Trees are of course very heavy, they are like putting a two foot deep slab of concrete in your garden,
it will compress the soil, anything built close to that soil will eventuall move down too, and not in
a nice uniform manner.
The trees will take the damping charachteristics out of the earth surrounding them, vibrations
will not get dampened, though the trees will still dampen sound, certain sounds better than others,
the grounds ability to dampen vibrations will be heavily impacted.
Wet ground will vibrate a lot, stand in a swamp and jump on the ground that is water logged
and the whole area around you will move, that is only with the weight of a person, try putting
a road down and lace it with cars trucks and busses, it will viberate, there are a few car parks in
my area that were built on such ground, they are very un nerving to sit in your car in, as the movement
of cars in the park, or within a half a mile outside the park can be easily felt, especially
lorries on the road, they push a shokwave that gives one the feeling jack daniels was in town the night before.
So the sepric tank is the culprit in your case water logging the area and transmitting the vibs.