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Abstract: In trying to mitigate natural hazards, society plays a high-stakes game against nature. Often nature surprises us when an earthquake, hurricane, or flood is bigger or has greater effects than expected from detailed natural hazard assessments. In other cases, nature outsmarts us, doing great damage despite expensive mitigation measures. These difficulties are illustrated by the March 2011 earthquake off Japan’s Tohoku coast. The earthquake was much larger than anticipated from hazard maps and generated a tsunami much larger than anticipated, which overtopped coastal defenses, causing more than 15,000 deaths and US$210 billion damage. Similar situations occur in predicting earthquake ground shaking (Stein...
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Recent large earthquakes that did great damage in areas predicted to be relatively safe illustrate the importance of criteria to assess how well earthquake hazard maps used to develop codes for earthquake-resistant construction are actually performing. At present, there is no agreed way of assessing how well a map performed and thus whether one map performed better than another. The metric implicit in current maps, that during the chosen time interval the predicted ground motion will be exceeded only at a specific fraction of the sites, is useful but permits maps to be nominally successful although they significantly underpredict or overpredict shaking, or to be nominally unsuccessful but do well in terms of predicting...
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Following the 2011 magnitude 9.1 Tohoku earthquake, Geller (2011) argued that “all of Japan is at risk from earthquakes, and the present state of seismological science does not allow us to reliably differentiate the risk level in particular geographic areas,” so a map showing uniform hazard would be preferable to the existing map. We explore this by comparing how well a 510‐yr‐long record of earthquake shaking in Japan is described by the Japanese national‐hazard (JNH) maps, uniform maps, and randomized maps. Surprisingly, as measured by the metric implicit in the JNH maps (i.e., a metric that requires only a specific fraction of the sites during the chosen time interval to exceed the predicted ground motion), both...
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Earthquake forecasts often prove cloudy. Seth Stein and Anke Friedrich peer through the mist to examine the challenges and prospects for earthquake hazard mapping and how we should decide how much to spend on mitigation.
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Recent large earthquakes that caused ground shaking larger than anticipated have generated interest in how to improve earthquake hazard mapping. Issues under discussion include how to evaluate maps’ performance, how to assess their uncertainties, how to make better maps, and how to best use maps given their limitations. An important question is what to do after an earthquake yielding shaking larger than anticipated. Hazard mappers have two choices. One is to regard the high shaking as a low‐probability event allowed by the map, which used estimates of the probability of future earthquakes and the resulting shaking to predict the maximum shaking expected with a certain probability over a given time (Hankset al.,...
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