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

爆炸性火山喷发在近火山口地区造成毁灭性影响，火山碎屑密度流可造成重大的生命损失，但大量灰烬注入大气并随后扩散到数百至数千公里，造成重大而深远的危害。火山灰降落是一种严重而广泛的火山灾害；造成屋顶倒塌、健康（呼吸系统）和农业问题以及基本基础设施大面积中断（例如电力、粮食/水供应；道路和铁路关闭）。就像2010年Eyjafjallajökull火山喷发（冰岛）所证明的那样，即使是在中度爆炸性喷发期间排放的火山灰也可以使空中交通中断。如此广泛的火山灰扩散带来了巨大的经济和社会成本。令人不安的是，全球有8亿人居住在活火山100公里范围内，然而，对详细喷发数据库（如日本）的统计研究表明，对过去更深时间的火山喷发的记录严重不足。我们对某座火山喷发的规模和频率的理解通常是偏向于最近的活动，因为较早的喷发记录是碎片化的，通常是由于最近的喷发的侵蚀和/或掩埋。然而，保存较好的短期记录并不一定反映火山活动的全部范围或活动速度的变化。这是长期火山危险评估的一个主要障碍，妨碍了我们以下方面的能力：1)确定随时间变化的喷发率，2)评估震级-频率关系，3)预测危险火山灰扩散的复发间隔。这项研究将克服这一僵局，重建生产弧火山爆发火山活动的全面长期记录。它将开发保存在密集的海洋沉积物岩心网络中的未充分利用的灰层记录。这些连续的序列代表了前所未有的灰烬落库（以可见和微观沉积物的形式保存），它们不容易受到破坏性近源过程的影响。使用最先进的地球化学“指纹”技术，可以确定海洋火山灰层的火山来源，同时通过岩心网络追踪这些火山灰下降事件，为计算模型和绘制火山灰扩散图以及计算喷发强度提供了独特的机会。尖端的氩-氩测年技术可以直接测定火山灰沉积物的年代，这将揭示单个火山过去爆发爆发的速度，并重要地确定这些火山广泛的危险火山灰扩散的复发间隔。由于有证据表明日本火山喷发记录中有近源低报的爆炸性火山活动，这项研究将从利用日本岛屿周围丰富的海洋沉积物记录开始，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，这项研究将扩展到其他多产的火山弧环境，特别是那些由于爆炸性火山活动记录不足而易受伤害的火山弧（例如环太平洋火山弧）。这些新的近海火山记录将与直接负责个别火山灾害/风险评估的人员和在这一领域工作的决策者合作审查。

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.