For example, let’s just say there had been a big shake 20 kilometres offshore from Kaikoura*. One that caused a chunky land uplift deep under the sea. How long would it take for the resulting tsunami wave to reach the coast if the depth of water was 1500 feet on average (500 metres)?
The physics is well understood. A tsunami wave is not like a surface wave caused by wind. It gets its energy because of the enormous pressures that exist deep under the sea.
Tsunami wave speed = Square root (Water depth x Gravity)
= Sqrt (500 x 10) (Gravity is more accurately 9.8 metres/second/second.)
= Sqrt (5000) (approx)
= 70 metres/second (approx)
= 250 kilometres/hour. (roughly - the deeper the water, the faster the wave)
So it would take the first tsunami wave less than 5 minutes to hit the Kaikoura shoreline. (In an earthquake there are usually several waves produced **). 5 minutes is a very short time, and if the average depth was 1000 metres the wave would take just over 3 minutes to hit.
That’s why it’s important to get tsunami warning systems right.
Otherwise they are useless.
* The Kaikoura Canyon is a submarine canyon situated 500 metres off the coast to the south-east of the Kaikoura peninsula. It is 60 km long, up to 1200 m deep, and is generally U-shaped. It is an active canyon that merges into a deep-ocean channel system that meanders for hundreds of kilometres across the deep ocean floor.
** You will have seen video footage of tsunami, and seen that they don't come ashore at anything like 250 kph. In fact they come ashore at more like 20 to 30 kph. So what happens? As the sea depth close to land gets shallower, tsunami waves that have been travelling at 100s of kilometres/hour and a hundred kilometres peak to peak only a metre high, get higher and closer and slower, as the energy of the deep water gets pushed up by the sloping sea floor. This is termed the "run up" phase.
What happened at Fukishima, Japan?
On March 11, 2011, at 2:46 p.m. local time (05:46 Universal Time, or UTC), a magnitude 9.0 earthquake struck off the east coast of Japan. The epicenter was 130 kilometers (80 miles) east of the inland city of Sendai, and 373 kilometers (231 miles) northeast of Tokyo. The earthquake took place around 67 km (42 mi) from the nearest point on Japan's coastline, and initial estimates indicated the tsunami would have taken 10 to 30 minutes to reach the areas first affected, and then areas farther north and south based on the geography of the coastline.
According to the Japan Meteorological Agency there were only about 4 minutes between the earthquake and the initial tsunami arrival at the observation buoy just off the coast of Kamaishi. Assuming the estimated arrival time is correct (i.e. it took another 5 minutes for the wave to travel from the buoy to the coast) that would give them somewhere between 9 to 24 minutes between the earthquake and the arrival of the first tsunami at the closest points and various points north and south.
One minute before the earthquake was felt in Tokyo, Japan’s Earthquake Early Warning system, which includes more than 1,000 seismometers, sent out warnings of impending strong shaking to millions. It is believed that the early warning by the Japan Meteorological Agency (JMA) saved many lives. The warning for the general public was delivered about 8 seconds after the first P wave was detected, or about 31 seconds after the earthquake occurred.
Assuming that the earthquake time reported is the time at which the JMA became aware that there was an earthquake occurring (most likely/logical time to report in my opinion), taking into consideration that it takes them at least 3 minutes to issue a forecast AND assuming that this time is when people in the affected areas actually receive the tsunami warning, it leaves only 6 to 21 minutes for people to register the warning, to take action, and to get to high ground. And that was for an earthquake that occurred 67 kilometres offshore.
If New Zealand had its act together in the same way Japan did - ie 3 minutes to figure out what was happening, decide, and issue a warning to the Kaikoura sirens - and if the first wave hit in between 3 and 5 minutes.... well, go figure...
This gives the heights of tsunami recorded around Japan's coastline. Dark red and bright red mark the highest waves, and the closest points to the epicentre.
And for some of those locations, here are the timings of arrival of the biggest tsunami waves, after the actual earthquake that triggered them:
On 13 March 2011, the Japan Meteorological Agency (JMA) published details of tsunami observations recorded around the coastline of Japan following the earthquake. These observations included tsunami maximum readings of over 3 m (9.8 ft) at the following locations and times on 11 March 2011, following the earthquake which remember was at 14:46 JST:
• 15:12 JST – off Kamaishi – 6.8 m (22 ft) - (ie 26 minutes later with epicentre 67 kilometres away)
• 15:15 JST – Ōfunato – 3.2 m (10 ft) or higher
• 15:20 JST – Ishinomaki-shi Ayukawa – 3.3 m (11 ft) or higher
• 15:21 JST – Miyako – 4.0 m (13.1 ft) or higher
• 15:21 JST – Kamaishi – 4.1 m (13 ft) or higher
• 15:44 JST – Erimo-cho Shoya – 3.5 m (11 ft)
• 15:50 JST – Sōma – 7.3 m (24 ft) or higher
• 16:52 JST – Ōarai – 4.2 m (14 ft)
While the kiwi tsunami message of “If it’s long and strong, get gone” is a good one, we need to better educate our population and civil emergency people about what can happen, and we need better systems in place that the public can trust.
For example, I am aware that people living at Mangawhai Heads, at the top of the North Island, more than 10 metres above mean high water and more than 100 metres inland from the estuary, were woken by volunteers and told to move to higher ground after the Kaikoura earthquake which had occurred just after mid-night. They hadn’t heard the alarm either.
Otherwise it’s a bit like crying “wolf”.
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