Could we have a tsunami here?

One of our neighbours asked a most relevant question, and I thought I would answer it to the best of my ability.

He asked: "To my Japanese Friends: I pray that your friends and family are safe and escaped the worst of this disaster. Our own Koeberg is not that elevated above sea level and a Tsunami in the Atlantic would surely wipe it out? Just imagine the Cape Town Foreshore after a 7 meter high wall of water hits it? Imagine the Cape Flats?"

Excellent question. In fact, Koeberg is not at all elevated, and the Cape Flats are below sea level and are home to over a million people.

Another neighbour then posted the following image which shows a great faultline on the continental shelf south of the Cape:

We did in fact have an earthquake in 1969 which devastated Ceres, Wolseley and Tulbagh and woke all of us up in Cape Town. It was caused by this fault.

It was not accompanied by any meaningful tidal activity, as I recall.

The Cape is, indeed, the area with the greatest geological activity:

http://www.geoscience.org.za/index.php?option=com_content&task=view&id=307&Itemid=442

There has not been an earth tremor reported for 40 years. This is not to say it won't happen, but given the lack of any tidal wave, ever, I would suggest we can worry about other things. We have never had a tidal wave in South Africa. Well, not true ... there was one recorded at Kommetjie five years ago, but that came from the Falklands, and it crested harmlessly over the beach. It was the highest wave ever recorded in South Africa. I cannot find the record, but a giant wave was spawned by a massive storm around the Falklands in the South Atlantic. There are two buoys stationed off Kommetjie, and they recorded a wave of something like 20m. It broke harmlessly but spectacularly.

Every winter sees the most spectacular storm-driven waves in False Bay, especially at Kalk Bay and Sea Point, but they have no ability to travel inland.

I grew up in Muizenberg, where the waves would race up Sandvlei, but they were contained. Very little damage ever, other than to the Victorian changing rooms, which were destroyed every year .

Storm-driven waves are totally different to earthquake-driven waves, which travel faster than a Jumbo Jet. They are triggered by the stored power caused by two tectonic plates grinding against each other. When the one plate has been pressing against another and is released, that stored power is analogous to a spring being released. It sends the water out in invisible waves at supersonic speeds. When the water reaches the land, the energy must go somewhere. It does so in the form of pulling all the water out from the shorefront, and then cresting. That is why the Japanese tsunami crested at 10m. The laws of physics demanded the energy had to go somewhere.

Then the water goes inland.

Finally, there are no records, anywhere, of a seismic-driven tidal wave, as I showed. I think Koeberg and the Cape Flats are safe!

As I say, I'm no expert, but the fault south of our coast is not a subduction zone (where two plates are sliding against each other, with related volcanic activity). Our last active volcano was Saltpeterkop near Sutherland, and it has been extinct for millions of years:

http://linbropark.blogspot.com/2011/02/south-africas-last-active-volcano.html

My own pic, I'm delighted to say. Are we ready for a tsunami? No. Do we need to be? No. In all of South Africa's recorded history, no reason to worry.

Happy to take any comments or questions.

Addendum: How do I know all of this and what is my authority? I have no degree, but I study seismology to an amateurish level of detail, and have since I was 10.

Second addendum: Remarkably, Kommetjie/Long Beach also holds (or held) the Guinness Book of World Records for the largest whale shark and beached whale in history. Glorious place, and second only to Sutherland as a spiritual sanctuary for Karen and me.

http://linbropark.blogspot.com/2011/01/flashback-to-2005.html