because removal of a lot of dirt and rock at the foundation would have lightened the load at that spot.
A cubic yard of granite weighs abbout 2.25 tons. If the building has a base of 100 yards by 100 yards and the hole is 31 yards deep then the weight of material removed should be about 700,000 tons.
Anyway, why should a building be causeing tremors or earthquakes other than tiny ones just under the building? All buildings have some settlement.