How would a colossal explosion affect an ocean-based planet?

Question

In my book series, a space station called the T-Ray that is capable of causing explosions the size of the Chixculub impact explosion just fired at a planet that is completely covered by a massive ocean, with the exception of a few hundred tiny islands no bigger than Hawaii's big island dotting the surface. The blast from the T-Ray exploded on contact with the seabed, causing an explosion as powerful as the one caused by the Chixculub impact.

Here are some things to note about this planet:

• It is called Ishga Atlantea

• The climate ranges from tropical to Mediterranean (not much range)

• The islands are inhabited by the Dolphinesian Tribe (think native Hawaiians), Platypusian Tribe (think Maori), Kangaroo Tribe (think Aboriginal Australians), and a Viking colony called Eyjagard.

• The shallower parts of the ocean used to be home to the Atlantean Civilization, but the Atlanteans were wiped out when the planet flooded millennia ago after a magic ritual gone awry. Now, 3 main civilizations dwell among the ruins. The most populous are the Ishgas, colonists from the planet Ishgabangaloodoo (think Victorian Britain with Star Wars technology), who originally came to the planet searching for artifacts from the Atlantean Civilization to put in the Ishga Museum. However, many of them really liked the idea of living underwater, so they set up air-filled, transparent domes where they have fashioned a new city from the ruins of Atlantea. They get drinking water through desalination filters and they farm fish (note: there are both sentient and non-sentient versions of the same animal in this universe, so it is perfectly acceptable for the Ishgas or even sentient fish to eat non-sentient fish, but any eating of sentient fish or any other sentient creature is considered cannibalism), coral and kelp for food. The second civilization is Reef City, a huge metropolis carved into a coral reef by sentient fish and other sentient sea creatures. It is the undersea center of trade on Ishga Atlantea, with all the routes between the islands and underwater areas passing through here. Reef city has a culture and a technology level similar to Venice at its peak. The third civilization is the Lobster Empire, a huge, but sparsely populated empire covering most of the seabed. Its capital is the Barbiclaw and it culturally and technologically resembles Rome at its peak. All 3 civilizations feed and water themselves in the same way.

• Ishga Atlantea is an Ishga colony under the occupation of the Tatians, the Ishgas' enemies in The Great War. They have constructed a massive base on Dolphinesian Island, which serves mainly as a file archive and a communications relay tower (communications between planets can only travel a few light-years at a time without stopping at a relay tower to be re-strengthened).

• All of the civilizations on the planet, especially the Ishga colonists, detest their Tatian overlords and will overthrow them the first chance they get.

  Anyway, what happens is this: the Ishgas land a strike team and raid the base, trying to gain access to all the classified files in there and send them to their commanders using the communications tower. Since the raid took out the garrison, the Tatians panic and use the T-Ray to blow up their own base.

Now for my question: how would this negatively impact the civilizations both above and below the surface, and would it help or hurt a potential anti-Tatian revolution on Ishga Atlantea?

Answer 1

Water breathers did the best when Chixculub came.

I am not sure how the T-Ray works or what sort of ray this is, so I will just think of it as a Chixculub meteor. My understanding of that event is that all the crap thrown up into the sky and on re-entry heated the atmosphere to about 150C for a day. Anything that had to breathe that air died.

Stuff that lived through it had air supplies to tide them over (because they were in burrows like crocodiles or early mammals) or they were water breathers like sharks and fish. Probably buried eggs would let a species survive - I bet that is how sea turtles made it thru.

With everyone in your world being underwater, they sidestep the thing which made Chixculub such a killer - the hot air. Your water breathers might have to rebuild but they should be ok. I am less sure about how an air-filled subsurface dome will fare with the shock wave; that is an interesting question. But your Atlanteans too have their own air protected from the heat above and so will do better than dinosaurs did.

Answer 2

I'm not too familiar with the actual event, but looking at the Wikipedia article, its going to be a life ending event. Firstly, the tsunami would likely be 4.6Km tall so thats most of your islands gone. Secondly, it would broil the earths surface, so any dry areas are going to experience fires if not outright roasting to death. Then there are the global earthquakes and volcanos explosions. Since these are islands, they are likely next to volacnos and fault lines and would feel the largest impact.

It would basically ruin any civilizations you have above and below the surface.

This isn't even considering the effects of the giant dust cloud an attack of that magnitude will throw up into the air.