OT Tongan volcano underwater effects

[Jeff Layman]

I haven't been able to find out anything about this, even conjecture. All that has been reported so far is that the submarine cable between Tonga and the world has been cut.

It was a massive explosion by all accounts. All the satellite photos show the size of the immediate explosion and the shock wave spreading outwards at the speed of sound, followed by the billowing ash cloud. Yet as most of the explosion was underwater, that shock wave would have travelled 4 - 5 times faster than the aerial one. I would not be surprised if there were a number of submarines in the area from several navies, but it is very unlikely that we will ever hear anything about them. So what would have happened to them, especially if they were side-on when the shock wave hit? And what about the effect on sea-life in the area? Surely that couldn't survive such a shockwave.

[newshound]

My understanding is that there was a fair bit of warning, and presumably sub technology gives them a very good idea where the problem is even if they didn't get updated from land-based systems. I'd guess subs would turn tail and run. Although it was obviously a big bang (my guess caused by the classic "steam" explosion from water / molten material interaction), surely subs would be safe at 100 miles?

[Andy Burns]

I gather the "footage" represents about a 6 hour timelapse?

[Colin Bignell]

There is very little known because the cable has been cut. However, two deaths have been reported so far and teams of Tongans are sweeping ash off the airport runway, to make it possible for aircraft to land. That may be finished tomorrow allowing fresh water to be flown in. Aerial surveys also show substantial damage on two outlying islands, including a village that appears to have been destroyed.

The shock wave was recorded in the UK, twice.

I would, as there are usually well marked hazard zones around submarine volcanoes. They can even sink surface ships, by creating gas bubbles that reduce the density of the water.

That would depend upon how much pressure they were designed to survive compared to how much was imposed by the shock wave.

And what about the effect on sea-life

Apparently, although there will be a lot of dead fish about in the immediate aftermath, undersea eruptions give rise to some very interesting new microbial life.

[Jeff Layman]

I'd guess they'd be safe at less than that, but I haven't seen estimates of the power of the explosion. It was probably equivalent to several megatons of TNT. Of course, we'll never find out as where those subs go and when, and how they are communicated with, as it is no doubt a closely-guarded military secret. We are told that underwater communication cables are prime targets if hostilities develop, so perhaps subs hang around them.

[NY]

My weather station recorded two blips (increase in pressure by 2 mb followed by a very rapid fall in pressure by 3 mb then a return to normal). The first one was at about 1900 (14 hours after the eruption) and the second was at

0200 (7 hours after the first) the following morning. I presume I was seeing the same pressure wave, first travelling one way round the earth and then the other. The only thing that doesn't fit with that theory is that Tonga is almost diametrically opposite the UK (actually longitude 175 W) so you'd expect both pulses to arrive at almost the same time. Or does the rotation of the earth have an effect?

[Jeff Layman]

Perhaps, I haven't checked over what area they extend. But that's not what matters; it's the fact that the wavefront expanded as around the speed of sound. So, in air, in 5 minutes it would travel about 60 miles, but it would take only a minute to travel that far in seawater. I doubt that any sub would get a warning and be able to do anything about it in one minute.

[NY]

>> The shock wave was recorded in the UK, twice. > > My weather station recorded two blips (increase in pressure by 2 mb > followed by a very rapid fall in pressure by 3 mb then a return to > normal). The first one was at about 1900 (15 hours after the eruption) and > the second was at 0200 (7 hours after the first) the following morning. I > presume I was seeing the same pressure wave, first travelling one way > round the earth and then the other. The only thing that doesn't fit with > that theory is that Tonga is almost diametrically opposite the UK > (actually longitude 175 W) so you'd expect both pulses to arrive at almost > the same time. Or does the rotation of the earth have an effect?

I meant to attach a link to the pressure graph from my weather station:

[Andy Burns]

It's been giving warnings since december, I bet the people who filmed it from a boat a couple of miles away are glad they didn't do so a fortnight later ...

[Jeff Layman]

That's the problem with trying to predict volcanic activity. Wasn't there a volcanologist on the news who said that they'd noted activity, but were totally surprised by the size and timing of the eruption. Perhaps any subs would have got general warnings over the past few weeks about increased activity, but if their preplanned route was intended to take them near, then that's what they would have had to follow.

[Jeff Layman]

That's local land-based info. So far as I know there's been nothing reported by any shipping, if any, in the area. Is there something like the website showing the position of aircraft at any time which shows the position of commercial shipping world-wide?

The size of the explosion can be calculated from the size of the ash cloud and pressure measurements.

I think that theory (originally linked to methane release in the Bermuda Triangle) has been disproven, BICBW. In any case, why would it worry a submarine? With regard to military intelligence gathering, I doubt there are any "no go" zones in international waters.

[The Natural Philosopher]

submarines can survive depth charges at anything above a few tens of metres

They will be fine

[The Natural Philosopher]

I think they would be safe at one mile

[Andy Burns]

They declared it dormant on the 11th ...

[newshound]

Remember that the shock front might start of circular, but will divert when there are air temperature gradients (both vertical and horizontal). So the paths to you in "opposite" directions may well not be the same lengths.

I did wonder whether you had missed the first one, so were seeing the second and third at 7 and 14 hours. But back of envelope calcs say that sound at the antipodes takes ~ 16 hours to reach you.

Tonga is only 21 degrees south, so it is significantly closer one way than the other.

[newshound]

[Colin Bignell]

That mirrors what the Met office trace showed. Thinking about it, I wonder whether the initial explosion had been followed by something like a collapse of the ash column, causing both a positive and a negative pressure wave.

[Andrew]

BBC news showed some aerial photos taken by an Australian recon plane. Blast tipped shipping containers on their side in a port area.

The subsea cables are no great secret. While I was working in Fiji, Cable and Wireless owned a cable laying ship called the Pacific Guardian which was moored in Suva bay ready to dash off and fix a cable break. Once a month it came into the docks for refuelling and they held a disco in the below deck cable area for thje locals. Very impressive kit used to pull repeaters on board and splice them back in.

You need to build houses to very high standards in Fiji if you want full insurance cover for cyclones and earthquakes.

[Andrew]

It's an earthquake zone though. Expect the unexpected.

[Colin Bignell]

It is based upon the 1952 sinking of Japanese research vessel Kaiyo Maru No. 5 while investigating an underwater volcano.

I think the Bermuda triangle link may be that a methane gas release was suggested as a possible reason for the unexplained disappearance of the Island Queen on a voyage to Saint Vincent and the Grenadines in 1944. That remains an unsolved mystery.

In any case, why would it worry a

I didn't say no go zone. I said well marked hazard zone. I would expect any sensible boat commander to keep clear of such a zone around a volcano that had been showing signs of activity for a while. I am also unclear what sort of intelligence gathering a submarine would be engaged in off Tonga.