Understanding risk is a prerequisite to effectively preparing and warning endangered people of potential and imminent tsunamis. To that effect, officials must understand in advance (1) where and to what extent tsunami inundation may occur and the amount of time before waves arrive, (2) the characteristics of the population and communities in tsunami-prone areas, and (3) how prepared individuals and communities are for evacuation.

Risk, as used in this report, is a concept used to give meaning to things, forces, or circumstances that pose danger to people or what they value (see also Box 2.1). It takes into account

BOX 2.1


Risk is a concept used to give meaning to things, forces, or circumstances that pose danger to people or what they value. Risk descriptions are typically stated in terms of the likelihood of harm or loss of a vulnerable thing or process (e.g., health of human beings or an ecosystem, personal property, quality of life, ability to carry on an economic activity) due to a physical event (i.e., hazard) (National Research Council, 1996a). Some researchers have used the term risk to quantify the likelihood of future tsunamis, while others have defined it as a product of the probability of tsunami-attributable social damage (e.g., buildings, lives, businesses) and the magnitude of that damage. Research shows that managers, policy makers, and members of the public rarely define risk as an objective calculation; instead, perceptions vary according to differences in awareness, experiences, and social context (Fischhoff et al., 1984; Weichselgartner, 2001).

Hazard is the physical characteristics of an event (e.g., tsunami: speed of onset, impact forces, currents, inundation area) that can pose a threat to people and the things they value.

Vulnerability is the personal or situational conditions that increase the susceptibility of people or resources to harm from the hazard.

Inundation refers to the process of coastal flooding due to tsunamis or storm waves regardless of the impact to human activities.

Run-up height is the vertical elevation of the most landward penetration of the tsunami wave with respect to the initial sea level (figure opposite page). Run-up is a vertical distance, while inundation is a horizontal distance.

Inundation models determine the areas likely to be flooded by a tsunami and involve numerical computations of tsunami evolution for specific tsunami scenario or consider an ensemble of tsunami scenarios that might affect the map area.

Hazard maps depict inundation areas on base maps that typically include contours, imagery, buildings, roads, and/or critical infrastructure and take into account local geologic knowledge.

Evacuation maps depict areas that need to be evacuated in the event of a tsunami and to show evacuation routes to safe havens. Evacuation maps are based on the same inundation zones in hazard

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