How can putting a building on rubber pads protect it from an earthquake?

Rubber pads protect a building by changing the rhythm at which it sways. Every structure has a natural period, the time of one back-and-forth swing. Short buildings sway in under a second, and that is exactly where the shaking of many earthquakes is strongest. Put the building on flexible bearings and its period stretches out, so it rocks slowly while the ground jerks quickly beneath it. The building 'feels' far less of the earthquake.

The proof came in Los Angeles in 1994. After the Northridge earthquake, eleven hospitals in the area were fully or partly closed. USC University Hospital, built three years earlier on an isolated foundation, stayed open: patients felt a gentle sway rather than violent jolts, and it could take new patients. Ordinary buildings are only designed not to collapse; they are allowed to be damaged. For hospitals, that isn't good enough.

Modern bearings sandwich rubber between steel plates, soft sideways but stiff under the building's weight, often with a lead core that soaks up energy so the building doesn't keep ringing like a bell. The idea is old: the Tomb of Cyrus the Great, around 550 BC, sat on a foundation designed to slide. But isolation is not earthquake-proofing. Buildings need room to move, and if the ground itself shakes slowly, as Mexico City's soft lakebed did in 1985, a badly tuned isolated building could swing even harder.