Measurable tsunami waves hit New Zealand on cue after Japan earthquake

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) 

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) 

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