Gusman, A.R.; Lukovic, B.; Wang, X.; Burbidge, D.R. 2024 Tsunami action maps and coastal tsunami warning area breakpoint names update. GNS Science report 2024/36. 82 p.; doi: 10.21420/6JWC-6G15
Abstract
The National Emergency Management Agency (NEMA) has asked GNS Science to create an updated set of Tsunami Action Maps similar to those previously made for the Central Kermadec Zone. The Tsunami Action Maps delineate areas for immediate evacuation and those under assessment. They are designed to be used shortly after a local or near regional earthquake occurs in order to help facilitate a rapid response to any potential tsunami. This project extends the previous Kermadec Zone Action Maps to include an additional five earthquake source zones. This brings the number of earthquake source zones with Action Maps to a total of six and covers all offshore plate margins within a 90-minute tsunami travel time of New Zealand. The project created a total of 44 Action Maps for this purpose. The new Action Maps produced in this report are based on the maximum offshore tsunami height expected from an earthquake in that zone within a given magnitude range. We call these new maps an “ensemble” Tsunami Action Map since they are based on assuming that the worst possible case tsunami is generated by any earthquake in that source zone in that magnitude range from an ensemble, or set, of possible earthquakes. Each ensemble Action Map is accompanied by a text file that describes the map in words. Since these maps are static and relatively few in number, they have the advantage of allowing a very rapid response, since they can be set up within NEMA’s system well before the earthquake. However, these static ensemble Action Maps are often more conservative than the dynamic ones currently generated by the Rapid Tsunami Threat Mapper (RTTM) algorithm in the National Geohazards Monitoring Centre (NGMC). The NGMC Action Maps are based on specifics of the earthquake’s location and magnitude, rather than using the worst case across an ensemble of possible earthquakes. We call the maps currently produced by the NGMC the “event-specific” Action Maps to differentiate them from the ensemble Action Maps provided here. We show in the report that these ensemble Action Maps can often result in more “Coastal Tsunami Warning Areas” (also referred as “coastal zones”) requiring immediate evacuation than what would be required if an event-specific Action Map was used.The decision on the magnitude threshold for use of the ensemble maps is a matter of balancing the advantage of speed of response with the disadvantage of potentially over-evacuating. We provide and discuss four options for implementing these maps. Given that using the event-specific Action Maps is not currently viable as they are too slow to use in the current NEMA system, we recommend that the event-specific Action Map produced by the NGMC is used as a trigger for when these ensemble maps are used. If the event-specific map indicates that one or more Coastal Tsunami Warning Areas requires immediate evacuation, then the pre-loaded ensemble map would then be used instead to determine which areas should be immediately evacuated. This would mean that immediate evacuation does not occur more frequently than current practice but that a larger area would be immediately evacuated when it does occur. In this case, it would also be much faster to send out the immediate evacuation alert since all possible Action Maps could be pre-loaded into NEMA’s system. There are other options for their implementation though and the final decision of how this should be implemented is a matter for NEMA and the GeoNet Programme. Long-term, we recommend that the event-specific maps are used instead of these ensemble Action Maps once the event-specific maps can be efficiently and easily ingested into NEMA’s Tsunami Warning System with minimal delay. Additional to these Action Maps, NEMA sought to revise the names of breakpoints between Coastal Warning Areas based on feedback from Civil Defence Emergency Management (CDEM) groups. GNS Science has collaborated with NEMA to address concerns and propose improved breakpoint names, ensuring clarity and specificity across all coastal regions in New Zealand. The new names are also presented here and appear on the new ensemble Action Maps. (auths)
