The surface of the planet isn't much the point. The mass of the stuff on the surface of the planet is orders of magnitude less than the mass of the planet itself. This, and the fact that we operate in the broad strokes and margins of error, means that my opinion is firm. We do not need to micromanage calculations for One Piece, nor any other verse with a weirdly high calculated planet size. If they are basically Earth-like, then we can treat them as such.
I'm returning to this so as to offer my thoughts, if it is not implicitly understood; this middle bit is added later. Basically, our EQ calcs look at two things, seismic activity and radiated waves. Seismic activity has to do with the tectonic plates and their movements, whereas radiated waves have to do with the surface materials themselves shaking. Presuming that the phrase "earthquake" is not applied inappropriately (I assume the One Piece thing, for example, is not shaking water, but rather solid ground), then I can think of no reason as to why the EQ calc method wouldn't be usable.
I don't care about these verses, and indeed actively wish to avoid conversation on them. I feel I have made my distinction rather clear at this point, and would like to move past them barring the stated votes/beliefs of voting staff, or the offering of some other context to sway my stance.