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Tsunami research 2013-14

Caption: Tsunami height (maximum amplitude) in metres at 50th percentile at return period of 2,500 years. Degrees longitude shown on horizontal axis; degrees latitude on vertical axis. Data: GNS Science.

Review of Tsunami Hazard in New Zealand

A report summarising New Zealand’s tsunami hazard was prepared in 2013 for the Ministry of Civil Defence and Emergency Management. The ‘Review of Tsunami Hazard in New Zealand (2013 Update),’ examines all likely sources of tsunami that could affect New Zealand. The review builds on the previous 2005 report and summarises the current state of knowledge and highlights new research. A substantially revised probabilistic hazard model has been constructed for this report, which for the first time estimates the tsunami hazard for all parts of the New Zealand coastline.

The review focusses on quantifying tsunami hazard caused by earthquakes ─ the likely size of resulting tsunami over specified timescales, along with estimates of uncertainty. The hazard posed by tsunami generated by landslides and volcanic activity has also been carefully considered. It is not yet possible to quantify the hazard from these sources, though research towards this goal is being undertaken.

Japan tsunami Stuart Fraser 400

Aftermath of Tohoku tsunami 2011. Photo: Stuart Fraser and IStructE Earthquake Engineering Field Investigation Team

The 2011 Tohoku tsunami in Japan illustrated some of the key changes in scientific knowledge. The tsunamis of the Indian Ocean (2004) and South Pacific (2009), followed by Tohoku were produced by earthquakes substantially larger than considered likely to occur at those locations. These earthquakes contradicted previous geophysical assumptions about the maximum magnitudes of earthquakes that could be created on tectonic plate boundaries.

There are now far fewer restrictions on possible maximum magnitudes than previously thought, and the new probabilistic model attempts to account for this. It is now known that there was a similar tsunami in Japan in AD 869, indicating that the interval between the largest earthquakes there is over a thousand years. The tectonic plates in Japan are converging twice as fast as those around New Zealand, which suggests that the interval between the largest earthquakes on our local plate interfaces could be in excess of 2,000 years. The important implication here is that our brief historical record of 200 years can, on its own, provide very little guidance in estimating the magnitude of the largest earthquakes that New Zealand may experience.

To improve estimates of the earthquake potential of subduction plate interfaces around New Zealand, where one plate is pushed below another, we must study the evidence of prehistoric tsunami and earthquakes (paleotsunami and paleoearthquakes) in the geological record, and work with the global community to find new, statistically valid, geophysical methods for estimating earthquake potential.

The movement between the tectonic plates in the Tohoku tsunami was very non-uniform—in some areas the plates moved more than 50 metres whereas in many other areas the movement was much less, typically around 5 to 10 metres. This ‘non-uniform slip’ has important implications for tsunami, as the distribution of movement between the plates affects the motion of the seabed, which determines the size of tsunami. The probabilistic model in the report attempts to incorporate the effects of this phenomenon to a first level of approximation - this is at the cutting-edge of current science and the analysis represents a first attempt at tackling this important problem.

The greater uncertainty that now exists regarding the maximum size of earthquakes on plate boundaries has led to an increase in the estimated hazard from tsunami triggered by local and regional sources. While the overall levels of estimated tsunami hazard did not change greatly from the 2005 report, the estimated hazard generally increased in those areas most exposed to tsunami from local subduction zones – notably the east-facing coasts of the North Island, and the southwest corner of the South Island.

Links:

  • John Hamilton, MCDEM Director, interviewed by Radio NZ about the National Tsunami Hazard Model and actions needed.
  • Development of MCDEM's National Tsunami Hazard Model led by William Power (GNS Science), which for the first time includes all coasts of New Zealand.

Contact: William Power, GNS Science

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Last updated 10 Nov 2014