Modeling tools for the real-time evaluation and historical reconstruction of tsunami events in New Zealand

Abstract:

We assess tsunami hazards in New Zealand through a review of historical accounts, analysis of water level and current speed data and detailed numerical modeling. The tsunamis of 2010 (Chile) and 2011 (Japan) were recorded on tide gauges throughout New Zealand, providing a rich water level data set for model comparison and calibration. Furthermore, a current meter at the entrance to Tauranga Harbor also captured these tsunamis providing a unique current speed data set augmented by several concurrent water level records. Analysis of the current data from 2011 shows that although port operations were not adversely affected, tsunami currents may have exceeded thresholds for the navigation of large vessels through the narrow harbor entrance. Harmonic analysis of the current speed data also illustrates the effect of tidal flows on tsunami currents. This information was then used to calibrate numerical models using the ComMIT modeling tool. A sensitivity study for tsunamis generated from around the Pacific Rim indicates the relative hazards from different source regions. Deterministic scenario modeling of significant historical tsunamis provides a quantitative estimate of the expected effects from possible future great earthquakes. These models were tested in April 2014 after the Mw 8.2 earthquake offshore of Iquique, Chile – an event of particular concern given that the August 1868 Arica earthquake generated a tsunami of ~7 m in Lyttelton Harbor as well as runup of up to 10 m in the Chatham Islands. As the April 2014 event unfolded, it was initially unclear if an evacuation or other emergency response would be necessary in New Zealand given that a tsunami was observed and recorded on tide gauges and deep ocean tsunameters close to the source region. Models run in real time, using sources based on inverted tsunameter data and finite fault solutions of the earthquake, suggested that a damaging far-field tsunami was not expected. As a result, emergency response teams and port authorities were advised that is was safe to stand down. These results were ultimately confirmed when the tsunami was recorded in Lyttelton Harbor with a maximum amplitude of ~15 cm. Nevertheless, this event reminded us of New Zealand’s far-field tsunami exposure as well as the hazard from sources in South America and the Peru/Chile border region in particular.