Earthquake Risk and
U.S. Highway Infrastructure:
Frequently Asked Questions
William J. Mallett
Specialist in Transportation Policy
Nicole T. Carter
Specialist in Natural Resources Policy
Peter Folger
Specialist in Energy and Natural Resources Policy
March 30, 2011
Congressional Research Service
7-5700
www.crs.gov
R41746
CRS Report for Congress
P
repared for Members and Committees of Congress
Earthquake Risk and U.S. Highway Infrastructure: Frequently Asked Questions
Contents
Introduction ................................................................................................................................ 1
What Are the Components of Seismic Risk?................................................................................ 1
How Vulnerable Is the U.S. Highway System to Earthquakes? .................................................... 2
How Vulnerable Are Highway Bridges to Earthquakes? .............................................................. 4
How Vulnerable Are Other Highway Components to Earthquakes? ............................................. 5
What Are the Options for Improving the Seismic Resilience of Existing Highway
Infrastructure?.......................................................................................................................... 5
How Much Would It Cost to Retrofit Vulnerable Highway Infrastructure?................................... 6
Who Pays to Retrofit Highway Infrastructure? ............................................................................ 6
What Role Does the U.S. Department of Transportation Have in Enhancing Seismic
Resiliency of the Highway System? ......................................................................................... 7
What Role Do Other Federal Agencies Play?............................................................................... 7
Figures
Figure 1. U.S. Bridge Seismic Hazard ......................................................................................... 3
Contacts
Author Contact Information ........................................................................................................ 8
Congressional Research Service
Earthquake Risk and U.S. Highway Infrastructure: Frequently Asked Questions
Introduction
The 9.0 magnitude earthquake that struck off the coast of Sendai, Japan, on March 11, 2011, has
renewed concerns about 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 cost-effectively constructed 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.1
The report also discusses federal and nonfederal actions to reduce seismic risk to the U.S.
highway system.
Japan is generally regarded as the country with the most earthquake-resilient infrastructure.
Research into the performance of Japan’s highway structures and systems during and after the
massive Sendai earthquake and resulting tsunami is ongoing. Japanese and U.S. earthquake
science and engineering researchers have well-established collaborations aimed at gathering
empirical data and drawing lessons from significant events. It is too soon to know how the events
in Japan will affect understanding of the seismic risk of U.S. highway infrastructure, and whether
changes in U.S. seismic highway design standards and guidance will result.
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).
1 Wen-Huei (Phillip) Yen, “Earthquake!,” Public Roads, vol. 74, no. 2 (September/October 2010).
Congressional Research Service
1
Earthquake Risk and U.S. Highway Infrastructure: Frequently Asked Questions
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.2 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 periodically are updated 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.
How Vulnerable Is the U.S. Highway System to
Earthquakes?
No national database exists on the seismic design and retrofit status of highway system
components; thus, a national level perspective on vulnerability 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 decision-making 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
21st 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
2 For more on earthquake hazards, see CRS Report RL33861, Earthquakes: Risk, Detection, Warning, and Research,
by Peter Folger.
Congressional Research Service
2
Earthquake Risk and U.S. Highway Infrastructure: Frequently Asked Questions
to federal and nonfederal entities for use in their evaluations and prioritizations of highway
investments.3
Figure 1. U.S. Bridge Seismic Hazard
Shaking Expected for Tall Structures like Bridges from Seismic Events with 1,000-Year Return Period
Source: Map produced for CRS by U.S. Geological Survey (USGS), March 2011, with minor formatting changes
made by CRS.
Note: The color legend shows the shaking expected in tall structures—expressed as a percentage of the
acceleration due to gravity (g).
3 The software is called REDARS (Risks from Earthquake Damage to Roadway Systems).
Congressional Research Service
3
Earthquake Risk and U.S. Highway Infrastructure: Frequently Asked Questions
How Vulnerable Are Highway Bridges to
Earthquakes?
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.4
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.5
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).6 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 (see Figure 1).7 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.8
4 Some scientists have called into question whether the earthquake hazard as depicted in the USGS maps is too high for
the New Madrid Seismic Zone. These researchers challenge whether the benefits of building structures to conform with
earthquake probability estimates merit the costs, in light of the uncertainty in making those probability estimates. For a
fuller discussion of the scientific controversy, see CRS Report RL33861, Earthquakes: Risk, Detection, Warning, and
Research, by Peter Folger.
5 Timothy Wright, Reginald DesRoches, and Jamie E. Padgett, “Bridge Seismic Retrofitting Practices in the Central
and Southeastern United States,” Journal of Bridge Engineering, January/February 2011, pp. 82-92.
6 The probability of flooding events is commonly referred to in similar terms. For example, the 100-year flood event
has a 1% annual probability, or a 1/100 chance of occurring each year. If the 100-year flood occurs one year, the
probability of the same magnitude flood remains 1/100 for the next year. It does not mean that another 100-year flood
will not occur for 99 more years, a common misperception.
7 Figure 1 produced by USGS for CRS is a simplified and color version of the map produced by USGS and used by
AASHTO.
8 South Carolina Department of Transportation, 2008 Seismic Design Specifications for Highway Bridges, Version 2,
July 2008, http://www.scdot.org/doing/bridge/pdfs/specs_2008.pdf; California Department of Transportation, Seismic
Design Criteria, November 2010, http://www.dot.ca.gov/hq/esc/earthquake_engineering/SDC_site/2010-11-
17_SDC_1.6_Full_Version_OEE_Release.pdf.
Congressional Research Service
4
Earthquake Risk and U.S. Highway Infrastructure: Frequently Asked Questions
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.9
How Vulnerable Are Other Highway Components
to Earthquakes?
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.10 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.
What Are the Options for Improving the Seismic
Resilience of Existing Highway Infrastructure?
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
9 Wright, et al.
10 U.S. Department of Transportation, Federal Highway Administration, Seismic Retrofitting Manual for Highway
Structures: Part 1- Bridges, FHWA-HRT-06-032, McLean, VA, Jan. 2006, and Federal Highway Administration,
Seismic Retrofitting Manual for Highway Structures: Part 2- Bridges, FHWA-HRT-05-067, McLean, VA, January
2006.
Congressional Research Service
5
Earthquake Risk and U.S. Highway Infrastructure: Frequently Asked Questions
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.11
How Much Would It Cost to Retrofit Vulnerable
Highway Infrastructure?
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. California’s retrofit program, which has
been ongoing for several decades, involves about 4,700 bridges and is estimated to cost about $14
billion.12 Nearly $10 billion of the total is to retrofit just nine very large, complex toll bridges.13 A
recent 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.14 The study did not estimate the cost of these retrofits.
Who Pays to Retrofit Highway Infrastructure?
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 Program and
the Highway Bridge Program, and as preventive maintenance under other federal-aid highway
programs.15 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 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 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.
11 Yen.
12 Several elements of the program are complete. Much of the remaining work involves several hundred local agency
bridges and a few large toll bridges.
13 Frieder Seible, “Safety of California’s Transportation Structures” Chair, Caltrans Seismic Advisory Board,
Presentation to the California Transportation Commission, January 14, 2009, http://www.dot.ca.gov/hq/transprog/
ctcliaison/2009/0109/PP_Tab51_Seible_Seismic.pdf. See also, California Department of Transportation, Fourth
Quarter 2010, Non-Toll Seismic Retrofit Program Quarterly Report, http://www.dot.ca.gov/docs/reports/4thqtr_non-
toll_seismic_report.pdf.
14 Wright, et. al.
15 U.S. Department of Transportation, Federal Highway Administration, “Preventive Maintenance Eligibility,”
Memorandum, October 8, 2004, http://www.fhwa.dot.gov/preservation/100804.cfm.
Congressional Research Service
6
Earthquake Risk and U.S. Highway Infrastructure: Frequently Asked Questions
What Role Does the U.S. Department of
Transportation Have in Enhancing Seismic
Resiliency of the Highway System?
Apart from its role in administering federal-aid highway funds that may be used for seismic
retrofitting, DOT and its FHWA also play a role in seismic hazard research and training, and
coordinate with other federal agencies working on enhancing resilience. The Safe, Accountable,
Flexible, Efficient Transportation Equity Act: A Legacy for Users, enacted in 2005 (SAFETEA;
P.L. 109-59), directed DOT to work with the Multidisciplinary Center for Earthquake
Engineering Research (MCEER) at the University of Buffalo and the Center for Civil
Engineering Research at the University of Nevada, Reno. Congress authorized $2.5 million per
year for seismic research and also designated $3 million to MCEER. FHWA’s National Highway
Institute (NHI) currently offers several courses on bridge seismic design and retrofitting, and,
according to FHWA, several additional courses are nearing completion and will be available
shortly.
What Role Do Other Federal Agencies Play?
Numerous federal agencies perform activities that contribute to improving the highway system’s
seismic resiliency. Under the National Earthquake Hazards Reduction Program (NEHRP),16 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 U.S. Geological Survey (USGS), the National
Science Foundation (NSF), the Federal Emergency Management Agency (FEMA), and the
National Institute of Standards and Technology (NIST). FHWA coordinates with and contributes
to the activities of the NEHRP agencies.
As part of a NEHRP reauthorization in 1990 (P.L. 101-614), Congress authorized USGS to
monitor seismic activity in the United States. To this end, 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 its pre-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,
16 The authorization for appropriations for this program expired in 2009; legislation for its reauthorization was
proposed but not enacted in the 111th Congress (H.R. 3820 of the 111th Congress).
Congressional Research Service
7
Earthquake Risk and U.S. Highway Infrastructure: Frequently Asked Questions
hurricane winds, and floods before a disaster occurs. 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.
Author Contact Information
William J. Mallett
Peter Folger
Specialist in Transportation Policy
Specialist in Energy and Natural Resources Policy
wmallett@crs.loc.gov, 7-2216
pfolger@crs.loc.gov, 7-1517
Nicole T. Carter
Specialist in Natural Resources Policy
ncarter@crs.loc.gov, 7-0854
Congressional Research Service
8