Interesting article on the Tonga underwater volcano with short videos of the tidal wave around the pacific rim.

[Concerto]

Surprised there has been no comment here on the Tonga volcano. This article is definitely worth reading and watching the short videos.

Watch: Aftermath of powerful tsunami as 50+ boats damaged and dozens sunk - Marine Industry News

 

[capnsensible]

Thanks for posting. The scale of the event is hard to take in.

 

[NormanS]

I know Tutukaka marina, and thought of it as soon as I heard about the explosive eruption at Tonga. It seems strange that apparently no tsunami warning was issued, as it's something that the authorities are very aware of.

 

[Biggles Wader]

The harbour at Nucualofa must have been directly hit by the Tsunami including the Royal Palace which is jut next to the port. I wonder how much damage was done there.

 

[Concerto]

Just been reading some reports from Tonga about the extensive damage in Tonga.

Aid crews survey ‘extensive damage’ in Tonga as island nation remains largely cut off
Distress signal detected in low-lying Tongan island

 

[Sandy]

I follow Scott Manley for all things rockets and space. An interesting view from space.

 

[AntarcticPilot]

One of the issues here is that the Tsunami was unexpected and unforecast - despite the eruption showing up on seismometers, and the volcano being known to be dangerous and having been monitored by Nasa since 2015 (that's probably why there is such excellent coverage of the eruption)

At present the cause of the Tsunami is unknown. Most such tsunamis are caused by the collapse of the volcanic edifice into the sea - when Anak Krakatau went last year, it was a vast underwater landslide that caused the tsunami. And Earthquakes cause tsunamis by displacing vast slabs of rock. It appears possible that this one was caused by the volcanic explosion on its own - which is not a mechanism that has been observed before. If you watch the videos of the explosion, you can see a circular shock wave in advance of the ash cloud.

 

[Humblebee]

Good link, thanks for posting.
Chris

 

[Concerto]

Further information, this time from Australia, about the damage in Tonga.

‘First official update’: Volcano horror

 

[richardh10]

I was sitting in the cockpit of my boat in Fiji when it happened, and couldn't work out what it was. It sounded like a cross between thunder and an explosion, and it was extremely loud! So much so that I thought it may be something on the boat and actually got up to look around. It went on for around 5 minutes. About 40 minutes later I was sitting having a beer in a bar by the river - as you do - and suddenly the water absolutely shot up the river for a couple of minutes and then shot back out again. It did this once more about 5 minutes later. All very strange until we were told that a volcano had erupted in Tonga. Oddly, there doesn't seem to be any damage at all in Fiji, and thankfully my boat was still safely at anchor and not washed up onto the beach!

Edit. Having looked into it a bit more there has been a lot of flooding in the Lau group of Islands which are nearest to Tonga.

 

[Concerto]

I was sitting in the cockpit of my boat in Fiji when it happened, and couldn't work out what it was. It sounded like a cross between thunder and an explosion, and it was extremely loud! So much so that I thought it may be something on the boat and actually got up to look around. It went on for around 5 minutes. About 40 minutes later I was sitting having a beer in a bar by the river - as you do - and suddenly the water absolutely shot up the river for a couple of minutes and then shot back out again. It did this once more about 5 minutes later. All very strange until we were told that a volcano had erupted in Tonga. Oddly, there doesn't seem to be any damage at all in Fiji, and thankfully my boat was still safely at anchor and not washed up onto the beach!

Edit. Having looked into it a bit more there has been a lot of flooding in the Lau group of Islands which are nearest to Tonga.

Seems like you had a lucky escape despite not being in Tonga but nearly 500 miles away. I just cannot imagine what it must have been like for yachts nearer Tonga. Do you have any news from other cruisers?

 

[Slowboat35]

One of the issues here is that the Tsunami was unexpected and unforecast - despite the eruption showing up on seismometers, and the volcano being known to be dangerous and having been monitored by Nasa since 2015 (that's probably why there is such excellent coverage of the eruption)

At present the cause of the Tsunami is unknown. Most such tsunamis are caused by the collapse of the volcanic edifice into the sea - when Anak Krakatau went last year, it was a vast underwater landslide that caused the tsunami. And Earthquakes cause tsunamis by displacing vast slabs of rock. It appears possible that this one was caused by the volcanic explosion on its own - which is not a mechanism that has been observed before. If you watch the videos of the explosion, you can see a circular shock wave in advance of the ash cloud.

Not sure how a landslide could cause an explosion...
Likely scenario surely? is the eruption had steadily blown away most of the island over the last few weeks until what was left was all but at sea-level - there are many photos of this - and when the magma chamber emptied it collapsed allowing the ocean to flood it. The explosion was caused by the resultant super-heated steam. Far from not being seen before I though this was a pretty standard volcanic process. Isn't that what's thought to have caused the similar 19th century Krakatoa blast?
Clearly possible the explosion set off undersea landslides too, it must have displaced a great deal of rock in any case but the magnitude of that immense blast was surely big enough to cause those rather modest 'tsunamis'?Frankly I'm surprised they weren't much bigger, and that Tonga and Fiji, being so close, got away with it so lightly.

 

[AntarcticPilot]

Not sure how a landslide could cause an explosion...
Likely scenario surely? is the eruption had steadily blown away most of the island over the last few weeks until what was left was all but at sea-level - there are many photos of this - and when the magma chamber emptied it collapsed allowing the ocean to flood it. The explosion was caused by the resultant super-heated steam. Far from not being seen before I though this was a pretty standard volcanic process. Isn't that what's thought to have caused the similar 19th century Krakatoa blast?
Clearly possible the explosion set off undersea landslides too, it must have displaced a great deal of rock in any case but the magnitude of that immense blast was surely big enough to cause those rather modest 'tsunamis'?Frankly I'm surprised they weren't much bigger, and that Tonga and Fiji, being so close, got away with it so lightly.

Tsunami's are usually caused by mass movement - landslides or earthquakes shifting the sea bed up or down. As you say, it appears that seawater and vulcanism conspired here to make a massive explosion - but that's rare, as the conditions are quite specific. If the water is too deep, the explosion is overcome by water pressure; too shallow and the blast all goes up. The hypothesis - and that's all it is at this moment - is that the water depth at which seawater met magma was in the sweet spot - around 200m - where it was shallow enough for there to be an explosion, and deep enough for the energy to be coupled to the sea rather than the atmosphere. But until detailed surveys can take place, we won't know. For obvious reasons, that won't be for a while!

The recent tsunami caused by Anak Krakatau was caused by a landslip. There's insufficient evidence to say what caused the tsunami after Krakatoa went up in the 19th century, but almost certainly land movement, not explosion - the water there is probably not deep enough to couple the explosive energy of the eruption to the sea.

 

[Frank Holden]

I know Tutukaka marina, and thought of it as soon as I heard about the explosive eruption at Tonga. It seems strange that apparently no tsunami warning was issued, as it's something that the authorities are very aware of.

They _ the NZ authorities - issued a warning at 0415Z which was about two hours after the eruption but that isn't much use unless you sound the hooter thingamabobs. Emergency agency defends not issuing tsunami alert

If you want to see serious tsunami effect in NZ then the geography of Tutukaka makes it the place to be - think Severn Bore.

Here in Ecuador the sirens went off but no-one paid much heed hereabouts. They also sounded them in Chile - Chile does good tsunami sirens - but Peru didn't bother as they didn't want to 'alarm the population'. The two ladies who drowned after their car was swept into the sea probably would have been quite happy being alarmed. That was in the north of the country. Tsunami leaves two dead in Peru

 

[richardh10]

As its cyclone season in Fiji there are no cruisers in the Lau group, and there has thankfully been no damage that I'm aware of to any boats

 

[Concerto]

This article by National Geograhic on the Tonga volcano tries to answer some of the questions about the explosion, but also raises many more.
The volcanic explosion in Tonga destroyed an island—and created many mysteries

From rock studies, they know there were major volcanic explosions 900 and 1800 years ago, and frequent minor ones as recent as 2014.

The explosion was the equivalent to 5 to 6 million tons of TNT. The sonic boom from the explosion went twice around the world and ash and gas up to 34 miles high. The tsunami was also strange as it was minor on the local islands but higher further away, possible caused by the air pressure of the explosion.

All the activity was only about an hour long. The cause of the sonic boom is not really known, but could be a combination of escaping gases and sea water mixing. The other strange this is there has been very little ash fall on the local islands.

Read the article (free access) and see what you think.