Cape Town Earthquake!

Sort of, anyway:

That email to me (personally, see the header) from the USGS ENS, telling me about a quake just down the road ocean from Cape Agulhas. I never felt a thing. But then it was about 2000km away.

Here are the details – phew, it looks like we dodged a bullet:

Although, has anyone heard anything from Bouvet Island this morning?
Their twitter account (here) doesn’t mention anything. Mind you, it seems to have given up when Trump became US President. Perhaps understandably.

An aside: Bouvet Island looks amazing. I’d certainly visit there if it wasn’t for the earthquake danger.

Johannesburg Tremor “M5.3” – USGS

Preliminary indications suggest that the tremor felt by most people in Gauteng this lunchtime was of Magnitude 5.4 and was centred approximately 6km east of Orkney on the border of North West and Free State provinces.
The recorded depth of the tremor was 10km – that’s quite shallow and explains why it was so widely felt.

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Interestingly, zooming in on that epicentre lands you right in the middle of the Vaal Reefs Mine complex.

M 5.3 – 6km E of Orkney, South Africa

Time: Location: 26.986°S 26.741°E
Depth: 10.0km

Earthquake mashups

Fortunately, we don’t really have earthquakes here in SA, but if we did have a semi-active geological fault, we’d almost certainly build a nuclear power station on it.
Not so California, of course. They’re well used to living alongside the infamous San Andreas Fault. Fairly recently (January 1994), LA was rocked by a 6.7Mw quake, killing 57 and injuring almost 9,000.

But that was nothing on the April 1906 San Francisco earthquake of 1906. No exact figures have ever been calculated as to how many died that day, but it’s widely believed to be over 3,000, with over 275,000 left homeless.

Now, Shawn Clover of… er… shawnclover.com has done some blends or mashups of modern day San Francisco and the city after the 1906 earthquake. And they’re really rather good.


Shawn’s captions are also clever – seamlessly describing both the 1906 and current scenes.

Go and see more in the two parts of his 1906 Earthquake blends, here and here.

Photo credits: Shawn Clover, durr

Physics of a tsunami

With my parents still in New Zealand and on the coast in Greymouth (in the direct line for any tsunami emanating from the Honshu earthquake) I was reading around the speed of Tsunamis with some personal interest. However, I didn’t have to, since the Pacific Tsunami Warning Center [sic] has all the predicted “hit” times for the arrival of the wave or, more often, waves.

SEA LEVEL READINGS CONFIRM THAT A TSUNAMI HAS BEEN GENERATED WHICH COULD CAUSE WIDESPREAD DAMAGE. AUTHORITIES SHOULD TAKE APPROPRIATE ACTION IN RESPONSE TO THIS THREAT. THIS CENTER WILL CONTINUE TO MONITOR SEA LEVEL DATA TO DETERMINE THE EXTENT AND SEVERITY OF THE THREAT.

ESTIMATED INITIAL TSUNAMI WAVE ARRIVAL TIMES AT FORECAST POINTS WITHIN THE WARNING AND WATCH AREAS ARE GIVEN BELOW. ACTUAL ARRIVAL TIMES MAY DIFFER AND THE INITIAL WAVE MAY NOT BE THE LARGEST. A TSUNAMI IS A SERIES OF WAVES AND THE TIME BETWEEN SUCCESSIVE WAVES CAN BE FIVE MINUTES TO ONE HOUR.

Apologies for the SHOUTING, but this is obviously a rather important message.
UPDATE HERE and again HERE

And there is NZ on the list, with a predicted arrival time of 1930 GMT this evening. That’s 2130 SA time and 0830 tomorrow local time: over 12 hours after the earthquake hit. And that gives you an idea of how massive the scale of this is, because tsunami waves can top 900kph.

The wave speed is the square root of the product of the gravity constant (g) and water depth.

Tsunamis are normally produced by an earthquake or displacement of the seafloor due to plate shifts, etc. This produces a very large wave very rapidly, which then possesses significant energy.

The energy is distributed through the depth of the water initially, as it is displaced, but because of gravity and friction with the seabed, tends to decrease with increasing depth after a short while.
In deep water, the frictional affect on the wave speed is negligible near the surface. The more shallow the water (for instance as it approaches shore), the greater the affect of friction in slowing the mass of water above the seabed; most of the energy of the wave is transferred to the seabed, a small portion is lost to the atmosphere and in heating of the water.

Therefore, the more shallow the water, the slower the wave speed.

And with the Pacific generally being rather deep, the waves are travelling rather fast.

Thankfully, given the distances involved, that still gives my parents significant time to ensure their safety. Sadly, others nearer the epicentre, or without access to this information will probably not be so lucky.

Quotes from Christchurch

The Christchurch earthquake and its aftermath have been undeniably tragic. Boston.com’s always impressive The Big Picture has probably the best collection of photos of the devastation, but they’ve been justifiably beamed around the world so many times that you’ve probably seen a lot of them already.

My Mum and Dad are touring New Zealand at the moment and were due to visit Christchurch next week.  Obviously, they have had to change their plans and we’re all hoping that their flight from Christchurch airport via Sydney and Jo’burg to Cape Town in a couple of weeks time will still be able to go ahead.

This has obviously been an emotional time for those involved. However, that doesn’t mean – no matter how unreal the situation seems – that you can just make stuff up to describe it.

Firstly, I’m looking at you, 63-year-old teacher’s assistant Kevin Fitzgerald and your

I thought the devil was coming up out of the earth.

quote. Is that really what you thought, Kevin? Because it sounds a bit lunatic ninja to me. Look, I appreciate that it must have been a harrowing experience, but was that really your first thought as your “building undulated menacingly” (love that image)?
Not that there had been, perhaps, an earthquake (one of over 5,000 in the Christchurch area since September), but that a mythical nasty bloke was on his way up from the home of the DA?
And then – once the dust had actually and metaphorically settled – you stood by that thought and told the entire world about it.

Seriously, even in the extremely unlikely event that I had thought that the devil was coming up out of the earth, once I had worked out that there had been an earthquake in this earthquake zone known for its earthquakes and a reporter shoved a microphone under my nose, I’d probably have chosen not to mention it. My quote would probably have been more along the lines of:

I thought it was a really big earthquake.

It might be a lie (it might not) but at least I wouldn’t look as silly as you do now.

And in that vein, Kevin, thank the good lord above (not for the devastating earthquake, the horrendous damage to livelihoods, the needless loss of loved ones and scenes of utter desperation, obviously) but thank him anyway, Mr Fitzgerald, for South African clergyman and Christchurch resident Dr Tienie Bekker. For Dr Bekker and his good lady wife were put on this earth to make your “devil coming up out of the earth” quote seem entirely reasonable.

As Dr Bekker told SAPA:

We were lucky that we could buy food as we were on the outskirts. Many have had to be helped. Drinking water is minimal in quite a few areas. A lot of people are without power.

Which all seems entirely reasonable. But then he describes the actual moment the earthquake hit and that’s where things get a bit odd:

I was just out on my way to an appointment after midday, had just finished lunch, when I felt this wave. My house started moving up and down. My wife compared the experience to a tumble dryer on a roller-coaster.

Wait. What? She compared it to what, Tienie?

My wife compared the experience to a tumble dryer on a roller-coaster.

Right. So she did.

Where to begin? I guess the obvious place would be how does she know?
Is there some weird sub division of roller-coaster fanatics which gets off on taking household appliances on rides?
Is Mrs Bekker one of these people?

And even if there was and she is, how does the seatbelt fit? Surely there is some significant danger in large consumer durables not being properly anchored when traversing several hundred metres of track replete with terrifying bends and gravity-defying loops.
Presumably there is a optimum size for these things. A simple kettle or iron would likely fall out almost immediately, while a mid-size appliance such as microwave might lend itself to being correctly and safely strapped into the seat.
But a tumble dryer is a large item and my fear would be that its uneven weight distribution together with its dimensions would probably prevent it from being adequately affiliated with the carriage of the ride in question.

Where would one plug it in? Because, if we presume that Mrs Bekker’s description is suggesting that a tumble dryer on a roller-coaster moves somehow more violently than any other item on a roller-coaster, then we must surely assume that it is switched on.
I have never seen a roller-coaster with plug points and that fact has never bothered me before because I had also never seen the need for them.

All that has changed.

In addition, does Mrs Bekker’s description merely refer to the motion of the said appliance on the roller-coaster? Or is there heat involved too? Did things suddenly get noticeably warmer as the quake hit? Because that’s part of tumble drying as well.
As is the ubiquitous lint filter. So are we looking at just movement, movement and heat or movement, heat and a mysterious purple-grey felt covering everything in Dr and Mrs B’s house?

I’m not aware of these things, because I don’t live in an earthquake zone – but neither do a lot of other people and they also need enlightening, Mrs Bekker. Should we just expect to feel a bit giddy with a slight sense of elation after an earthquake has hit or should we expect to feel hot and a bit fuzzy?

And could her description help in designing better quake-proof buildings? After all, their house is still standing, they are unharmed. Should all houses in earthquake zones come with a height restriction, a long queue and the need to be accompanied by a large alternative to a washing line?

Because if that is how the Bekkers survived – think how many other lives could be saved.