Evaluation of Australian Tsunami Warning Thresholds Using Inundation Modelling

Tsunami warnings issued by the Joint Australian Tsunami Warning Centre (JATWC) are derived from a database (T2) consisting of more than two thousand pre-computed tsunami scenarios. Following any potentially tsunamigenic earthquake, warnings are issued for individual coastal zones with three different levels of threat: Land Threat, Marine Threat or No Threat. The decision is based on the 95th percentile (P95) of the maximum wave amplitudes (over time) of the relevant T2 scenario within each coastal zone. Threshold values for P95 have previously been derived through analysis of observed impacts for recent events. Given that historical records are available for only a short time period and no observations exist for which a Land Threat would have been issued for Australia, it has been difficult to determine the appropriate threshold for a Land Threat. Several recent tsunami hazard assessment studies have used inundation models nested within T2 scenarios. These modelling results are used to evaluate the threshold values for JATWC tsunami warnings and provide guidance on a possible further warning tier—Major Land Threat. The optimal Land Threat threshold for P95 is found to be 48.5 cm, however, it is not recommended that any changes are made from the existing operational threshold of 55 cm. The optimal threshold for P95 a Major Land Threat is found to be 150.5 cm.