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.