How much weaker are moves like earthquake in double battles?

Question

Moves like earthquake and explosion are weaker in double battles, but how much weaker?

Answer 1

If a move is targeting more than one pokemon (either both opponents or both opponents and partner), its power against each pokemon is 75% its normal value.

So for example, Earthquake has power 100. In a double battle it will be 75. The exception is if the move only hits one pokemon, like if the others have fainted. Then it will be the usual full power.