The First Wave May Not Be the Largest

The tsunami generated by the recent Chile earthquake nicely illustrates the warnings issued by Civil Defence authorities when New Zealand coasts are threatened by dangerous waves.

Saturday 6th March 2010

The Pacific-wide tsunami generated by last Saturday’s magnitude 8.8 earthquake in Chile was not as destructive as first feared, but large waves did reach New Zealand shores.

The National Institute of Water and Atmospheric Research has analysed measurements made on New Zealand’s east and west coasts, and finds that the highest waves didn’t occur for some time. The first territory of interest to New Zealand to experience the wave was Scott Base, but the waves there were small.

The first wave then reached Kaingaroa in the Chatham Islands at 7:10 a.m., 11 hours and 36 minutes after the earthquake in Chile. About an hour later, the wave reached East Cape, the first part of the North Island to experience the event.

Lyttelton experienced the largest waves, with a peak wave (crest to trough) of 1.9 metres. This wave arrived mid-afternoon, six hours after the first wave arrived there. Waves over a metre in height were experienced at Gisborne, the Chatham Islands, Sumner, Timaru and Whitianga.

The wave came ashore all along the western coasts of New Zealand as well, where peak heights were between 0.3 and 0.5 metres.

GeoNet’s tsunami gauges on March 1st

GeoNet’s tsunami gauges clearly show the disturbances that occurred at many locations on our coasts after the massive earthquake off the coast of Chile.

Last Sunday’s tsunami was, in fact, a teletsunami – a long-distance wave generated at some distance from our shores. The waves are very stable and can travel a long way. Writing in “Awesome Forces” Willem de Lange notes, “Over large distances the wave initially spreads out (like ripples in a pond), but eventually the curvature of the Earth starts to concentrate the wave again. At this point the tsunami increases in height and can become very destructive.”

The waves travel at about 700 km/h in the deep ocean but slow down when they encounter shallow water. De Lange adds, “When the sea is only 20 metres deep … their speed drops to 50 kilometres per hour … close to the coast tsunami start to increase in height. Measurements of tsunami waves in the deep ocean show that the wave height is usually less than half a metre. Given the right conditions, the same waves at the coast can reach heights of 30 to 35 metres.”

The problem facing New Zealand scientists and Civil Defence officials last weekend is now clear. The location of the earthquake gave several hours to monitor for a teletsunami and calculate its likely effect. However, the lack of deep ocean wave measuring instruments between here and Chile meant that coastal wave measurements made near the epicentre and at the Marquesas Islands had to be analysed to gain an impression of the size of the deep ocean wave approaching our shores.

Because the wave was generated by a displacement of the ocean floor off the Chilean coast, past experience suggested that the first wave would not necessarily be the largest. However, local conditions would dictate the experience of communities along our coasts. Poverty Bay is known to amplify certain tsunami waves, and Banks Peninsula has suffered more damage than other New Zealand locations during notable teletsunami generated near Chile in the past.

There is no universal rule for tsunami waves, however. Local waves generated by mud eruptions off the coast often feature two waves of similar height before the waves rapidly abate. Local tsunami generated by earthquakes close to New Zealand also behave differently, and can be larger than the earthquake magnitude might suggest. Observations made during last weekend will greatly improve the risk assessment for future tsunami events.

GeoNet wants to hear about the public’s experience of last week’s tsunami. They have created a questionnaire, and are keen to locate photographic and video material. Information gained will be used to enhance their modelling of tsunami originating near South America in the future.

Earthquake activity has continued at a high level at many locations along the Chilean coast over the past week. There have been thirteen quakes of sixth magnitude, and larger aftershocks are possible. Only one tsunami bulletin has been issued by the Pacific Tsunami Warning Centre for Chilean aftershocks since Sunday’s tsunami. It was for the magnitude 6.6 earthquake that struck 35 km north of Concepcion at 47 minutes after midnight this morning, New Zealand time. A widespread tsunami threat was not triggered by this event.

[Compiled from data provided by the GeoNet project and its sponsors EQC, GNS Science and FRST, National Institute of Water and Atmospheric Research media release, Pacific Tsunami Warning Centre, and the US Geological Survey and its contributing agencies. “Awesome Forces” Te Papa Press, Wellington, 1999.]

One Response to “The First Wave May Not Be the Largest”

  1. Flying Deldas says:

    An aftershock of 6.6 this morning!! Me muchos grazias that I am not living in Chile!

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