This feat happens in the Canon comic Darth Vader: Dark Lord of the Sith, Issue 17.
The Sovereign, an Imperial I-class Star Destroyer owned by Wilhuff Tarkin, and two other ISDs carry out an orbital bombardment of Mon Cala. The three warships devastate most of the planet's northern hemisphere in the time it took Vader to settle a lightsaber duel i.e. not very long.
I shall be attempting a calc of this blast:
^ That was most likely done by the XX-9 heavy turbolaser model.
Dividing by two
According to the Explosion Yield Calculations page, it is standard practice to divide by 2 while calculating the yield of an explosion, but this can be ignored if an explosion is nuclear in nature.
The Canon novels Tarkin, Catalyst, and Phasma make it clear that turbolaser blasts are nuclear.
The Tarkin novel notes that years of orbital turbolaser assaults had raised the temperatures of the atmosphere of the planet Murkhana, with the background radiation of the atmosphere being at barely tolerable levels for humanoid life:
"Atmosphere is a mess, but breathable," the comm officer said, her eyes fixed on the sensor board. "Background radiation is at tolerable levels." Swiveling to Tarkin, she added, "Sir, you might want to consider wearing a transpirator."
~ Star Wars: Tarkin
The Catalyst novel notes that an Imperial facility on the planet Hypori, which was destroyed by star destroyer turbolasers, was still hot a few weeks after the incident, and likely also radioactive. And the Phasma novel has this line:
Calliope Station was made to withstand anything, and it had already survived one radioactive event.
~ Star Wars: Phasma
Context: This line was made in regard to Calliope Station coming out intact after being hit by a single bolt from one of the turbolasers of the Resurgent-class Star Destroyer Finalizer. Such shots are powerful enough to reduce large cities to a "black smudge" visible from orbit.
In conclusion: Since turbolaser blasts are nuclear in nature, dividing the calculated yields by 2 should not be necessary.
So total yield is roughly 800 Gigatons = High 6-C (Large Island level+)
Per shot yield is roughly 266.67 Gigatons = High 6-C (Large Island level)
Anyway, now I shall be doing a calc of my own for this.
Here be a shitty attempt at pixel-scaling:
I shall use spherical cap method for this.
Here is a simple spherical cap calculator: Link
Chord AB (red line) = 614 px
Spherical cap radius (horizontal blue line) = 307 px
Spherical cap height (vertical blue line) = 30 px
According to the calculator, total sphere radius would be 1585.40 px (which would be the planet's radius).
Mon Cala diameter = 11030 km
Mon Cala radius = 5515 km
Explosion diameter (purple circle) = 65 px
Explosion diameter in km = 65*(5515/1585.80) = 226.05 km
Explosion radius in km = 113.03 km
I had a discussion with Kepekley23 about which method to apply here. Since turbolaser blasts are stated to be nuclear in nature, he said it would be okay to use the Fireball radius on this calculator:
Now, the question is: Whether to go with Fireball radius (ground contact airburst) or Fireball radius (minimum)?
The former obviously makes sense given how this explosion happened when those shots hit an underwater city.
But the latter makes sense to me because of how highly focused directed energy weapons are in this verse. A perfect example would be the laser cannons of the Patrol Transport Gunship. They have a calc'ed feat from a Canon comic at 1.16 Kilotons. They also have a potential 1.6 Kiloton feat in the Canon Servants of the Empire novel series. Yet the AOE of each shot is like 10 metres in diameter at the most (highest power setting).
Refer this blog to see how highly focused directed energy weapons are in this verse:
Anyway, I'll just go with both as a Low end and High end respectively.
According to the above calculator, Fireball radius (ground contact airburst) of 113 km requires a yield of 335000 Megatons.
That is 335 Gigatons = High 6-C (Large Island level)
Per shot yield is 111.67 Gigatons = High 6-C (Large Island level)
According to the above calculator, Fireball radius (minimum) of 113 km requires a yield of 1104000 Megatons.
That is 1104 Gigatons = 1.104 Teratons = Low 6-B (Small Country level)
Per shot yield is 368 Gigatons = High 6-C (Large Island level)
XX-9 heavy turbolaser yield per shot:
Dodo's calc = 266.67 Gigatons = High 6-C (Large Island level)
My calc (Low end) = 111.67 Gigatons = High 6-C (Large Island level)
My calc (High end) = 368 Gigatons = High 6-C (Large Island level)
Looks like it's High 6-C or Large Island level regardless of which end is acceptable.
In the Canon novel Aftermath: Life Debt, shots from the heavy turbolasers of the Imperial-class star destroyers Dominion, Neutralizer, and Vitiator cause the entire planet of Kashyyyk to experience violent earthquakes:
Everything shakes and rumbles. Kashyyyk is caught in the throes of tectonic spasms.
~ Aftermath: Life Debt
According to this blog by Gwynbleiddd, that shit requires Island level+ to Large Island level+ power.
There are also some good feats in other Canon novels and comics. Refer this blog for more information.