NZ’s Tsunami Monitoring Network

New Zealand is establishing a tsunami monitoring network, due to be completed by 2010.

GNS Science is partnering with Land Information New Zealand and the National Institute of Water and Atmospherics to establish a tsunami monitoring network to cover New Zealand territory. The network will consist of 20 stations located in three distinct areas – off-shore islands, at-risk coasts and vulnerable population centres.

Each station will monitor relative sea level using two submerged pressure sensors, and attached logging equipment will record the readings ten times per second. Associated communications gear will transmit the data to the GeoNet Data Management Centre in Lower Hutt using satellite, radio or terrestrial communication networks depending on the station’s location and priority.

The network is intended to provide information on incoming and regional tsunami events as well as detect the first landfall of a wave on the main islands of New Zealand. By interpretation it will also allow an “all clear” to be issued after a wave has been recorded near one of the vulnerable population centres.

Five stations will be operated at off-shore islands at Norfolk, Raoul, Macquarie, Antipodes and Chatham islands. The stations at Norfolk Island and Macquarie Island will be installed by Australia.

Evidence of past tsunami events indicates that the northern, eastern and southern coasts of New Zealand are most at risk from tsunami events, compared with studies of the west coast. The location of eight coastal monitoring stations reflects this with sites at North Cape, Hokianga, Great Barrier Island, East Cape, Mahia, Castlepoint, Kaikoura and Puysegur Point.

Population centres identified as being at risk of tsunami are Auckland, Tauranga, Gisborne, Napier, Wellington, Christchurch and Dunedin. Seven stations will be installed at these locations to provide confirmation that a remotely detected wave has arrived and thus provide an “all clear” notification when the threat has passed.

This last stage is a key component that is currently provided by a mixture of calculation and guesswork owing to the complex nature of our varied coastal environment and the relative rarity of tsunami events for study. Being able to confirm that a wave has indeed been recorded at a specific location will allow people to get back to normal after an alert.

Installation work has already commenced, and a pilot site in Wellington Harbour successfully detected the tsunami wave generated by the magnitude 8.1 Solomon Islands earthquake of the 2nd of April 2007. The earthquake struck at 8:40 a.m. New Zealand time, and generated a tsunami wave over 3 metres high at Gizo and nearby islands. The wave was measured at 14 cm at Honiara about 2 hours later and 11 cm at Vanuatu just over 3 hours later. The trial gauge at Wellington detected the arrival of the wave from the Solomons, and confirmed that resonance occurred within the harbour.

Hazardwatch reported that the peak-to-trough heights of the tsunami in New Zealand coastal waters reached a metre at susceptible west coast sites, with this height being measured at a NIWA recorder at Charleston. It also noted that a witness reported seeing 20 cm changes every half hour in Wellington Harbour at lunchtime on April 3rd.

Work on installing the monitoring stations is underway, with the Napier station built in September, the Chatham Island station established in December and the Gisborne station was installed last week.

Wellington is to host two pilot stations to test the two different installation styles. As mentioned, the harbour site at Queens Wharf was completed early last year. A more complex ocean bottom site will be installed off Wellington’s south coast near the airport soon. This site will test the strength and integrity of equipment bolted to the ocean floor in a region susceptible to large swells.

Once commissioned, data from the network will be made available to the public via the GeoNet website, and raw data may be available by May this year. In operation, the network will be used to alert the Ministry of Civil Defence and Emergency Management of a tsunami threat. It will then be the ministry’s responsibility to issue civil defence warnings and organise an appropriate response. It is not anticipated that the network will replace the warnings currently supplied by the Pacific Tsunami Warning Centre. It will, instead, augment this information and enable the issuing of local alerts and tracking the progress of tsunami near our coasts.

Data collected will be made available to the international tsunami community improving our knowledge of tsunami and enhancing management of actual events.

Earthquakes are responsible for generating 75% of Earth’s tsunami waves, with the remainder being caused by volcanic activity, landslides and meteorological activity. Ten percent are still unexplained. Geoscience Australia has several interesting pages regarding tsunami on their website. The causes of tsunami are well-explained here but dial-up users should plan a coffee break if they intend to download the two animated maps showing the Sumatra quakes of 2004. The progressive layers of aftershock activity is fascinating, but rather a large download. The other images load quickly, however.

One apparent gap in New Zealand’s tsunami monitoring network is the absence of a station on either the Snares or Auckland Islands to detect a tsunami wave generated in the Puysegur Trench subduction zone to the south of New Zealand. I queried this with GNS Science, remembering the magnitude 7.3 and 6.6 earthquakes in the area on September 30th last year. These quakes were strongly felt in the lower South Island and caused some public alarm over the possibility of tsunami waves.

The siting of a station in the area was considered as part of the project. However, it was determined that the transit time of a tsunami wave generated in the southern part of the Puysegur Trench was likely to be 25 minutes to the Auckland Islands and 35 minutes to Puysegur Point. When the cost and benefit of the Auckland Island station was weighed, it was decided that the 10 minutes gained from a station on the island was insufficient to enhance the warning given to the public.

This highlights the fact that warning networks can only be used to alert the public of tsunami threats for the more distant events. Waves generated by seafloor movements or undersea landslides near the New Zealand coast would wash ashore before any useful warning could be issued. This was illustrated in March 1947 when two waves swept ashore on the East Coast 7 minutes after a heavy off-shore earthquake.

In any case, not all undersea earthquakes generate tsunami. The quake needs to be relatively large, displace the seafloor (or trigger landslides), and be relatively shallow. The Puysegur Trench quake of September did generate a small wave whereas the magnitude 6.8 quake off the coast of Gisborne (also near a subduction zone) on December 20th did not.

[Compiled from data provided by the Geonet project and its sponsors EQC, GNS Science and FRST; the US Geological Survey and its contributing agencies; GeoNet News Issue 9, January 2008.]

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