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

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.