Major earthquakes around the world, such as those in Japan in 2011 and 2016, periodically renew attention to the seismic risk to America's infrastructure, including its highways. 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. Seismic resilience of the U.S. highway system has improved in recent decades as investments have been made to build new, more resilient infrastructure and to retrofit existing structures. However, not all existing highway infrastructure has been retrofitted, and no infrastructure can be constructed in a cost-effective manner to be immune from the most intense earthquakes, so some seismic risk to the U.S. highway system remains. Although earthquake hazards in the United States generally are well documented, little national or federal data exist about the seismic risk to U.S. highway infrastructure. Instead, seismic highway risks typically are assessed and addressed by state and local entities which are generally responsible for building and maintaining that infrastructure. The federal government supports these nonfederal efforts by providing data on the seismic hazard for different locations, assisting in the development of construction standards and guidelines, and undertaking research, training, and the development of tools to assist in risk reduction. In limited circumstances, the federal government invests directly in improving resiliency of specific highway structures. This report addresses frequently asked questions about the risk from earthquakes to highway systems, including bridges, tunnels, pavements, and other highway components. Particular attention is given to highway bridges, which often are the most vulnerable highway structures. The report also discusses federal and nonfederal actions to reduce seismic risk to the U.S. highway system. 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 hazard is assessed by determining the probability of different intensities of shaking of a highway structure caused by earthquakes. Although some seismic hazard exists everywhere in the United States, the magnitude of the hazard varies greatly across the country and within individual states (see Figure 1 ). Seismic hazards are greatest in the western United States, particularly in California, Washington, Oregon, and Alaska and Hawaii. While the areas of the country most prone to earthquakes are well known, the timing, magnitude, and other characteristics of a specific seismic event, such as the intensity and duration of shaking, cannot be accurately predicted. Maps depicting U.S. seismic hazards are updated periodically as more and better data are obtained and earthquake science improves. 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. Vulnerability is determined by the design and current condition of the specific highway element. Consequences depend on the role of a highway in the transportation system and how its availability or damage affects public safety, recovery efforts, economic and social disruption, and national defense. While seismic hazard is a function of plate tectonics and cannot be controlled, actions can be taken to manage vulnerability and consequences. Human-induced earthquakes, also known as "induced seismicity," are an increasing concern in regions of the United States where wastewater from oil and natural gas production activities is injected into the subsurface through deep disposal wells. The immediate concern is that injection of these fluids into underground formations may be responsible for damaging earthquakes in regions that typically do not experience much seismic activity. The induced seismicity hazard from deep-well injection represents what might be considered a short-term hazard, compared with the perennial seismic hazard from natural tectonic forces, because to some degree the chance of an earthquake caused by deep-well injection depends on the injection activity. In response to the increase in earthquake activity in some locations that appears to be associated with deep-well injection, the U.S. Geological Survey (USGS) recently updated its seismic hazard maps. The updated maps include a one-year seismic forecast for the central and eastern United States that factors in contributions from both natural and induced earthquakes ( Figure 2 ). Compared to natural tectonic forces responsible for earthquakes, which constitute a perennial hazard, the hazard from human-induced earthquakes is likely temporal in nature. However, the USGS considers the hazard sufficient to update its earthquake hazard maps, and the hazard could increase the vulnerability of U.S. highway infrastructure where the newly depicted earthquake hazard and highways intersect. 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. However, many states with large seismic hazards have compiled data on the vulnerability of highway components within their borders, ranked highway infrastructure based on this vulnerability, and used these data as part of the decisionmaking process for distributing highway funding. The industry standards and guidelines for construction and retrofitting of highway components also now call for increased seismic resiliency of highway infrastructure. These standards were developed and adopted by the American Association of State Highway and Transportation Officials (AASHTO) in consultation with the U.S. Department of Transportation (DOT) and other stakeholders, and incorporated by reference into the Code of Federal Regulations (23 C.F.R. §625). Seismic design guidelines for bridges, developed by Federal Highway Administration (FHWA) and the California Department of Transportation, were adopted by AASHTO in 1983, and these guidelines became a national standard in 1992. There are no national seismic design standards for tunnels, culverts, pavements, and other highway components, although there have been retrofitting guidelines available from FHWA since 2006. From the early 1970s to the late 1990s, much of the effort to reduce seismic vulnerability focused on improving the resiliency of the most vulnerable highway structures . More recently, efforts to understand and address highway risks have been broadened from structures to highways systems . For example, under a program authorized by Congress in the Transportation Equity Act for the 21 st Century (TEA-21; P.L. 105-178 ), FHWA developed a software package to estimate the loss of highway systems capacity in particular localities due to earthquakes. The software is available to federal and nonfederal entities for use in evaluating highway investments. The performance of highway bridges is a critical determinant of the seismic performance of a highway system. Some western states have been following seismic design practices since the early 1970s, after the 1971 San Fernando (CA) earthquake. By contrast, states in the New Madrid seismic zone (AR, IL, IN, KY, MO, MS, TN), which have serious hazards but less recent experience with significant earthquakes than the West Coast, did not adopt seismic design standards until the early 1990s. National seismic bridge design standards have been in place since 1992, based on guidelines developed in the 1970s and 1980s. These standards are regularly updated to incorporate lessons learned from the behavior of bridges during earthquakes in the United States and abroad. Because many highway bridges predate national standards, numerous bridges remain vulnerable to more frequent seismic hazards. For example, roughly 73% of all bridges in the New Madrid seismic zone were built prior to 1990 and likely were not designed to withstand the region's seismic hazards. The current industry minimum standard for new U.S. bridges and retrofits is to design for no collapse in a span or part of a span for the most intense earthquake anticipated during a 1,000-year period, which is known as the 1,000-year return period event (or 0.1% annual probability event). The magnitude of the assumed 1,000-year earthquake varies by location depending on the earthquake hazard present, and, in general terms, determines the need for and cost of seismic bridge design and retrofitting ( Figure 1 ). If the projected magnitude of the 1,000-year event is correct, the probability of an earthquake that exceeds that magnitude during the 75-year theoretical design life of the bridge is roughly 7%. This seismic design standard was adopted by AASHTO in consultation with DOT and other stakeholders, and is incorporated into the Code of Federal Regulations by reference (23 C.F.R. §625). Some states, such as South Carolina and California, have adopted stricter standards. 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. While seismic damage to bridges often is more visible than damage to other highway structures, the performance of these other structures may significantly influence system performance and the consequences of a seismic event. Tunnels are generally considered to be less vulnerable to seismic hazards than bridges, unless the tunnel crosses a geologic fault. If a tunnel is damaged, the consequences can be significant. Tunnels often link highway systems with little or no redundancy if the tunnel fails. While pavements, retaining walls, and embankments may not pose the greatest risk to lives during a seismic event, these structures and their rapid repair are receiving increasing attention as significant for highway system performance. Damaged culverts through or beneath highways can contribute to erosion of highways or flooding of the highway and surrounding area. In 2006, FHWA published a manual providing guidance on the seismic evaluation and retrofit for these other types of highway structures. The FHWA guidance provides for a two-tier design approach. The guidance recommends a performance level of fully operational for the more likely 100-year return period event (a 1% annual probability event) for most infrastructure components. Fully operational means that damage is negligible and full service is available for emergency and non-emergency vehicles after inspection and clearance of debris. Also, any damage is repairable without interruption to traffic. The FHWA guidance also recommends a performance level of providing for life safety (i.e., no collapse and no loss of life) for most infrastructure, and for some essential infrastructure to be operational, for the more rare 1,000-year return period earthquake. AASHTO has no formal seismic standards for highway structures other than bridges. 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. Highway structures are assessed on structural vulnerability, site characteristics, and other factors to determine the priority for retrofitting. Other factors may include a structure's importance to the highway system, its non-seismic deficiencies, and its remaining useful life. 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. California's Bridge Seismic Retrofit Program, undoubtedly the largest in the country, provides some indication of the magnitude of the task. Some retrofitting was done in the 1970s and 1980s, but larger retrofitting programs involving about 2,200 state-owned bridges were launched after the 1989 Loma Prieta earthquake and the 1994 Northridge earthquake. According to the California Department of Transportation (CalTrans), these programs cost about $11 billion, although this total cannot be adjusted to 2016 dollars to account for inflation. Around three-quarters of the funding went to retrofit just nine very large, complex toll bridges. Most of this work has been completed, but more effort and funding are going to retrofit several hundred bridges owned by local governments. A study of the New Madrid seismic zone found nearly 13,000 vulnerable bridges in seven states that would likely require retrofitting to satisfy the current seismic bridge design standards for new bridges. The study did not estimate the cost of these retrofits. 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. For a specific project, a state generally must provide at least 20% of the money, or at least 10% if the project is on the Interstate Highway system. Although federal funds may help states with the cost of retrofitting, any federal money directed to this purpose by a state is not available for other uses; that is, there is an opportunity cost of using the funds for retrofitting in lieu of other highway or bridge improvements. State and local governments and other highway and bridge owners also may use their own funds for seismic retrofitting. In some situations additional federal funds have been provided for infrastructure improvements, including seismic retrofitting. For example, TEA-21, enacted in 1998, authorized $25 million in FY1998 for the seismic retrofit of the Golden Gate Bridge. 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. The Fixing America's Surface Transportation (FAST) Act ( P.L. 114-94 ) provided funding for the Highway Research and Development Program to sponsor research including "to study vulnerabilities of the transportation system to seismic activities and extreme events and methods to reduce those vulnerabilities." The FAST Act allows funding for the "maintenance of seismic research activities, including research carried out in conjunction with other Federal agencies to study the vulnerability of the transportation system to seismic activity and methods to reduce that vulnerability" (23 U.S.C. §503). The FAST Act authorized $125 million annually for FY2016 through FY2020 for the Highway Research and Development Program. The FAST Act also authorized $67 million for FY2016 and $67.5 million annually for FY2017 through FY2020 for the Technology and Innovation Deployment Program, which funds projects to accelerate the implementation of highway-related innovations and technologies. Other planning and research are funded through the State Planning and Research (SPR) Program. States are required by the FAST Act to set aside 2% of highway funds from five core highway programs for the SPR Program. FHWA's National Highway Institute (NHI) currently offers several courses on bridge seismic design and retrofitting. Under the National Earthquake Hazards Reduction Program (NEHRP), the federal government supports efforts to assess and monitor earthquake hazards and risk in the United States. Four federal agencies responsible for long-term earthquake risk reduction coordinate their activities under NEHRP: the USGS, NSF, FEMA, and NIST. FHWA coordinates with and contributes to the activities of the NEHRP agencies. As part of NEHRP reauthorization in 1990 ( P.L. 101-614 ), Congress authorized the USGS to monitor seismic activity in the United States. To this end, the USGS operates a nationwide network of seismographic stations; it also operates a notification system to disseminate information regarding the location, magnitude, and epicenter of earthquakes. These data have multiple uses related to highway systems. For instance, data from the network can be used soon after an earthquake by those responsible for bridges and highways to prioritize inspections and response efforts, especially for critical lifeline highway systems. This can improve highway services for local, state, and federal emergency responders. Another example of how federal agencies can enhance seismic resiliency of highway systems is through informing pre-disaster and post-disaster efforts. For example, FEMA developed a methodology and software called the Hazards U.S. Multi-Hazard (HAZUS-MH). The software combines existing scientific knowledge about earthquakes, available engineering information (including for highway structures), and other data to produce maps and estimates of economic losses. The program can be used by local and state decision makers to estimate losses from damaging earthquakes, hurricane winds, and floods, and the spatial relationships between the risk locations, populations, and highway and other assets for preparedness, mitigation, and recovery. For preparedness, the output of HAZUS-MH can inform the development of mitigation plans and policies, highway and other infrastructure investment decisions, and emergency preparedness and response actions.

Summary: 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).