Day 323 – Shaky

I know that there are lot of other things going on at the moment: loadshedding, football, some virusy thing (apparently, anyway), but something else just caught my interest and I thought it worth sharing here.

I’m on the USGS mailing list. They tell me whenever there’s a “big” (> M6.0) earthquake anywhere in the world so that I can be ready in case the Cape Town tsunami is on its way.

The good news is that we’re safe (for the moment anyway), but things on the seismological front have been very busy – much busier than usual.

Including that M7.5 Chunky Boi (which was later upgraded to M7.7!). And it’s continued into yesterday (when I’m writing this post).

If you lower your expectations (and let’s face it, you’re here, so they can’t have been that high anyway) and set your earthquakeometer to M5.0 and above, the screen is filled (and more) with shaky things happening all around the Loyalty Islands.

Which are located thus, by the way:

I would not want to be there right now.

Although the beaches do look quite nice:

Anyway, I don’t want to fill you with more existential dread, but it does seem that the world is shaking itself to pieces as well as all the ice melting and everyone dying from The Virus.

Otherwise, you well?

Day 104 – Tenuous tsunami parallels

Let’s make things clear right from the start here.

I’m not saying that Cape Town is going to get hit by a tsunami.
We’ve covered that concern here: something I would strongly advise you to read if you think that people are paranoid about Covid-19. You ain’t seen nothing yet.

They walk among us.

But I digress. Often.

One of the enduring images of the 2004 Indonesian tsunami was the drawback: the water disappearing from the shorelines of beaches and resorts about to be hit by the tsunami, effectively (some would say ‘exactly’) like a huge low tide.

In some places, this drawback was up to a kilometre. And in most cases, there was good correlation between the size of the drawback and the extent of the damage caused by the tsunami that followed.

If we ignore all scientific reason for a moment and apply this clear inverse proportionality to tomorrow’s predicted storm, I think we’re in trouble. Because today’s weather in Cape Town could not be calmer or more beautiful. So still. So clear. So utterly perfect.

Not a cloud in the sky. Not a breath of wind.

If only there were some term that one could use to describe such a period of placid weather ahead of a predicted tempest.

And yet… just out there in the bottom left corner:

A frothing mass of low pressure, general misery, howling winds and all the rain we didn’t get in 2017. All due to begin tomorrow afternoon/evening and then be followed in by a second front on Sunday into Monday.

And our eyes are already on another vicious lump of nastiness heading out of Uruguay towards SA like Luis Suarez’s poor sportsmanship and bad temper in 2010.

And possibly every bit as bitey.

But let’s just get through this weekend first. Here’s what we’re expecting to see at 0800 local time on Friday morning:

Big winds, much rain, huge waves, general unpleasantness.

In all seriousness though, it does look like quite a nasty one, so please look out for your community and maybe consider helping out your local shelter, which will obviously be under more pressure than usual over the next few days.

Notes on Japanese ship-naming conventions

Yeah, I know. That title. You’re already disinterested, but hey – hang tight – you might just learn something today.
I know I did.

Japanese fishing vessels have been all over the news lately. If you count the one that ran aground on Clifton Beach last month and the one that was found drifting off the coast of Canada in April, that is.
The former has sadly dropped out of the news and even now, no-one is really sure how it ended up parked among the holiday homes of the German elite. The latter was a victim of the March 2011 tsunami and has been drifting across the Pacific ever since.

Their names: the Eihatsu Maru and the Ryou-Un-Maru. And I’ll use this handy opportunity to chuck the name of the only other Japanese fishing vessel I know in there too: the Meisho Maru 38. Some of that one lies aground near Cape Agulhas and has surely featured in many photographs, but most notably, this one:

Eagle-eyed readers should really give the eagle its eyes back, but in the meantime, they will have noticed the common “Maru” in the names of all these vessels, because eagles are good at spotting that sort of thing.

When you look  up Maru on Google translate, it tells you in mean “circle” and also, if you look a little below that, “suffix for ship names”. But why?

Well, god bless the internet, because Wikipedia can help us out with an answer on their helpfully named: “Japanese ship naming conventions” page, which discusses and explains Japanese ship naming conventions. And it tells us:

The word maru (meaning “circle”) is often attached to Japanese ship names. The first ship known to follow this convention was the Nippon Maru, flagship of daimyo Toyotomi Hideyoshi’s 16th century fleet. There are several theories which purport to explain this practice:

  • The most common is that ships were thought of as floating castles, and the word referred to the defensive “circles” or maru that protected the castle.
  • That the suffix -maru is often applied to words representing something that is beloved, and sailors applied this suffix to their ships.
  • That the term maru is used in divination and represents perfection or completeness, or the ship as a small world of its own.
  • A legend of Hakudo Maru, a celestial being that came to earth and taught humans how to build ships. It is said that the name maru is attached to a ship to secure celestial protection for it as it travels.
  • For the past few centuries, only non-warships bore the maru ending. It was intended to be used as a good hope naming convention that would allow the ship to leave port, travel the world, and return safely to home port: hence the complete circle arriving back to its origin unhurt.
  • Note also that Hinomaru or ‘sun-disc’ is a name often applied to the national flag of Japan.

Today commercial and private ships are still named using this convention.

Of course, there are many superstitions and traditions in Japanese society and there are probably (at least) an equal number in the seafaring community, so it seems perfectly reasonable that when these two behemoths of folklore come together, we get this well-observed custom of nomenclature.

That said, many of the reasons given above are centred around the protection of the vessel and its safe return to port and that hasn’t really held true for any of the ships I am aware of (n=3). Let’s not forget that one ended up on a local beach, another ended up on some fairly local rocks and another was sunk by the US Coastguard “for safety reasons” (and, let’s be absolutely honest here, fun).

Look, I recognise that it’s Friday afternoon and you aren’t in the mood to learn stuff. But you’ll be thanking the Japanese Seagods and 6000 miles… at your next pub quiz, believe me.

Assuming there’s a question about this sort of thing, of course.

Not much?

Thankfully, aside from in Japan, there seems to have been very little damage from yesterday’s tsunami. However, the waves did make it across the Pacific and even though they were small – tiny even – by the time they arrived in New Zealand, the systems set out to detect them still detected them, as can be seen from this graph:

For anyone interested, the gauges listed are located here.

Noting that the waves that reached NZ were an average of about 30cm, it demonstrates the extraordinary sensitivity of their system.

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.