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New Zealanders in coastal areas are right now preparing for potential wave surges as tsunami warnings are in force for the entire coast of the country.

This follows the 8.8 magnitude earthquake in Chile last night – my colleagues at the Science Media Centre rounded up some analysis of the quake from British scientists last night.

Preparedness for possible tsunamis has been good this time, following a patchy response from Civil Defence in the wake of the earthquake late last year which triggered a tsunami off the Samoan islands. It looks like New Zealand will escape any major damage from this tsunami, but a tsunami originating closer to home or resulting from a higher magnitude quake as far away as South America could have devastating consequences for the country, according to a tsunami risk assessment by Civil Defence updated just a few days ago.

As the report states:

A 2005 review showed estimated damages of $12-21 billion nationally from a 500-year return period tsunami — approximately 10% probability of occurrence in 50 years, or annual probability of 0.2%.

The risk in terms of mortality in the 19 urban centres assessed in the review (for the same return period) arises from losses in many towns and cities in New Zealand, but is predominantly from those along the east coasts of the North and South Islands as a result of large earthquakes in South America (about 60% of total deaths) or along the Hikurangi subduction margin of the eastern North Island (about 34% of total deaths). Tsunamis generated by offshore local faults are likely to account for 5% of deaths and those generated by regional sources 1%.

So what are the chances of a massive earthquake triggering a one in 500 year tsunami from the South American region? Well, researchers up until now have really had the massive 1960 earthquake in Chile to model things on:

…the 1960 tsunami, although caused by a much larger earthquake than the 1868 event (magnitude 9.4, possibly 9.5), occurred on a part of the South American plate boundary that is not as well oriented to New Zealand as the 1868 location. It produced a smaller tsunami in New Zealand than would have occurred had the location been ideally oriented.

Nevertheless, the 1960 tsunami caused run-ups of up to four metres in parts of the North and South Islands. The magnitude of the 1960 earthquake probably represents the upper limit for earthquakes for the whole South American coastline (and worldwide).

Computer models (Power et al., 2004), combined with historical observations, suggest that South American earthquakes with magnitudes less than 8.5 generate a minimal risk of a damaging tsunami in New Zealand. The historical record of Peru and Chile, which is hundreds of years longer than New Zealand’s, indicates that large earthquakes and tsunami have occurred relatively frequently in the past 450 years. Nine earthquakes with estimated magnitudes >8.5 caused nearsource run-up heights near to, or greater than, those produced locally by the 1868 event, and hence probably produced significant tsunami in New Zealand prior to
European settlement.

The Civil Defence report notes that while there has been little damage and few deaths from tsunamis in New Zealand since European settlement, Maori traditions make note of tsunamis that killed many people. Potentially then, the devastation in the last 1000 years or so was caused by larger that 8.5 magnitude quakes in South American that triggered tsunamis.

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A presentation from Victoria University’s Dr John Townsend on the Chilean earthquake: