Feature

Tsunami Surprise

We may be more vulnerable to tsunami than has been realised.

New Zealand may be more prone to tsunami than previously thought, judging from recent geochemical and sedimentological studies by a husband-and-wife team of Wellington scientists.

Institute of Geological and Nuclear Sciences Limited environmental geochemist Catherine Chagué-Goff and her husband James Goff, a geographer at Victoria University, have attracted international scientific attention by finding evidence of up to four previously unknown tsunami events while studying wetlands in the Abel Tasman National Park in the northwest of the South Island.

"While trying to establish the age of the wetlands and measure changes in sea level, we were astonished to find evidence of large and sudden saltwater inundation events," Goff says.

The changes in sea level were expected to be in the order of one or two millimetres per year, but instead evidence was found of of huge and sudden -- if temporary -- changes of sea level from tsunami.

At least 32 recorded tsunami have hit New Zealand since 1840, but the pair believe the real figure is well over 100, although most of these were relatively minor. Even less is known about New Zealand's pre-European tsunami history because there has been no serious attempt to find evidence of this phenomenon, but that is now changing.

The study involved taking core samples up to 1.4-metres long, radiocarbon-dating them and subjecting them to chemical and physical analyses. The oldest sediment in the cores was dated at AD 300. The analysis shows that since then the tidal flats and salt marshes have experienced several catastrophic saltwater inundation events, or tsunami.

Goff says Abel Tasman National Park is an ideal place to study sea level changes because there appears to have been no significant uplift there for the past 10,000 years. Other parts of New Zealand have experienced significant tectonic movement during that period.

"Studying sea level changes in New Zealand presents problems because many parts of New Zealand are either going up or down because of seismic activity," James says. "But the Abel Tasman National Park seems to be on the fulcrum -- the west is going up and the east is going down, while the park is an area in the middle where nothing happens. Several reports have concluded that there has been no uplift of any significance there for the last 10,000 years, since the last glaciation."

Wetlands are very sensitive areas ecologically, easily upset by changes in sea level. This will affect the plant life, for example, especially if rising levels turn a freshwater area into a saltwater one. Some plants favour fresh water and others salt water, and the same applies to micro-organisms, fossils of which can be found in the sediment. Geochemical analysis of sediment will reveal other clues; for example traces of sulphur and other "signatures" indicate when salt water has been present.

Goff and Chagué-Goff have investigated three inlets in the park area, Wainui, Awaroa and Totaranui, by taking cores from the sediment and peat there. Among the indicators discovered was an increase in zinc levels showing the arrival of Europeans with their corrugated iron roofs and buildings, and increases in the amount of sediment which can be attributed to the start of logging upriver from the wetlands. But the big surprise was the evidence of tsunami at all three sites. In this study, a tsunami event is characterised by a sharp increase in organic material and fine particles of silt and clay. There is a similar rise in the percentages of iron and sulphur.

The Lotteries-funded study, a joint effort between the institute and Victoria University's Geography Department, marks the first time that geochemistry has been used to detect tsunami events. The alignment of geochemical clues with a suite of tsunami "signatures" has produced a compelling picture of inundation.

Radiocarbon dating has helped to produce a tsunami chronology that matches the last two, perhaps three, major ruptures of the Wellington Fault, and the 1855 earthquake on the West Wairarapa Fault. Two tsunami -- dated at AD 1440 and AD 1220 -- were recorded clearly at more than one site. Two others -- at AD 350 and AD 1855 -- were less clearly defined.

Because the three wetland areas in the study are so sheltered, it would not be easy for a tsunami to affect them. At Awaroa Inlet on the east coast of the park, a tsunami travelled about 2.5 km into a sheltered estuary which is itself situated in a large, sheltered bay -- Golden Bay.

Chagué-Goff and Goff estimate that a big tsunami hits New Zealand once every few hundred years. They believe New Zealand's coastal wetlands may hold records of many pre-European tsunami.

"Tsunami are not necessarily huge, but they can cause serious damage. Perhaps we have been a little complacent regarding our coastal management work on these events to date," Goff says.

The pair call Cook Strait "Tsunami Alley" because its funnelling effect can increase the size of tsunami. Accounts of the 1855 tsunami in Cook Strait suggest it was 9-10m high. Goff and Chagué-Goff are now undertaking more detailed analysis of the cores in order to achieve more precise conclusions. They acknowledge that it is possible that these events were not tsunami.

"They might be cyclones that have coincidentally hit the coast around about the same time as a known earthquake has taken place. It is strange that there are no other such cyclone events recorded in the sediments, though," they say.

Major tsunami affecting New Zealand are usually caused by relatively close earthquakes, submarine landslides or submarine volcanic eruptions, while minor ones tend to be caused by distant earthquakes -- for example in Chile.

Using geochemical, radiocarbon and a raft of other techniques, it may eventually be possible to produce a map of New Zealand showing where tsunami have occurred, when they occurred and an estimate of their impact. It may indicate that some coastal areas are much more vulnerable to tsunami than others. Information such as this will be extremely useful for property owners, engineers, insurance companies and planners.

Victoria University of Wellington, Institute of Geological & Nuclear Science