Tonga Tsunami at Kaingaroa 

Data kindly supplied by National Institute of Water and Atmospheric Research, NIWA

Commentary

Kaingaroa, on the north eastern tip of Chatham Island, was not affected by meterologically generated long waves at the time of the event, so the tsunami waves stand out. The arrival time of the first waves was 06:50 NZST, 3.4 h after the earthquake, indicating that the waves took a direct route south from Tonga.

Figure 1. Tonga tsunami at Kaingaroa, Chatham Island showing: A. long wave record; B. significant (red) and maximum (blue) wave heights, and high tide times (green diamonds); C. period of significant waves; and D. distribution of significant wave height with time and period. EQ denotes the time of the earthquake. Times are in NZST (UTC + 12).

Notice in plots B and D that waves appear to have occurred before the arrival time of the first waves indicated in plot A. This is because in calculating the significant wave heights, a 6-hour centered-window is used. Such a strategy works for meteorologically generated long waves which gradually build up over a day or two, then die away over a similar period. In contrast, the tsunami waves in this event have arrived all of a sudden. This raises a question as to how representative wave heights should be calculated for tsunami. The plot below shows the individual wave heights. Notice that the wave period shows no perceptible change with the arrival of the tsunami waves.

Figure 2. Tonga tsunami at Kaingaroa showing: A. long wave record; B. height of each wave; and C. period of each wave. EQ denotes the time of the earthquake (03:27 on 4 May 2006, NZST), TOA denotes time of arrival of first waves (06:50 on 4 May 2006, NZST).

How were these calculated?
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