It depends on countless factors, so you can really do anything you like.
You can get tremors from a footstep two ways:
- Waves emitted from the step
- Waves emitted because of non-linear effects within the ground.
The waves emitted from your steps you can actually feel on human scales. Just go to the 2nd story of a house and walk around. You'll hear the waves emitted when you step. In fact, if you can find an industrial building, many of them have very large distances between supports (so you can have large open work areas). I work in one where there's points that you can hop up and down at and people can feel the vibrations in the floor 20 feet away.
These effects are very dependent on how the creature chooses to step. Go to your 2nd story building and clomp around for a bit. Then try to walk like a cat stalking its prey. The kind of stepping you do dramatically changes what tremors you might find.
The second effects are the non-elastic effects which occur when you push the ground too far. In our 2nd floor house example, these are the creaks and squeaks that the floor emits when you push on it. Naturally, as you've noticed that some floorboards creak more than others, this is very dependent on the exact terrain. A nice granite mountain could withstand a much larger creature than, say, wet sandstone.
Of course, with this many variables you can get all sorts of interesting effects. Trees can cause tremors, for instance. In fact, they can be downright creepy.