Evidence-based volcanology: application to eruption crises W.P. Aspinall a;  , G. Woo b , B. Voight c , P.J. Baxter d a Aspinall and Associates, Woodside Close, Beacons¢eld, Bucks HP9 1JQ, UK b Risk Management Solutions, 10 Eastcheap, London EC3M 1AJ, UK c Department of Geosciences, Penn State University, University Park, PA 16802, USA d Department of Public Health and Primary Care, Cambridge University, Fenner’s, Gresham Road, Cambridge CB1 2ES, UK Abstract The way in which strands of uncertain volcanological evidence can be used for decision-making, and the weight that should be given them, is a problem requiring formulation in terms of the logical principles of Evidence Science. The basic ideas are outlined using the explosion at Galeras volcano in Colombia in January 1993 as an example. Our retrospective analysis suggests that if a robust precautionary appraisal had been made of the circumstances in which distinctive tornillo signals were detected at Galeras, those events might have been construed as stronger precursory evidence for imminent explosive activity than were the indications for quiescence, given by the absence of other warning traits. However, whilst visits to the crater might have been recognised as involving elevated risk if this form of analysis had been applied to the situation in January 1993, a traditional scientific consideration of the available information was likely to have provided a neutral assessment of short-term risk levels. We use these inferences not to criticise interpretations or decisions made at the time, but to illustrate how a structured, evidence-based analysis procedure might have provided a different perspective to that derived from the conventional scientific standpoint. We advocate a formalism that may aid such decision-making in future: graphical Bayesian Belief Networks are introduced as a tool for performing the necessary numerical procedures. With this approach, Evidence Science concepts can be incorporated rationally, efficiently and reliably into decision support during volcanic crises. ß 2003 Elsevier B.V. All rights reserved. Keywords: Bayes’ Rule; Bayesian Belief Network; decision support; Evidence Science; expert judgment; Galeras volcano; risk assessment; volcanic eruption 1. Introduction In one of his last volcanology publications, Bruno Martinelli (1997) pointed out that methods to detect and to quantify the dynamics of the pro- cesses suspected of triggering eruptive activity are practically non-existent. In this context, and as a physicist, he argued that research should be fo- cused on the physical aspects of volcanic activity, and that more observations and experiments should be performed. It was his view that today’s geophysical approach to the investigation of vol- canic activity allows, at best, only a static recon- struction of the internal structure of the volcanic complex, and that geological and petrological in- vestigations, as well as the analysis of volcanic gases, while representing substantial sources of information towards accomplishing the task of short-term prediction, are not substitutes for a 0377-0273/03/$ ^ see front matter ß 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0377-0273(03)00260-9 * Corresponding author. Tel.: +44-(0)1494-672746; Fax: +44-01494-671239. E-mail address: willy@aspinall.demon.co.uk (W.P. Aspinall). VOLGEO 2696 4-11-03 Journal of Volcanology and Geothermal Research 128 (2003) 273^285 R Available online at www.sciencedirect.com www.elsevier.com/locate/jvolgeores