Chapter 4 . Tsunami Probability

Evaluating the probability of tsunami occurrence is a crucial step in the assessment of tsunamis hazards. Deterministic tsunami hazard studies involve hydrodynamic modeling of tsunami propagation, runup, and inundation from a particular source, usually defined as the maximum credible earthquake, landslide, or another tsunami trigger. Scenario-based modeling such as this is useful in emergency planning, but transferring the modeling results to other applications, such as estimating risk, is difficult. Risk assessment relies heavily on determining the probability that a tsunami of a certain size will occur within a given time frame. A tsunami hazard curve that plots tsunami size against probability for a given exposure time (T) is a central concept in such analyses (Fig. 4.1). There are two ways in which a tsunami hazard curve can be used. The most common way is specifying a particular probability and exposure time of interest, and then determining the magnitude that a hazard variable (e.g., runup R) that will be met or exceeded (Fig. 4.1a). An example of such an approach is flood hazard analysis for insurance applications in which the wave height and extent are determined for annualized probabilities (T=1 yr.) P=0.01 and P=0.002 (Houston and Garcia, 1978). Alternatively, engineering applications may specify a risk tolerance value for a particular structure and use the hazard curve to determine the probability that that value will be met or exceeded during the exposure time (Fig. 4.1b). In addition to tsunami hazard curves for a particular site, probabilistic-based hazard assessment tools commonly include regional assessments (Rikitake and Aida, 1988; Geist and Parsons, 2006) and probabilistic inundation maps (Tsunami Pilot Study Working Group, 2006).