When the tsunami warning was issued on Monday morning, substantial confusion reigned about the warning system. Sirens sounded in some places nearly two hours after warnings were formally issued by the Ministry of Civil Defence and Emergency Management.
Three sources of tsunami are known to exist in New Zealand and are:
- Near field tsunami that originate just off the coast and are caused by either under sea landsliding, tectonic uplift or in the case of the Bay of Plenty volcanic activity
- Mid field tsunami that originate from sources that are 2-5 hours travelling time from the coast of New Zealand – Samoa, Tonga-Kermadec Trench tsunami are in this category
- Far field tsunami that originate from places whose travel time is 6+ hours from New Zealand – the west coast of the America’s, Aleutian Islands and Kamchatka tsunami are all included in this category
In a far or mid field tsunami event, there is time for proper warnings. When a significant earthquake or other event capable of displacing a large volume of water occurs, a tsunami watch is instituted. This means capable of triggering a tsunam has occurred and to watch for signs of one. When a tsunami warning is issued, it is because a tsunami has been detected by instruments such as pressure gauges such detecting waves passing, or by observation of wave activity consistent with a tsunami – abnormal rises or falls of the sea level outside the 12 hourly tidal regime. As well as being passed on to local authorities, the information is passed on to the Pacific Tsunami Warning Center in Hawaii, which collates the information and estimate arrival times and potential wave run ups for individual nations around the Pacific Basin.
But the case is quite different if one is dealing with a localized event where the source of tsunami is nearby. In some cases the first sign of an approaching tsunami might be the tsunami itself. In that case the only thing to do is to immediately go to higher ground. On Monday morning, in the pre dawn darkness, with the knowledge a quake had occurred, but with significant confusion about tsunami warnings, many people who should have evacuated as soon as the shaking had stopped, were found waiting for warnings did not come until 2 hours after the tsunami.
The failure is unacceptable. Acting Minister of Civil Defence, Gerry Brownlee admits that the delays are not tolerable and could have led to loss of life. And if one keeps reading, it is not difficult to see why there must be a good working tsunami warning system.
Since European settlement tsunami from all three sources have occurred. As yet, no lives have been lost to tsunami since European settlement of New Zealand, but significant damage has been caused.
In January 1855 a magnitude 8.2 earthquake in Cook Strait caused a significant tsunami to occur in the Wellington area, which. Damage was minimal because of the small population in Wellington at the time. In March and May 1947 magnitude 6.9-7.1 earthquakes off the coast from Gisborne triggered damaging local tsunami events, but no lives were lost. A local tsunami was also observed in July 2009 following a magnitude 7.8 earthquake in Fiordland. No damage was caused and no lives were lost. It may have also triggered landslide tsunami in adjacent fiords.
In 2009 a magnitude 8.0 earthquake off the coast of Samoa triggered a tsunami that killed over 100 people in Samoa and caused widespread damage. It was observed in New Zealand as well, but no lives were lost and no damage was caused because of a combination of people deciding to evacuate and the authorities having time to warn of the incoming tsunami.
Possibly as early as 1868, tsunami from far field sources have been observed on the New Zealand coastline. In that year a magnitude 8.8 earthquake in Chile caused widespread damage and a large number of deaths in South America. The tsunami was witnessed all over the Pacific Basin. In May 1960 a magnitude 9.5 earthquake triggered a tsunami that was witnessed all over the Pacific Basin. Damage but no loss of life occurred in New Zealand. Heavy damage and loss of life occurred in Chile, Japan and the United States, and caused major disruption to shipping around New Zealand. In March 1964 a magnitude 9.2 earthquake in the Aleutian Islands off the coast of Alaska triggered a tsunami that was recorded across much of the Pacific.
Since 2009 large tsunami have been generated by earthquakes in Indonesia (2004 – magnitude 9.3 earthquake), Chile (2010 – magnitude 8.8 earthquake), and Japan (2011 – magnitude 9.1 earthquake) which have caused large scale damage and loss of life in those countries. None of these caused any damage or deaths in New Zealand, but warnings were issued for all them.
As detailed above, tsunami pose a significant risk to the New Zealand coast line. The absence of loss of lives has been a combination of people acting on their own intuition, the absence of seriously damaging wave sequences and significant warning time. One day there will be a tsunami from one of the sources mentioned that causes both damage and deaths.
We must be ready for this.