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

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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.