Three main factors together determine seismic risks: the level of seismic hazard, the number of people and amount of property that are exposed to seismic hazards and how vulnerable these people and property are to the hazards.
Seismic hazards and risks and how they are measured
Seismic hazards are sources of potential harm or loss during earthquakes. They can be natural phenomena, such as landslides or tsunamis, that are generated by earthquake ground shaking. They can also be elements of the built environment, such as vulnerable buildings, brittle piping or loose equipment, which can become hazards when exposed to earthquake shaking. For more information about seismic hazards and how they can affect people, visit Why Earthquakes Occur.
Because stronger ground motions generate more seismic hazards or hazards that are potentially more dangerous or damaging, scientists measure seismic hazards in terms of potential ground shaking. That is, the level of seismic hazard in a region is measured as the likelihood that ground shaking exceeding a specified strength will occur in the region during a specified period. The probability of such shaking is estimated by analyzing past earthquake activity in the region, evidence of stress building up within area faults and how seismic waves are likely to move through the earth’s crust and overlying soils in the area.
Seismic risks are the harm or losses that are likely to result from exposure to seismic hazards. They are usually measured in terms of expected casualties (fatalities and injuries), direct economic losses (repair and replacement costs) and indirect economic losses (income lost during downtime resulting from damage to private property or public infrastructure). Other, more specific measures of risk are also used for disaster planning, such as probable volumes and durations of utility outages and displaced households and amounts of debris likely to be generated.
Risk in Your Area = Hazard + Exposure + Vulnerability
In any geographic area, three main factors together determine seismic risks: the level of seismic hazard, the number of people and amount of property that are exposed to seismic hazards and how vulnerable these people and property are to the hazards.
Hazard
Seismic hazard levels differ significantly across the United States, both between and within states. To view the levels in your region, go to Earthquake Hazard Maps. These and other maps that depict earthquake hazard levels throughout the country are produced by the U.S. Geological Survey (USGS). For more maps and related information, visit the USGS Earthquake Hazards Program.
Information about seismic hazards in particular parts of the nation is available from regional earthquake consortia supported by FEMA. Hazards in the central United States, home of the New Madrid Seismic Zone, are tracked by the Central United States Earthquake Consortium; those in many western states by the Western States Seismic Policy Council; those in the Pacific Northwest by the Cascadia Region Earthquake Workgroup and those in northeastern states by the Northeast States Emergency Consortium.
Exposure
Earthquake casualties are limited by the number of people present in stricken areas and losses are constrained by the quantity and value of the buildings, infrastructure and other property in those areas. Seismic risk increases as earthquake-prone regions become more densely populated and urbanized. Although local planning and zoning activities can help shape regional growth over time, additional development is generally (and understandably) promoted as a means of strengthening local economies.
Vulnerability
The vulnerability of property to seismic hazards is determined by the prevalence of earthquake-resistant construction. Buildings, lifelines and other elements of the built environment that have been constructed in compliance with the latest seismic building codes and standards will be more resistant to earthquake damage. Older structures that were built under earlier, less-effective codes and have not been retrofitted to meet later standards are likely to sustain more damage. Building Codes further explains this critical factor.
Levels of earthquake preparedness and disaster resilience determine how vulnerable people are to seismic hazards. Individuals, organizations and communities that have invested in assessing their risks and in formulating and implementing responsible preparedness and mitigation measures are likely to experience fewer casualties, less damage and less disruption from earthquakes. Earthquake-resistant construction is a preeminent example of such measures. To learn about measures you should consider, visit Earthquake Safety at Home, at Earthquake Safety at School and Earthquake Safety at Work. Learn how FEMA is working to reduce seismic vulnerability at FEMA Advances Earthquake Safety.