A framework and guidelines for volcanic risk assessment

Natural hazards such as earthquakes, floods, volcanoes and tsunamis constitute a significant source of risk in several regions of the world and are often associated with widespread loss of human lives, damage to the qualities of the environment as well as to property and infrastructure. It is hence a great challenge for the engineering profession to provide methods and tools enhancing decision making for the purpose of efficient management of natural hazards. Since our understanding of the aspects involved in such decision problems is often less than perfect and that we are only able to model the involved physical processes as well as human interactions in rather uncertain terms, the decision problems in engineering are subject to significant uncertainty. Due to this, it is not possible to assess the result of decisions and consequences in certain terms. However, what can be assessed is the risk associated with the different decision alternatives. If the concept of risk as the simple product between probability of occurrence of an event with consequences and the consequence of the event is widened to include also the aspects of the benefit achieved from the decisions, risk may be related directly to the concept of utility (von Neumann and Morgenstern 1944, Raiffa and Schlaifer 1961) from the economic decision theory. A whole methodological framework is thus made available for the consistent identification of optimal decisions. This framework is considered to comprise the theoretical basis for risk based decision making. Based on these principles, a document (JCSS 2008) describing the framework and principles for risk based engineering decision making has been recently developed by the Joint Committee on Structural Safety (JCSS). In this paper, the main features of this framework are first briefly described. Then, aspects related to the modelling of the hazard process due to volcanoes are discussed. A system of classification of structures and identification of different building characteristics that could be used for volcanic vulnerability and risk assessment is then proposed. This is followed by a discussion on the fragility and vulnerability modeling of structures relevant for seismic analysis. Finally, general issues dealing with the quantification of risk and their treatment and communication are covered