It's a simple fix.
A quick search on google give us 627,6 km^2 for square root of the surface area of Tokyo.
627,6 km^2 = 627,600,000 m^2
Radius = 627,600,000/2 = 313800000m
W = 313800000^3 × ((27,136 × 1.37895 + 8,649)^(1 ÷ 2) ÷ 13,568 - 93 ÷ 13,568)^2 = 2.4833866e+21 joules or 593 Gigatons