In base isolation systems, what is the fundamental mechanism by which seismic forces are reduced in the superstructure?

The fundamental mechanism by which base isolation systems reduce seismic forces in the superstructure is by lengthening the building's fundamental period of vibration and decoupling the building from the ground motion. Base isolators are installed between the foundation and the superstructure. When an earthquake occurs, the ground moves rapidly, but the base isolators allow the superstructure to move more slowly and smoothly. This is achieved by using flexible bearings that significantly increase the building's fundamental period (the natural frequency at which it tends to oscillate). Because the lengthened period shifts the building's response away from the high-acceleration region of the earthquake's response spectrum, the superstructure experiences lower accelerations and, consequently, lower seismic forces. The isolators essentially act as a filter, preventing much of the ground motion's energy from being transmitted into the building. In simpler terms, the building 'floats' above the ground motion, reducing the forces it experiences.