Weather does not affect the magnitude of earthquakes. Earthquakes are caused when two big pieces of earth slide against each other ( this happens at a place we call "plate boundaries"). When the plates slip, they jolt the earth around them. Whether or not its sunny, raining or snowing the size of the earthquake is affected only by the amount of slip that happens at these plate boundaries.

No, weather has nothing to do with the strength of an earthquake. If the ground is soggy and wet, an earthquake may cause more damage than it would had the ground been dry, but that's because dirt is less solid when it is wet, not because the earthquake itself is any stronger.