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.

– NIWA