Tonga Tsunami at Selected NZ Ports

At 15:27 Z on 3 May 2006 (03:27 on 4 May NZST), a large earthquake (Magnitude 7.8) occurred in Tonga. A tsunami warning was issued by the Pacific Tsunami Warning Center (PTWC) as shown here.

The direct path from the earthquake to the nearest port (Marsden Point) is only 2090 km, so we would expect the waves to arrive about 4 hours after the earthquake (07:30 NZST). In fact, the waves that arrived at Marsden Point were so small, they can hardly be distinguished from the background meteorologically generated waves (called Far Infra Gravity or FIG waves). However, the tsunami was felt at other east coast ports, though the main waves did not arrive until 12 or more hours after the earthquake, so they must have taken a roundabout route.

Subsequently, NIWA has kindly supplied data from their sea-level recorder at Kaingaroa, on the northeast tip of Chatham Island, almost due south from the earthquake location, and 900 km east of mainland New Zealand. Unlike the NZ coast sites, there is almost no continental shelf off Kaingaroa, so it receives waves from the Pacific first. In the deep ocean, the waves are travelling at up to 800 kph, but on the continental shelf, they slow to less than 250 kph. Thus, Kaingaroa is an excellent tsunami early warning site for New Zealand, as illustrated in this event.

You can see the waves at a particular port by clicking on its name.

How were the tsunami waves calculated?