What makes an extreme geological event a catastrophe?
by
David E. Alexander
University of Massachusetts at Amherst, USA

This paper reviews a varied set of definitions and measures of disaster. These include the mathematical concept of abrupt hysteresis and bifurcation, the earth science concept of accelerated and widespread change in environmental conditions, the social science concept of radical, if transient, mutation of organizations, the engineering concept of major technological system failure, and the medical concept of mass casualty situations. The terms 'hazard' and 'vulnerability' are fundamental components of disaster but cannot adequately be defined independently of one another.

Apart from disciplinary orientations, there are other reasons why it has proved very difficult to obtain a consensus on the meaning of the terms 'disaster' and 'catastrophe'. Some scholars regard them as synonymous, while others consider them as descriptive of different levels of impact. Attempts to impose a numerical threshold on disaster have not proved particularly successful. Likewise, geographical extent and other measures of size have not led to a better functional definition. This paper uses examples of differences in the sizes of various important earthquakes and landslides selected from recent history to show how the magnitude and frequency of natural hazards are poorly correlated with their disaster potential. Instead, in most situations of risk, vulnerability-coupled with its opposite, resilience-tends to be a better predictor of damage, destruction and casualties.

The significance of disaster is measured in terms of both its effects and its potential for destruction. In this respect, the catastrophe potential of very large, very infrequent events is the subject of much theoretical, as well as practical, uncertainty. This paper explores some of the dilemmas associated with assigning values of low potential to risks that are extremely infrequent but potentially very damaging, such as asteroid impacts or some of the largest Quaternary volcanic eruptions.

The paper ends by considering trends and predictions for global disaster potential in the forthcoming decades. Some of these are contradictory and most present a bleak picture that may, nevertheless, underestimate human ingenuity in the face of adversity.

Date received: April 11, 2002