The chances of a moderate to large earthquake depend on the seismicity of the region. The West Coast is the most active region in the United States for earthquakes since it is where two of the earth's tectonic plates, the Pacific Plate and the North American Plate, meet. However, this does not mean the other parts of the country do not experience earthquakes. The largest earthquake recorded in North America took place on the New Madrid Fault in Missouri in 1811. The Charleston, South Carolina region has also experienced significant earthquakes in the past hundred years.
The time between earthquakes is also important in determining the magnitude of an earthquake. The longer the forces build up along a fault, the more energy will be released when the fault ruptures, creating a larger earthquake. Recent building codes have required buildings to be life-safe from forces generated by an earthquake with an approximate return period of 500 years.
The greatest risk from an earthquake is that to life safety. In past earthquakes, many buildings have collapsed, killing thousands of people. Modern building code requirements are set with the intent of protecting life safety. The building may be damaged beyond repair, but the building has not collapsed, allowing safe evacuation, and the overall risk of life-threatening injury is low.
The problem today is that many buildings were designed and constructed before modern seismic codes were enacted. Therefore, there is still significant risk to life safety in the event of a major earthquake. Some types of buildings are more susceptible to collapse than others. Older, unreinforced masonry buildings are one of the most vulnerable types.
Traditional seismic codes have focused on ensuring a life safe performance and offered some expectation that the damage would be repairable. However, in recent moderate and large earthquakes, while modern buildings have performed as designed, structures have been irreparably damaged or too costly to repair. Economic losses due to property damage have been extensive in recent years and have led to the development of new performance-based methodologies with the intent on controlling property damage and losses.
Another economic factor in assessing damage following earthquakes is the risk of business interruption. Most of the revenue generated by companies is related to the products and services they provide to the public, rather than the physical assets of the company. Any significant interruption to the production of these goods and services can have an adverse effect on companies, including putting them out of business. Technology companies in the Silicon Valley are just such a sector where business interruption is a critical issue. The recent performance-based guidelines developed by the structural engineering community focus on the design of new structures and strengthening of existing buildings with the intent of minimizing business interruption.
The healthcare services industry is another sector where continued operation following an earthquake is critical. Recent legislation enacted by the State of California has required that hospital acute care facilities must upgrade their buildings to be operational following a major earthquake by the year 2030.