Reply on RC2

Susanna Jenkins

doi:10.5194/nhess-2021-320-ac2

Abstract

Regional assessments provide a large-scale comparable vision of the threat posed by multiple sources and are useful for prioritising risk-mitigation actions. There is a need for such assessments from international, regional and national agencies, industries and governments to prioritise where further study and support could be focussed. Most existing regional studies on the threat posed by volcanic activity have relied on concentric radii as proxies for hazard footprints and have focused only on population exposure, often using indices to make first-order estimates of exposure. However, this approach is an oversimplification of volcanic hazards and their associated impacts. We have developed and applied a new approach that quantifies and ranks exposure to multiple volcanic hazards for 40 high-threat volcanoes in Southeast Asia. For each of our 40 volcanoes, hazard spatial extent, and intensity where appropriate, was probabilistically modelled for four volcanic hazards across three eruption scenarios, giving 697,080 individual hazard footprints plus 19,560 probabilistic hazard outputs. We then developed a GIS framework to overlay the spatial extent of probabilistic hazard footprints with open-access datasets across five exposure categories. Finally, we used our calculated exposure values to rank each of the 40 volcanoes in terms of the threat they pose to surrounding communities. We present VolcGIS, an open-source Python code that implements all of the spatial operations required for exposure analysis, available at github.com/vharg/VolcGIS. We provide all our outputs - more than 6,500 geotif files and 70 independent estimates of exposure to volcanic hazards across 40 volcanoes - in user-friendly format. Results highlight that the island of Java in Indonesia has the highest median exposure to volcanic hazards, with Merapi consistently ranking as the highest threat volcano. Hazard seasonality, as a result of varying wind conditions affecting tephra dispersal, leads to increased exposure values during the peak rainy season (January, February) in Java, but the peak dry season (January, February, March) in the Philippines. A key aim of our study was to highlight volcanoes that may have been overlooked, perhaps because they are not frequently or recently active, but that have the potential to affect large numbers of people and assets. It is not intended to replace official hazard and risk information provided by the individual country or volcano organisations. This study and the tools developed provide a road map for future multi-source regional volcanic exposure assessments, with the possibility to extend the assessment to other geographic regions and/or towards impact and loss.

Highlights

Southeast Asia is one of the most densely populated regions on Earth; it is home to over 12% (n=173) of the world’s Holocene volcanoes and around 15% (n=1,543) of Holocene eruptions (Global Volcanism Program, 2013)
340 The multi-hazard and multi-exposure analysis presented here required nearly 700,000 individual simulations and produced 26,640 probabilistic outputs (15,240 hazard and 11,400 exposure estimates) that can be useful at the individual volcano scale
Ranking was performed using both a “shortterm” approach, where the exposure is conditional on the occurrence of the eruption scenario, and a “long-term” approach, which accounts for the probability of occurrence of a given eruption scenario at each volcano

Summary

Introduction

Southeast Asia is one of the most densely populated regions on Earth; it is home to over 12% (n=173) of the world’s Holocene volcanoes and around 15% (n=1,543) of Holocene eruptions (Global Volcanism Program, 2013). Of these Southeast Asian eruptions, 93% (n=1,435) have occurred since 1500 CE, showing the dominance of historical records in reflecting previous eruptive activity. There is a need for detailed geological studies to supplement short eruptive records; such studies are lacking for many volcanoes around the world because they can be time-consuming, costly and suffer from a lack of deposit exposure, especially in tropical regions such as Southeast Asia (De Maisonneuve and Bergal-Kuvikas, 2020). To identify volcanoes that pose a considerable threat to society, previous studies have applied consistent and transferable methodologies to rank multiple volcanoes according to their hazard (e.g. Aspinall et al, 2011; Auker et al, 2015) or their population exposure (e.g. Small and Naumann, 2001; Freire et al, 2019), with some studies combining the two to evaluate ‘threat’ (e.g. Ewert, 2007; Brown et al, 2015b; Scandone et al, 2016) on a regional

Methods

Results

Conclusion