and map ash dispersals, and calculate eruption magnitudes. Cutting-edge argon-argon dating techniques to directly date the ash deposits, will reveal the tempo of past explosive eruptions at an individual volcano, and importantly determine the recurrence intervals of widespread hazardous volcanic ash dispersals from these volcanoes. With evidence for near-source under-reporting of explosive volcanic activity emanating from Japanese eruption records, this research will begin by utilising a wealth of marine sediment records from around the Japanese Islands, including those of the International Ocean Discovery Programme (IODP) and the Geological Survey of Japan (AIST). This research will then look to expand into other productive volcanic arc settings, particularly those that are vulnerable owing to inadequate records of explosive volcanism (e.g., circum-Pacific volcanic arcs). These new offshore volcanological records will be examined in partnership with those directly responsible for hazard/risk assessments at individual volcanoes and policy-makers working in this field.