Last year scientists discovered earthquake faults on the sea floor in Pegasus Bay that were “too small, too lacking in vertical movement and moving too infrequently (once every few thousand or tens of thousands of years), to create a tsunami”. http://ecan.govt.nz/advice/emergencies-and-hazard/tsunami/pages/tsunami-info-chch.aspx
This report, bearing the logo of the Canterbury Council, explains that the reason there are no local tsunami warning systems in New Zealand is because the time between a tsunami being generated and it hitting the shore is too short to be able to give a warning.
How very peculiar then, that the same council which had said risk was too low to need any, has just installed 22 tsunami sirens on the coastline between Waimairi Beach and Sumner at a cost of $550,000, operational from July 1.
So now they are somehow a fact again, although not a local fact. They have decided the alarms may be useful in the event of a distant-source tsunami from across the Pacific Ocean, which would take 12-15 hours to reach New Zealand.
Given the 12-15 hours before anything arrived, a distant-source tsunami trundling its slow way across the Pacific would be a huge news story, well warned-of and endlessly milked to death by all media up and down the whole country, which means the sirens could be blasting away all day from lonely hilltops with no one to hear them.
There has never been a tsunami without first an earthquake. Earthquake risk has been thoroughly demonstrated by the 10,000 or so in the region since September 2010, but oddly there is still no warning system for earthquakes or even plans for one.
If there is half a million dollars available for tsunami warnings then perhaps another half million dollars could be spent on earthquake sirens, given that earthquake activity is still traumatising people and clearly not yet over, while tsunami activity is zero.
Although a warning system can provide peace of mind, putting up a system for something unnecessary may create fears that may not have been there in the first place. And most Christchurch residents, given the opportunity, would tell a council spending committee that regional anxiety has come from the earthquakes, not tsunamis.
We can rule out the probability of a tsunami with a massive degree of statistical confidence. We have never had a tsunami generated in local waters.
There has only been one tidal upsurge in Canterbury’s history that has come anywhere near a tsunami when in May 1960, after-wash waves 2m high from a Chilean earthquake entered Lyttelton Harbour, damaging boats and a hotel.
One can only come from South America, probably Chile.
This is because the land of which the visible South island is the above-water part extends about 2000kms to the east before it drops to the Pacific Plate. The water is too shallow to generate a large enough wave close enough to NZ shores to effect damage to the South Island coastline.
The North Island is more exposed, but the risk is mitigated by a 200 mile shallow buffer zone extending to the east off the Wairarapa coast.
It is why tsunamis are so very rare in NZ compared to earthquakes. 15,000 earthquakes occur on average in NZ every year. Out of the 30,000 over the past year, not one has morphed into a tsunami, or even a tsunami scare.
Looking back over the years since seismologists started keeping records there have been about 2 million earthquakes recorded in NZ; 60,000 of which have been in Canterbury.
In that whole time there have been 4 tsunami scares, all distant-source tsunamis from South America, and with no loss of life. The tsunami risk has been 0.0002%, or virtually nil.
The tsunami alarms would have to be triggered by offshore quakes, and so cannot detect local earthquakes. Meanwhile, powerful shakes are still regularly causing anxiety in the region, and there is continuing refusal to consider earthquake alarms.For more writing from Ken Ring, visit www.predictweather.com
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