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EARTHQUAKES

Measurable tsunami waves hit New Zealand on cue after Japan earthquake
Wednesday 16 March 2011, 3:52PM
By NIWA
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New data from NIWA shows waves generated by last Friday’s magnitude 9.0 Japan earthquake reached all coasts of New Zealand, as predicted, and even registered on a sea-level gauge at Scott Base, in Antarctica.

Results from 19 sea-level gauges show peak wave amplitudes (the height of the wave above predicted tide levels) ranged from 0.78 metres at Whitianga, Kaingaroa (Chatham Islands) and Timaru to 0.05 metres in Scott Base.

Whitianga and Kaingaroa (Chatham Islands) experienced the largest waves at 1.6 metres (from the crest to the trough of the wave). Mount Maunganui (Moturiki Island), Charleston (near Westport), and Timaru also recorded wave heights over a metre.

Understanding the peak wave amplitude is important to assess the tsunami hazard on land from a sudden surge in water above normal predicted tide levels that could cause significant inundation. The height of the wave (from the crest to the trough) provides information about the possible tsunami hazard in or on the water – for example, how boat moorings, mussel farms or ports will be impacted by surging currents generated by the waves.

The first waves to hit New Zealand as a result of the earthquake occurred at least 12 hours after the earthquake first hit Japan at 5.46pm New Zealand time on Friday 11 March.

At most sites the highest waves recorded didn’t occur for some time after the first wave arrived. In Timaru and Sumner, the largest wave height didn’t occur until 33 to 40 hours after the first wave hit.

The delay in arrival of the largest waves from a distant tsunami source is due to waves bouncing off continental shelves all around the Pacific including South America and local coastal headlands and offshore ridges, diminishing slowly over several days.

“It’s important to understand that tsunami waves can continue for some time after the first few waves hit. It’s not safe to assume that once you have seen one wave, the risk subsides and all returns quickly to normal. We are still recording obvious wave heights at our sea-level gauges now, days after the earthquake, which are affecting currents in harbours and estuaries,” says NIWA Principal Scientist, Dr Rob Bell.

“The size and timing of the waves recorded by the sea-level gauges are very similar to those estimated in the response phase of this event. The accuracy of this information is vital for civil defence and emergency management when they are making decisions about tsunami warnings, to ensure the hazardous impacts of a tsunami event in New Zealand are minimised.”

Sea-level gauges around New Zealand are operated by various agencies including NIWA, port companies, regional and district councils and complement the operational real-time monitoring undertaken by GNS Science through GeoNet (New Zealand’s geological hazard monitoring system).

NIWA and GNS are anchor partners in the Natural Hazards Research Platform, and undertake research that focuses on understanding the impact of tsunami on New Zealand.

Table1:  Location of source of New Zealand sea-level gauge datasets (shaded rows show gauges that record at 5 minute intervals, all other gauges record sea-levels each minute) 

Station Latitude (°N) Longitude (°E) Operating agency
Kaingaroa

Chatham Is.

–43.7315 183.733 NIWA
Moturiki Is. –37.6304 176.186 NIWA
Little Kaiteriteri –41.048 173.027 Tasman District Council
Marsden Pt. –35.842 174.5 Northland Regional Council
Whitianga –36.833 175.709 Environment Waikato
Sumner Head –43.570 172.773 NIWA
Lyttelton Port –43.6058 172.7222 Lyttelton Port Co. Ltd
Timaru Port –44.392 171.254 PrimePort Timaru Ltd
Green Is. –45.9523 170.3867 NIWA
Dog Is. –46.652 168.412 NIWA
Jackson Bay –43.957 168.616 NTC, Bureau of Meteorology
Charleston –41.908 171.433 NIWA
Port Taranaki –39.055 174.033 Port Taranaki Ltd
Kapiti Is. –40.842 174.938 NIWA
Kawhia Wharf –38.0659 174.8232 Environment Waikato
Anawhata –36.921 174.461 NIWA
Poutu Pt. –36.362 174.182 Northland Regional Council
Tararu (Firth Thames) –37.128 175.521 Environment Waikato
Kaikoura –42.415 173.703 NIWA
Scott Base (Ross Sea) –77.85 166.767 NIWA/Antarctica NZ

Table 2:  Primary measures of the recorded tsunami waves in NZ (where peak waves are recorded according to peak-elevation amplitude of de-tided wave) 

Station Elapsed arrival time

[hrs]

Elapsed time of peak from first arrival

[hrs]

Peak wave amplitude

[m]

Associated wave height: crest-to-trough (max wave height)

[m]

Marsden Pt. 12.89 4.00 0.34 0.59
Whitianga 13.04 7.05 0.78 1.53 (1.61)
Moturiki Is. 13.03 2.55 0.52 0.97 (1.04)
Kaingaroa

Chatham Is.

13.81 2.42 0.78 1.64
Kawhia Wharf 14.04 12.30 0.20 0.36
Port Taranaki 13.94 11.72 0.38 0.72 (0.75)
Kapiti Island 15.59 8.24 0.22 0.34 (0.38)
Little Kaiteriteri 16.03 21.80 0.33 0.46 (0.55)
Charleston 14.83 5.93 0.47 0.94 (1.10)
Jackson Bay 14.43 4.63 0.37 0.71
Sumner Head 16.94 32.50 0.55 0.76 (0.97)
Lyttelton Port 17.24 16.94 0.55 0.86
Timaru Port 16.44 40.15 0.78 0.97 (1.19)
Green Island 16.13 6.96 0.31 0.52 (0.55)
Dog Island 18.13 9.01 0.28 0.40
Anawhata (5 min) 13.39 5.92 0.34 0.67
Poutu Point (5 min) 13.48 7.83 0.17 0.29 (0.35)
Tararu (5 min) 14.56 4.58 0.29 0.50
Kaikoura (5 min) 16.06 31.58 0.38 0.64 (0.68)
Scott Base (5 min) 21.73 15.00 0.05 0.07 (0.10)

Note: Alternative (max wave height) is the maximum where it didn’t coincide with the peak amplitude.






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