Natural hazards, extreme events, and mountain topography Oliver Korup a, * , John J. Clague b a Swiss Federal Research Institutes WSL/SLF, CH–7260 Davos, Switzerland b Centre for Natural Hazard Research, Simon Fraser University, Vancouver, B.C. V6B 1R8, Canada article info Article history: Received 25 February 2009 Accepted 27 February 2009 abstract The hazard of any natural process can be expressed as a function of its magnitude and the annual probability of its occurrence in a particular region. Here we expand on the hypothesis that natural hazards have size–frequency relationships that in parts resemble inverse power laws. We illustrate that these trends apply to extremely large events, such as mega-landslides, huge volcanic debris avalanches, and outburst flows from failures of natural dams. We review quantitative evidence that supports the important contribution of extreme events to landscape development in mountains throughout the world, and propose that their common underreporting in the Quaternary record may lead to substantial underestimates of mean process rates. We find that magnitude–frequency relationships provide a link between Quaternary science and natural hazard research, with a degree of synergism and societal importance that neither discipline alone can deliver. Quaternary geomorphology, stratigraphy, and geochronology allow the reconstruction of times, magnitudes, and frequencies of extreme events, whereas natural hazard research raises public awareness of the importance of reconstructing events that have not happened historically, but have the potential to cause extreme destruction and loss of life in the future. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Over the past several thousand years, extreme geological events have killed tens of millions of people and severely damaged human infrastructure (Fig. 1). Some geological events may have even shaped the course of early civilisations, including, for example, the Minoans in the Mediterranean, the Vikings in Greenland, and the Mayans in Central America (e.g. Sparks et al., 2005; Cashman and Giordano, 2008; Nur and Burgess, 2008). Potentially harmful geological, climatic, and hydrologic processes are subject of the rapidly growing field of natural hazards research. The term ‘‘hazard’’ expresses the likelihood that a poten- tially damaging event will occur within a particular region during a specified period of time (e.g. Cruden and Varnes, 1996). Natural hazards can be quantified statistically from documented time series using historical data, but the time series are typically short and may not capture the full range of process magnitudes. For example, the 2004 Indian Ocean tsunami, which killed more than 225,000 people in 11 countries, is regarded as an extreme event based on the brief historic record, but geological data show that tsunami of the same size are common in the Indian Ocean on timescales of hundreds to thousands of years (Jankaew et al., 2008; Monecke et al., 2008). In this respect, the geological record provides a unique context for evaluating hazards posed by extreme events. The innate interdisciplinarity of Quaternary research offers many opportuni- ties for identifying traces left by extreme events in the geological record. Here, we amplify on the papers in this Special Issue by arguing that Quaternary science and natural hazards research, two cross- cutting but traditionally unrelated fields, have much in common, as well as considerable potential for synergism – in combination they can produce results of societal importance that neither discipline on its own can deliver. On one hand, Quaternary science provides the necessary tools for reconstructing ages and frequencies of prehistoric extreme events that are recorded in landforms and sediments. Geomorphology and stratigraphy provide additional information on the magnitude of past events and the underlying climatic and environmental conditions that prevailed at the time of the events. For its part, natural hazard research provides a context for raising awareness of the importance of reconstructing events that have not happened in the historic period, yet have the potential to cause much destruction and loss of life in the future (Messerli et al., 2000). We focus here on mountainous terrain, which is the product of an interplay among tectonic, climatic, and erosional processes. Pronounced tectonic and climatic forcing, manifest in high rock * Corresponding author. Tel.: þ41 81 417 0250; fax: þ41 81 417 0110. E-mail address: korup@slf.ch (O. Korup). Contents lists available at ScienceDirect Quaternary Science Reviews journal homepage: www.elsevier.com/locate/quascirev 0277-3791/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2009.02.021 Quaternary Science Reviews 28 (2009) 977–990