Tsunamis can hit coastal societies very shortly after large earthquakes. To mitigate their impact, tsunami early warning systems can alert the coastal population and civil authorities. This may allow evacuation, and mobilization of emergency services. Immediately after the earthquake notification, the destructive potential of an associated tsunami is highly uncertain. At the start, there is much uncertainty about the exact size, the exact location, and the exact mechanism of the earthquake and generation of the tsunami. This uncertainty means that tsunami early warning systems can occasionally issue false alarms, or may even fail to warn of an actual tsunami.
Current tsunami early warning systems provide single-outcome forecasts that can't account for this uncertainty. For the sake of safety, they will often overestimate the tsunami impact. This can however result in too many false warnings. The opposite could happen with a badly rigged system, with too many missed events.
Managing the risk of false alarms and missed alarms lies in the political sphere. However, decision-makers need to be informed by tsunami warning systems that accurately forecast the uncertainty. The PTF resembles modern numerical weather forecasts work by calculating multiple simulations – an ensemble – each with a slightly different starting point and slightly different model parameters. PTF works by simulating vast numbers of earthquake-tsunami scenarios, covering the full range of possible sources.
The principle is shown below. Using average forecasts, or incorporating a small part of the uncertainty, fewer false alarms are issued. However, the risk of missing events increases. Incorporating a very large uncertainty range, the missed alarms are almost eliminated. However, we thereby risk more false alarms. While the optimal solution may lie between these extremes, this selection process needs to be done through political choices by the responsible stakeholders. The PTF method provides a basis for this optimization, and links the optimal solution to the desired trade-off between false alarms, missed events, and correct warnings.
1) Istituto Nazionale di Geofisica e Vulcanologia, Le Grazie, 2) Italy, Department of Physics “Ettore Pancini”, University of Naples, Naples, Italy, 3) German Research Centre for Geosciences (GFZ), Potsdam, Germany, 4) Norwegian Geotechnical Institute (NGI), Oslo, Norway, 5) Grupo EDANYA, Universidad de Málaga, Málaga, Spain.
The Norwegian Geotechnical Institute (NGI) is a leading international centre for research and consulting within the geosciences. NGI develops optimum solutions for society, and offers expertise on the behaviour of soil, rock and snow and their interaction with the natural and built environment. NGI works within the markets Offshore energy; Building, construction and transportation; Natural hazards, and Environmental Engineering. NGI is a private foundation with office and laboratory in Oslo, branch office in Trondheim, and daughter companies in Houston, Texas, USA, and Perth, Western Australia. NGI was established in 1953.