Why can the tremor of the ground from a distant explosion be felt before the sound of the explosion can be heard?

Tremor will be felt first because both the shock (tremor) in the ground and the sound behave as waves -- the tremor is essentially a sound wave in the ground. The tremor will be felt first because the ground is more dense than the air, and the speed of sound increases directly in relationship to increases (although I don't know the specific mathematical relationship) in the density of the material through which it is traveling.