Sounds interesting, lets take a shot.
Ok so the idea is to vibrate the ground underneath the target to the point where they are unable to stand properly.
Using your research it would require a 3 to 4 richter earthquake to cause this to be effective.
So, you need to create a localised earthquake of up to richter 4 in order to destabalize a person to the point where they cannot stand easily.
So far, I'd say potentially not possible, however ima go with it anyway, it sounds cool.
- Not going to be widely useable. If you were to use this in a populated place the area of effect would probably cause a lot of collateral damage, probably damaging property and possibly actually hurting people, which based on your question is not the intention. It would likely be limited to use in open unpopulated areas, meaning it is not widely deployable.
Not going to be easy. Maybe if you were to use ultrasound or possibly just a giant hammer hitting the ground, you could POSSIBLY make a richter 4 earthquake - don't quote me on that I haven't researched that part - so I'm thinking a large machine designed to hit the ground super hard and cause an earthquake around it (sketch included later).
Not going to be remote. I think if you were to use ultrasound, it may not be effective, and could run the risk of harming the people around it, as you would have to use frequencies low enough to affect the ground. Something like a large hammer would be more effective I think.
Here I'll include a sketch if I can:
Ok so terrible sketch included, this would be my idea.
My thought process is that this is parachuted in or something and dropped into a warzone. This would initially cause an impact, staggering nearby enemies, but the main purpose of this drop is to set the machine in place so it does not fall over etc. The hammer would then raise (or maybe it would be raised before drop) and would drop as hard as it can (maybe with weights attached etc) and create as heavy an impact as possible, I'm not sure what kind of impact you would need to make that kind of scale of impact, I'm better at ideas than actual straight up calculations but hey, I'm sure you could do it.
The impact would then cause a short but intense shockwave, easily staggering everyone nearby depending on scale of the hammer impact.
As for specifically your idea of a continuous earthquake, I don't think it would be possible.
Then for the miniaturising of this, again no idea, maybe a sort of gauntlet with rocket boosters, similar to the Gypsy Danger Pacific Rim rocket punch, but the impact would likely be far less in terms of magnitude, and the shock reverberating back at the user could cause serious damage to the user I'm not sure, I don't have the knowledge to give you an accurate prediction there.
Just as a pointer, yes there are no numbers in this, I apologise, I don't have the expertise to give you the accurate data, but this could be a good method, even if there are no numbers to back it up YET (may add more if I can).