These strong-motion sensors provide the latest technology to help us do that. In addition to City Hall, accelerometers have been installed on other structures in the San Francisco area, such as the Golden Gate Bridge and San Mateo Bridge. In the event of a large earthquake, information gathered by the sensors can be analyzed by DOC computers and seismologists and help emergency crews determine the hardest-hit areas within minutes after the quake. During the 1989 Loma Prieta earthquake, the City Hall dome twisted like a bottle cap, moving two inches. Walls and concrete floor slabs cracked on all levels.
Base isolation helps buffer a building from seismic waves.
There are 590 rubber cylinders at the base of City Hall's support columns that dampen the effect of the seismic waves. Base isolation also allows the building to move more than two feet in any direction during an earthquake, further minimizing quake damage. Data gathered by SMIP sensors on how structures react to temblors is being applied by engineers to create stronger, safer building plans for new construction and retrofits. The Strong Motion Instrumentation Program is part of the DOC's Division of Mines and Geology, which is California's State Geological Survey. SMIP instruments, in combination with sensors from the U.S. Geological Survey, California Institute of Technology and UC Berkeley, are the backbone of a developing integrated seismic network that will cover most of the state's earthquake-prone areas.