The Loma Prieta earthquake of 1989 damaged the structure, and twisted the dome four inches (102 mm) on its base. Afterward, under the leadership of the San Francisco Bureau of Architecture in collaboration with Carey & Co. preservation architects, and Forell/Elsesser Engineers, work was completed to render the building earthquake resistant through a base isolation system, which would likely prevent total collapse of the building. City Hall reopened after its seismic upgrade in January 1999, and was the world's largest base-isolated structure at that time.