Spread nerf is basically where moves that hit multiple targets do less damage to a each Pokemon. These moves do 25% less total damage on a each Pokemon. That is why drill run ends up being more powerful than earthquake, because its BP is dropped 25% to 75 BP, while drill run still has 80 BP. Overall, even though earthquake is weaker, it is generally the better option still due to the fact that it hits multiple Pokemon.
Hope I helped :)