Concerns about the U.S. highway system's seismic risk stem from interest in protecting public safety, facilitating response and recovery efforts, and minimizing economic loss and social disruption. This report addresses frequently asked questions about the risk from earthquakes to highway systems, including bridges, tunnels, pavements, and other highway components. What Are the Components of Seismic Risk? Seismic risk to a highway system is determined by three factors:
likelihood of seismic events of varying magnitudes, and related physical events, often referred to as the hazard; vulnerability of highway structures to damage from such events; and potential consequences of that vulnerability (e.g., lives lost, economic disruption). Seismic hazards are greatest in the western United States, particularly in California, Washington, Oregon, and Alaska and Hawaii. Other events that can occur in response to an earthquake, including soil liquefaction, landslides, tsunamis, flooding, and fires, also contribute to the hazard exposure of a highway. No national database exists on the seismic design and retrofit status of highway system components; thus, a perspective on vulnerability at the national level is unavailable. The performance of highway bridges is a critical determinant of the seismic performance of a highway system. National seismic bridge design standards have been in place since 1992, based on guidelines developed in the 1970s and 1980s. Many of the most vulnerable older bridges, particularly in West Coast states, have been retrofitted to improve seismic resilience. In contrast, many older bridges in the New Madrid seismic zone have not been retrofitted. Existing highway structures vulnerable to earthquake hazards can be replaced, retrofitted, abandoned, or simply left alone. The decision with respect to each structure generally is up to state governments and other infrastructure owners; most importantly, state governments determine whether to pursue retrofitting or replacement as they set priorities for using federal and state highway funds. Because no national data exist on the status of retrofitting existing highway bridges or other infrastructure, no national estimates exist of what it might cost to retrofit the most vulnerable structures. Seismic retrofitting of highway infrastructure is an eligible expense for federal highway funds apportioned to states (i.e., distributed by formula), via the Surface Transportation Block Grant Program and the National Highway Performance Program, and as preventive maintenance under other federal-aid highway programs. State and local governments and other highway and bridge owners also may use their own funds for seismic retrofitting. What Is the U.S. Department of Transportation's Role in Enhancing Seismic Resiliency? Apart from its role in administering federal-aid highway funds that may be used for seismic retrofitting, DOT and its FHWA also fund seismic hazard research and training and coordinate with other federal agencies working on enhancing resilience. Four federal agencies responsible for long-term earthquake risk reduction coordinate their activities under NEHRP: the USGS, NSF, FEMA, and NIST.
Earthquakes and related events, such as soil liquefaction, landslides, tsunamis, flooding, and fires, pose risks to highway infrastructure. Concerns about the U.S. highway system's seismic vulnerability stem from interest in protecting public safety, facilitating response and recovery efforts, and minimizing economic loss and social disruption. This report addresses a number of frequently asked questions about earthquake risk and highway system components, especially bridges.
The three components of seismic risk to the highway system are the likelihood of an earthquake of varying magnitudes, the vulnerability of highway structures to damage, and the potential consequences of that vulnerability. Seismic hazards are greatest in the West Coast states, California, Washington, and Oregon, as well as Alaska and Hawaii. Although disputed by some geologists, the U.S. Geological Survey (USGS) identifies high hazard in portions of several southern and midwestern states associated with the seismic zone near New Madrid, MO. There is also growing concern about earthquakes induced by activities associated with oil and natural gas production.
Existing highway structures vulnerable to earthquake hazards can be replaced, retrofitted, abandoned, or simply left alone. The decision of how to manage each structure's seismic risk generally is up to state governments and other infrastructure owners. There is no national database of seismic design and retrofit status of highways. Many states at greatest risk have assessed their highway infrastructure's seismic risk and directed funding to mitigate these risks.
Bridges are a major determinant of the seismic performance of a highway system. National seismic bridge design standards have been in place since 1992. Many of the most vulnerable older bridges, particularly in West Coast states, have been retrofitted to improve seismic resilience. In contrast, many older bridges in the New Madrid seismic zone have not been retrofitted.
Because no national database exists on the status of retrofitting existing highway bridges or other infrastructure, no national estimates are available for what it might cost to retrofit the most vulnerable structures. Seismic retrofitting of highway infrastructure is an eligible expense for federal highway funds distributed by formula to states. However, when states choose to invest their federal funds in seismic risk reduction, less is available for other purposes. State and local governments and other highway and bridge owners also may use their own funds for seismic retrofitting.
In addition to its role in administering federal-aid highway funds that may be used for seismic retrofitting, the Department of Transportation (DOT) supports seismic hazard research and training and coordinates with other federal agencies working on enhancing resilience. Other federal agencies that contribute to improving the highway system's seismic resiliency include the USGS, the National Science Foundation (NFS), the Federal Emergency Management Agency (FEMA), and the National Institute of Standards and Technology (NIST).