Tsunamis have reportedly affected Southland or areas nearby since the early 1800s.
Prior to about the year 2000, most of the recorded tsunamis were relatively large ones, the effects of which were quite apparent to the eye.
In more recent times, as coastal water level recorders became more prevalent and recording intervals were decreased, it has become possible to record the occurrence of much smaller tsunamis, the effects of which were not readily apparent.
These measurements have been particularly useful. Firstly, they help confirm the tsunami travel times from a source regional to various places on the Southland coast (big tsunamis travel at the same speed as little ones) and, secondly, they give an indication of which places are most likely to be affected by tsunamis from the different source regions and by how much. However, no two tsunamis are the same so the tsunami size/earthquake magnitude relationship for any given source region varies from tsunami to tsunami.
When people think of tsunami, they normally think of waves or walls of water. They are the most obvious effects in smaller tsunami there is more likely to be a faster than usual fall then rise in water level, or vice versa, than a wave per se.
Any change in water level is associated with a current as the water moves in or out. In tsunamis, the water level changes much more rapidly than it does with the normal rise and fall of the tide. As such, the currents during a tsunami are much stronger than normal. Not only that, they also change direction much more frequently than they would with the normal rise and fall of the tide. So, instead of changing direction every six hours or so, the currents change direction several times an hour. This presents a significant risk to boating and shipping activity, especially in ports, enclosed waters and mooring areas.
These days, the threat from potential tsunamis is classified as a marine threat only (small waves and strong currents), or a land and marine threat (waves that flood normally dry land and very strong currents).
In large tsunamis the combined effect of flooding and strong currents gives rise to another hazard, that being the fast moving water borne debris that collides with any people and property in the path of the tsunami.
Tsunami warning network and sea level recorders
There are a number of international and regional tsunami warning networks that tend to be integrated via the Pacific Tsunami Warning Centre, based in Hawaii.
The New Zealand tsunami warning network, established in 2010, consists of 17 sea level tsunami gauges strategically placed around the New Zealand coast and offshore islands.
One gauge is located at Puysegur Point, on the south west corner of the South Island. Warnings of tsunamis from the north and east are provided by gauges located at Dunedin, Christchurch and the Chatham Islands.
In addition to the above network, there is a sea level network gauge located at Dog Island, Foveaux Strait. This gauge is operated by NIWA and since its inception in 1997 has recorded several tsunami from multiple sources. The gauge also monitors storm surge and rissaga, a term used to describe meteorologically induced tsunami.
Environment Southland also operates its own water level recorders in tidal locations at Stead Street, in Invercargill (New River) Estuary, the Waiau River lagoon and Freshwater Basin, at Milford Sound.
Areas most at risk
Tsunamis may be generated by distant regional or local sources, examples of which are South America, the Macquarrie Ridge to the south of New Zealand and the Puysegur subduction zone, respectively (see the tsunami inundation modelling report for Riverton and New River Estuary and the Review of Tsunami Hazard and Risk in New Zealand).
The biggest threats to the Southland coast are considered to be distant source earthquakes generated off the coast of Peru and local source earthquakes generated within the Puysegur subduction zone, south west of the South Island.
Peru sourced earthquakes are considered a greater threat then Chile sourced earthquakes because of the orientation of the seabed ruptures that create the tsunami.
The Chile tsunami was generated by a Mw 8.8 earthquake 60 miles NNE of Concepcion, Chile
It is interesting to note that in the past, the Southland coast has been affected by five South America sourced tsunamis, one Peru sourced and four Chile sourced, but two of the Chilean tsunamis were close to the Peru border.
It is difficult to be certain about which was the worst. Generally though, all were associated with water level changes of 0.5-1.5 metres. Nationally, the 1868 tsunami is considered to be the largest from South America.
There is no confirmed record of post tsunamis from the Puysegur subduction zone, but this area is possibly the cause of the early 1800s event recorded in Te Waewae Bay.
Local tsunami can also be generated in Fiordland, but past events suggest that the effects are mainly confined to the fjords, or directed towards Westland and Australia. Local tsunami travel times are very short – less than one hour. As such, the best warning is the earthquake shaking itself. Unlike distant sourced tsunami which have travel times of 3‑15 hours, there is insufficient time to distribute formal warnings.
Landslide induced tsunamis in lakes
Landslides, whether they are earthquake induced or otherwise, have historically created significant localised tsunami in some of the fjords. The August 2003 landslide induced tsunami in Gold Arm, Charles Sound, is well documented (see Environment Southland Tsunami and Seiche Study). The size of landslide induced tsunamis is dependent on both the size and speed of the landslide.
There has often been concern expressed that a landslide induced tsunami could occur on Lakes Te Anau or Manapouri. Consequently, Environment Southland arranged for GNS Science to investigate this possibility in 2011/12. Thirty-one potential landslide areas were investigated. The investigation found that most of the earthquake induced landslides around the two lakes were likely to be shallow, small to moderate rock fall and debris slides, capable of producing tsunami run up heights of 0.5-1.0 metres within one kilometre of the landslide areas. This is not a significant risk. However, a couple of sites on Lake Manapouri were identified as having the potential to generate 5-10 metres, or 10‑25 metres. While these landslides present a degree of risk, they area several kilometres away from the nearest land based facility. They certainly don't present any risk to any township.