Just how much do speakers suppose to vibrate?

If the speakers are vibrating due to interaction with the floor, as is usually the case with suspended plywood floors found in most modern home upper levels, then its often a good idea to try to isolate the speakers from the floor better by using isolation stands or platforms of some sort. The bad effects of floor interaction and resulting vibrations transmitted to the floor and then the room is muddied less articulate bass that also obscures detail in the mid-range compared to otherwise.

Vibrations detected from the cabinet may also well be a symptom of this and not the root cause of any resulting sound issues.

Many good speaker isolation products out there. Main thing is to find the one that fits your speakers and needs best.

As an example I use Isoacoustics brand isolation stands under my smaller monitors and Auralex Subdude platforms under my floor standers. Both cost a mere $100 or so a pair to try and are similarly effective in greatly  cleaning up the sound overall when needed.

Yes of course it’s the speakers that are the source of vibration. That’s what dynamic speaker drivers do. Who said anything about seismic waves? Not me.

Speaker interacts with floor and waves propagate through the floor. Anything in contact with the floor is vibrating to some extent from there including the entire speaker not just the drivers which are the true source.

If the floor is massive and rigid like a concrete foundation for example the waves produced by the speaker driver movement is not likely to make the floor vibrate so likely not a real issue that case.