Creating Probabilistic Multi-Peril Hazard Maps

An often overlooked component of natural hazards is the element of human involvement. Physical events--such as massive earthquakes--that do not affect society constitute natural phenomena, but are not necessarily natural hazards. Natural phenomena that occur in populated areas constitute hazardous events. Furthermore, hazardous events that cause damage--either in the form of structural damage or the loss or injury of lives--constitute natural disasters. Geographic areas that do not contain human interests, by definition, cannot suffer from hazardous events. Therefore, they do not contain a component of natural hazard. Note that this definition differs from the view of natural hazards as "unavoidable havoc wreaked by the unrestrained forces of nature". On the contrary, the burden of cause is shifted from purely natural processes to the concurrent presence of human society and natural events. Although individuals can do little to change the occurrences or intensities of most natural phenomena, they can mitigate their exposure to natural events and help ensure hazardous events do not become natural disasters. For example, choosing to build new settlements in known flood zones increases the exposure--and therefore risk--to natural flood events. Similarly, while volcanoes do erupt periodically, it is the conscious act of reappropriating the rich soils formed by ejecta as farmland that makes the volcanoes hazardous. Again, this empowers individuals and makes them responsible for their own exposure to natural hazards. Various local and governmental agencies--in particular, the United States Geographical Survey (USGS)--do a good job of identifying and listing various local natural hazards. These listings, however, are often treated individually and independently. Thus, it is often difficult to construct a "big picture" image of total natural hazard exposure. In this presentation, we discuss methods of identifying and combining different natural hazards for a given location. We then further refine our calculation into a single value--a hazard exposure index--and show how this value can be used to create multi-peril hazard maps. As an example, we contour and present one such map for all of California.