An earthquake is one of the most devastating natural phenomena known to man. Earthquakes such as the Lisbon earthquake of 1755 or the San Francisco earthquake of 1906 can devastate cities, causing thousands of casualties and untold damage to homes and businesses. Although no building can be completely protected against earthquake, modern architecture provides a number of strategies to reduce the risk of earthquake damage.
When an earthquake causes the ground to shake, older, weaker buildings crumble and collapse. Base isolation is one method of preventing buildings for collapsing. Base isolation involves installing a system of pads, springs and ball bearings in a building's foundation. When an earthquake hits, the building shakes and sways much less than a building resting directly on the ground. By being more mobile than other buildings, a base-isolated building actually resists the effects of the quake.
Buildings constructed too close to one another are highly vulnerable to damage from earthquakes. Some earthquake-safe buildings employ "moats" or "buffer zones," areas of empty space around the walls. If the building sways or tilts during an earthquake, the moat will prevent it from colliding with its neighbours and increasing the potential damage. In densely-populated urban environments, however, creating a protective buffer zone around a potentially vulnerable building is not always possible.
Building materials such as concrete, brick or stone can fall to pieces in the rapid shaking of an earthquake. Reinforced concrete and masonry are more resistant to earthquake stress than other types of building. Prestressed concrete with internal steel reinforcement bars is used in large constructions intended to withstand earthquakes, such as bridges and motorway overpasses. Steel structures or structures with steel frames are usually resistant to earthquakes, as long as they are built to modern safety standards.
Modern buildings in high-risk areas are usually required to adhere to strict earthquake safety codes. Cities that wish to preserve their architectural heritage face the challenge of bringing older buildings up to safety standards without damaging them. The practice of seismic retrofitting involves installing new reinforcement materials in historic buildings. Retrofitters must balance the need to keep older buildings safe with the desire to preserve the historic materials used in their construction.