Regional ash fall hazard II: Asia-Pacific modelling results and implications

In a companion paper (this volume), the authors propose a methodology for assessing ash fall hazard on a regional scale. In this study, the methodology is applied to the Asia-Pacific region, determining the hazard from 190 volcanoes to over 1 million km2 of urban area. Hazard is quantified at each km2 grid cell of urban area in terms of the Annual Exceedance Probability (AEP), or its inverse the Average Recurrence Interval (ARI) of ash falls exceeding 1, 10 and 100mm. A surrogate risk variable, the Population-Weighted Hazard Score: the product of AEP and population density, describes the relative risk for each grid cell. Urban areas in Indonesia are found to have the highest levels of hazard and risk scores, while Australia has the lowest. A clear demarcation emerges between the hazard in countries close to and those farther from major subduction plate boundaries, with the latter having ARIs at least two orders of magnitude longer for the same thickness thresholds. Countries with no volcanoes, such as North Korea and Malaysia, face ash falls from volcanoes in neighbouring countries. In each of the countries considered, modelled ash falls exceeding 1mm affect more than one million people living in urban areas; in Indonesia, Japan and the Philippines this situation could occur with ARIs less than 40 years.