There are five reasons I can think of: 1) vibration due to over stressed structural floor system, 2) continuous span structural floor materials, 3) high water table, 4) underlying soil conditions eroding away creating voids that the foundation rests on, and 5) footing poured on water in forms at time of construction.
1) The Building Code specifies "minimum" loading requirements. In order to save money, designers and builders will push the limits on these values causing the floor to feel springy. Good design will increase those minimums so that the floor feels "stiff". It makes the building "feel" like its high quality. However, this will be evident immediately at the completion of construction, not several years later.
2) New floor joists material (TJ-I's, engineered joists, etc.) are now used and they can be made up to 70-80' long. This allows a builder to install floor joists in multi - span configurations. (In olden times, a floor joists was a 2x8 or 2x10 and came in 16'-18' lengths and usually stopped under walls. ) Now continuous floor joists allows the structural calculations to include the weight in adjoining spans to "off-set" the required deflection so the live load is actually decreased, which allows for more deflection and vibration. However, again, this will be evident immediately at the completion of construction, not several years later.
3) A high water table will allow any vibration to transfer throughout the house. I live in a high water table area (swamp) in the U.S. and extra heavy trucks driving by our building will cause a vibration on the second floor of our building. If the tide is "out" and it's summer, we don't feel it so much. It's winter where you are and I'm sure the water table is high right now. Is it? However, I doubt if your cat could create such a phenomenon, unless Items 1) and/or 2) are occurring.
4) This is bad. Are there pockets of limestone in you neighborhood? You could contact your local Building Official to see if this is even possible. If it is occurring, you might be noticing some early "signs". If a void is created under a footing, then it could settle causing the wood floor joists to span an extra long distance. The "span" of floor joists is the most critical determination for deflection (vibration).
5) During construction, it could have rained in the footings and the contractor left water in the bottom of the footings when they poured the concrete...and didn't remove the mud so the footing did not set on undisturbed soil. This muddy mess set up when it dried out, but has turned back to mush due to a high water or leaky downspout, etc. Again, if a footing settles an inch or less, then the floor joists would need to span to the next support.
Note: Most of this does not make sense if your foundation system is a "slab on grade" system or if you have a basement, because you'd see cracks in your basement walls long before you noticed your cat causing the floor joists to vibrate. Are there doors binding and not closing? Are windows stuck that you can't open? Are there cracks in the plaster above the windows or doors? (Look close, because sometimes it's just a "hairline" crack.)
