Ocean Impact = No Tsunami

[Jonfliesgoats]

This is only based on 3D modeling.  Fundamentally, we don't really know what would happen with these impacts, and I a, not sure if this accounts for second order effects, like waves generated from a massive inrush of water to replace the vaporized material after an impact.  More educated scientists can contribute here, I am sure.

http://www.space.com/35081-asteroid-impact-ocean-computer-simulations-solar-system.html

[magnemoe]

Look like it need an serious big one to make an tsunami, makes sense an surface detonation of an large nuclear bomb would also not make an real tsunami. 
One who is kilometer sized would probably make an tsunami. 

[p1t1o]

Apparently its not just the mass of water displaced but *how* it is displaced, which affects how the energy disperses afterwards. It says that [as simulated] the impacts can produce initial waves up to 400m high and a central jet that can reach tens of kilometres in height, but that these waves would very quickly lose their energy. A tsunami is a movement of the entire water column (whereas high energy impacts tend to dump their energy into the surface, with penetration depth on par with the length of the impactor) and generates waves in a different manner - ones of a form that does not lose its energy very quickly to the surrounding water.

The simulation, however, dealt with rocks in the 250m range. I imagine that there would be a step-change in the effects when talking about rocks in the 1-10km range.

Edited December 20, 2016 by p1t1o

[Green Baron]

Sorry, if correct someone.

A tsunami, like any wave in water, actually does not move water. Particles of the water make an orbital movement, they circle. After a wave is through everything is where it was before the wave. Particles circle up on the back, forward on the crest, down on the front and back on the bottom of the wave. The orbital movement takes place up to a depth of a multiple of the waveheight (5-7 ?). If there is no seafloor the wave moves freely without overturning.

Compression and slowing of a wave in shallow and/or norrowing water is what causes a low but fast moving wave (such as one from a small/medium impact or a submarine landslide or volcanic eruption) on the open water to become a slow, towering and overturning wave on the shore. The lower part of the orbital movement is slowed by the ground, thus the upper part overtakes and overturns. An overturning wave on the beach moves just fast enough to take the illusion of an escape from a good sprinter ...

 

The above award winning video is not exactly what geology teaches about impact tsunamis (this is only the first result on the search list) or what a stone thrown into a paddy does to the spectators ... so, yeah, nice movie, i don't believe it as long i don't know what assumptions they made for the wave buildup, propagation and modelling of the shorelines. Seems to me that major effects in wave propagation have not been taken into consideration.

Again: a tsunami on the open water will barely be noticed, only very near the impact site or submarine landslide. Bad things happen only at the shores.

Another example is the Krakatoa eruption in 18813. Ships on sea near the site of the explosion didn't notice much of the waves that each explosion caused though they were covered with ashes, on the shores the waves built up 30-45m, the final big bang up to 60m.

 

Edited December 20, 2016 by Green Baron

[Shpaget]

50 minutes ago, p1t1o said:

Apparently its not just the mass of water displaced but *how* it is displaced, which affects how the energy disperses afterwards. It says that [as simulated] the impacts can produce initial waves up to 400m high and a central jet that can reach tens of kilometres in height, but that these waves would very quickly lose their energy. A tsunami is a movement of the entire water column (whereas high energy impacts tend to dump their energy into the surface, with penetration depth on par with the length of the impactor) and generates waves in a different manner - ones of a form that does not lose its energy very quickly to the surrounding water.

Exactly.

A tsunami is not just a big wave. It's a specific type of wave that behaves differently than those formed by wind or rock splashes (regardless of the size of the rock).

[Green Baron]

9 minutes ago, Shpaget said:

Exactly.

A tsunami is not just a big wave. It's a specific type of wave that behaves differently than those formed by wind or rock splashes (regardless of the size of the rock).

Insofar as a higher or deeper part of the water column is disturbed. The tsunami earns its name on shore.

Edited December 20, 2016 by Green Baron

[YNM]

Tsunami is a big wave near the coast ("harbour", lit. "harbour wave"). So if you have hundred-metre high wave from an impact in the middle of wide, deep ocean, that doesn't really count.

But, if the impact happens on shallow waters not that far from the coast... There's this and this. (edit) And this, which sounds... scary...

Edited December 20, 2016 by YNM

[magnemoe]

3 hours ago, Green Baron said:

Sorry, if correct someone.

A tsunami, like any wave in water, actually does not move water. Particles of the water make an orbital movement, they circle. After a wave is through everything is where it was before the wave. Particles circle up on the back, forward on the crest, down on the front and back on the bottom of the wave. The orbital movement takes place up to a depth of a multiple of the waveheight (5-7 ?). If there is no seafloor the wave moves freely without overturning.

Compression and slowing of a wave in shallow and/or norrowing water is what causes a low but fast moving wave (such as one from a small/medium impact or a submarine landslide or volcanic eruption) on the open water to become a slow, towering and overturning wave on the shore. The lower part of the orbital movement is slowed by the ground, thus the upper part overtakes and overturns. An overturning wave on the beach moves just fast enough to take the illusion of an escape from a good sprinter ...

 

The above award winning video is not exactly what geology teaches about impact tsunamis (this is only the first result on the search list) or what a stone thrown into a paddy does to the spectators ... so, yeah, nice movie, i don't believe it as long i don't know what assumptions they made for the wave buildup, propagation and modelling of the shorelines. Seems to me that major effects in wave propagation have not been taken into consideration.

Again: a tsunami on the open water will barely be noticed, only very near the impact site or submarine landslide. Bad things happen only at the shores.

Another example is the Krakatoa eruption in 18813. Ships on sea near the site of the explosion didn't notice much of the waves that each explosion caused though they were covered with ashes, on the shores the waves built up 30-45m, the final big bang up to 60m.

 

Yes it was not obvious if it was an fast and deep wave in the simulation on the other hand the linked impact was an 4 km asteroid rather than an 300 meter one and a 4 km one should make an major tsunami just by the water it displaces. 
It looks like the 300 meter one would be pretty much an surface explosion at worst, and would not blow an deep hole in the water who would the tsunami generator. 
An air blast or fragmentation will have even lower effect over water but would be worse over land for an so large asteroid. 

[Jonfliesgoats]

I think it's interesting either way.  Also, does anyone have insight regarding follow on effectsinnthe monents after impact?

 Itokawa is roughly the size of this model and has a mass of 3x10^9kg.  Impacting at 8km/s, it would carry 9.6x10^16 joules or the equivalent of 22.9 megatons of TNT.  That's roughly half the size of the largest acknowledged, deliverable nuclear weapon in history, the Soviet Tsar Bomba (57MT).

The reason I delve into the energy is that a literal hole would exist in the ocean after the impact.  Using the energy of vaporization of water (which I admit is of questionable utility regarding the mechanics of an impact) at an average depth of 3.5 kilometers, a 2km radius hole would be blown in the ocean.

Wouldn't the waves caused by the inrush of water to fill a 4km diameter hole in the ocean generate some pretty large tsunamis, even if the impact itself doesn't?  Is there a more informed scientist who has a better idea of how to calculate the mount of water column vaporized or ejected, assuming the asteroid does not make it to the sea floor or airburst?

Also, something .3 to .5km in size is big but not so big to discount it falling apart due to gravitational and aerodynamic stresses.  Are there formulae for this?

[p1t1o]

45 minutes ago, Jonfliesgoats said:

The reason I delve into the energy is that a literal hole would exist in the ocean after the impact.  Using the energy of vaporization of water (which I admit is of questionable utility regarding the mechanics of an impact) at an average depth of 3.5 kilometers, a 2km radius hole would be blown in the ocean.

This is assuming an implausibly high efficiency in converting water to steam. In reality, rather than a maximum volume of 100degree steam, you'd get a considerably smaller amount of much hotter steam - there would also be considerable kinetic energy in the fast moving debris. Ergo, the size of the "hole" is going to be far smaller. Initial waves of 400m were quoted for a 250m rock, if I read it correctly - if that equates to a "hole" of depth 400m, that would seem more plausible.

The absorption of a huge amount of energy by the formation of steam helps explain why so little of the energy is converted into large waves.

 

48 minutes ago, Jonfliesgoats said:

Wouldn't the waves caused by the inrush of water to fill a 4km diameter hole in the ocean generate some pretty large tsunamis, even if the impact itself doesn't?  Is there a more informed scientist who has a better idea of how to calculate the mount of water column vaporized or ejected, assuming the asteroid does not make it to the sea floor or airburst?

Isn't that precisely what this new simulation was calculating?

I also think that this represents something closer to a "megatsunami", a term coined to describe the very violent tsunami-like events which follow large landslides that enter water (such as @YNM mentions above), amongst other things - which produce very large waves initially but which are incapable of traversing large ocean stretches like a "classical" tsunami.

 

49 minutes ago, Jonfliesgoats said:

Also, something .3 to .5km in size is big but not so big to discount it falling apart due to gravitational and aerodynamic stresses.  Are there formulae for this?

I think because the structural makeup of any given large rock is probably unique unto itself, and given their size, impossibly complex, I presume accurate solutions would require accurate data on the makeup of the particular rock you are interested in, mainly because large rocks tend to be agglomerations of smaller rocks.

Newtons approximation for impact depth is my go-to for things like this, its very approximate but generally puts you in the right ball park for a lot of things.

***

Fun Fact: The Soviets had plans for a 4-foot diameter, 22m-long torpedo with a 30-40km range for attacking harbours and ports, equipped with a 100Mt warhead!

http://survincity.com/2012/02/russian-nuclear-torpedoes-t-15-and-t-5/

 

[Green Baron]

If you want to dive deeper into it i'd suggest to start with nature geoscience for instance or the egu. Elsevier has geoscience journals as well. Search result here. These are all peer reviewed papers, dry reading but informative.

Asteroid impact is, though, not a major research field of geoscience ;-)

Cheers

gb

[Jonfliesgoats]

Very cool!  I figured my estimate about the conversion of water to steam was off.  I just pulled that number off of google and was aware of no provisions to account for water turned, kineticallt into ejecta, etc.  

The Americans experimented with conventional and nuclear tsunami-bombs to be detonated on the sea floor as well.  They never turned out to be practical.

[kerbiloid]

One can make a test.

Giant asteroid.

Spoiler

Tsunami

Spoiler

Catzilla

Spoiler