Enki slams Gurren Lagann into the ground and shatters it considerably. This will be more or less straightforward. Gurren Lagann's main body is 10.5446154m tall.
1 pixel = 10.5446154m/13 = 0.811124262m
0.811124262m X 2876 = 2332.79338m
2332.79338m/2 = 1166.39669m
Now for the thickness of the part shattered.
Volume as a cylinder.
V = πr^2h
= π X 1166.39669^2 X 98.95716
= 422950607m^3
Violent fragmentation of rock is 69000000 joules/m^3.
E = 422950607 X 69000000
= 2.91835919e16 joules
= 6.9750458652007649718 megatons
Final Results
Enki slams Gurren Lagann into the ground = 6.975 megatons