U.S. States and Territories

National Tsunami Hazard Assessment:

Historical Record and Sources for Waves

Paula K. Dunbar

National Oceanic and Atmospheric Administration

 

Craig S. Weaver

U.S. Geological Survey

Prepared for the 

National Tsunami Hazard Mitigation Program

August 2008

U.S. DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric 

Administration

U.S. DEPARTMENT OF INTERIOR

U.S. Geological Survey
This publication was prepared for the 

National Tsunami Hazard Mitigation Program by:

National Oceanic and Atmospheric Administration

National Environmental Satellite, Data, and Information Service

National Geophysical Data Center 

325 Broadway

Boulder, Colorado 80305

U.S. Geological Survey

Department of Earth and Space Science

University of Washington

Box 351310

Seattle, Washington 98195

Table of Contents
iii

Acknowledgments .................................................................................................................. iv

Executive Summary .................................................................................................................v

Section 1. Introduction ..........................................................................................................1-1

1.1 References................................................................................................................1-2

Section 2. Known Historical Tsunami Record .......................................................................2-1

2.1 Validity of Tsunami Data ...........................................................................................2-1

2.2 NGDC Database Searches ......................................................................................2-2

	 	 2.2.1 Earliest Historical Accounts in the Pacific .......................................................2-2

 2.2.2 Earliest Historical Accounts in the Atlantic ......................................................2-2

 2.2.3 Runup Counts .................................................................................................2-2

2.3 Results .....................................................................................................................2-3

2.3 Discussion ................................................................................................................2-9

2.5 Qualitative Tsunami Hazard Assessment................................................................2-10

2.6 References .............................................................................................................2-12

Section 3. USGS Earthquake Hazards Assessment ............................................................3-1

	 3.1 Atlantic and Pacific Basin Differences ......................................................................3-3

3.2 Results of USGS NSHM Databases ........................................................................3-5

 3.2.1 Non-subduction Zones: Atlantic and Gulf Coasts ...........................................3-5

	 	 3.2.2 Subduction Zones: Pacific and Caribbean .....................................................3-7

 3.2.3 Hawaii ..............................................................................................................3-9

 3.2.4 Southern California ..........................................................................................3-9

 3.2.5 Alaska Arctic Coast .......................................................................................3-10

3.3 Discussion ..............................................................................................................3-10

3.4 References .............................................................................................................3-10

Section 4. Gaps in Knowledge of Tsunami Sources .............................................................4-1

4.1 Atlantic Basin ...........................................................................................................4-1

 4.1.1 Earthquake Sources .......................................................................................4-1

 4.1.2 Landslide and Volcano Sources ......................................................................4-3

	 4.2 Pacific Basin ............................................................................................................4-5

4.3 Discussion ................................................................................................................4-7

4.4 National Tsunami Research Plan .............................................................................4-7

4.5 References ...............................................................................................................4-8

Section 5. Next Steps ...........................................................................................................5-1

5.1 References ...............................................................................................................5-2

Section 6. Conclusion............................................................................................................6-1

Appendix A. What is a Tsunami? ......................................................................................... A-1

Appendix B. Risk Management Process ............................................................................. B-1

B.1 Hazard Assessment Module ................................................................................... B-1

B.2 Exposure and Vulnerability Assessment Module .................................................... B-2

B.3 Loss Assessment Module ....................................................................................... B-3

B.4 Mitigation Module ................................................................................................... B-3

B.5 References ............................................................................................................. B-3

Appendix C. Probabilistic Tsunami Hazard Assessment for Seaside, Oregon ....................C-1

C.1 Results ...................................................................................................................C-1

C.2 References .............................................................................................................C-1
iv

Acknowledgments

This report benefited from reviews and critical comments made by Mark Petersen and Eric Geist of 

the U.S. Geological Survey (USGS), Tim Walsh of Washington Emergency Management, George 

Priest of the Oregon Department of Geology and Mineral Industries, Chris Goldfinger of Oregon 

State University, Dwayne Meadows and Jenifer Rhoades of the National Oceanic and Atmospheric 

Administration (NOAA), Dale Dominey-Howes of Macquarie University, Sydney, Australia, and 

Laura Kong of UNESCO/International Tsunami Information Center. Lori Dengler of Humboldt State 

University provided reviews and comments, as well as help from her graduate students for qualitycontrol of the tsunami data.

We want to thank David Green of the NOAA Tsunami Program and David Applegate of the USGS 

Earthquake and Geologic Hazards Program, for their support of this study.

At the NOAA National Geophysical Data Center we thank Susan McLean, Chief of the Marine 

Geology and Geophysics Division, for her support of this study. Help with quality control of the 

tsunami data was given by Ruth Brocko, Joy Ikelman, Tanya Sazonova, Kelly Stroker, and Jesse 

Varner. Help with graphics and database support was provided by Jesse Varner. Technical editing 

and design were provided by Joy Ikelman.

This report was funded in part by the NOAA National Climatic Data Center’s Integrated Data and 

Environmental Applications (IDEA) Center through the Pacific Region Integrated Data Enterprise 

(PRIDE) budget committee.
v

Executive Summary

The National Science and Technology Council (NSTC) is the principal means for the President to 

coordinate science and technology policy across the Federal government. In 2005, the NSTC released 

a joint report by the sub-committee on Disaster Reduction and the U.S. Group on Earth Observations 

entitled Tsunami Risk Reduction for the United States: A Framework for Action. The Framework 

outlines the President’s strategy for reducing the United States tsunami risk. The first specific action 

called for in the Framework is to “Develop standardized and coordinated tsunami hazard and risk 

assessments for all coastal regions of the United States and its territories.” The National Tsunami 

Hazard Mitigation Program (NTHMP), a partnership between Federal and State agencies, provides 

the organizational framework needed to execute the President’s tsunami initiative. Since the National 

Oceanic and Atmospheric Administration (NOAA) is the lead agency for providing tsunami forecasts 

and warnings, the NTHMP requested that NOAA take the lead in conducting the first national tsunami 

hazard assessment. 

The first step in a tsunami hazard assessment is to examine the past record since it provides clues 

to what might happen in the future. NOAA’s National Geophysical Data Center (NGDC) catalogs 

information on global historical tsunamis. Earthquakes or earthquake-generated landslides caused 

more than 85 percent of the tsunamis listed in the NGDC tsunami database, with the remainder due 

to volcanic eruptions, non-earthquake generated landslides, and other sources. The United States 

Geological Survey (USGS) conducts research on earthquake hazards facing all of the United States 

and its territories. Therefore, NOAA/NGDC and USGS collaborated to conduct the first tsunami 

hazard assessment of the United States and its territories for the NTHMP.

The great diversity of coastal areas at risk—from metropolitan areas to beaches crowded with 

thousands of vacationers to quiet seaside towns—makes such a total risk assessment an essential, 

but daunting, task. A complete assessment as called for in the Framework consists of a hazard 

assessment, exposure, and vulnerability assessment of buildings and people, and loss assessment. 

In July 2006, a group of tsunami experts convened to review the current state of knowledge in 

areas essential to tsunami risk reduction. The results of this review, published as NOAA Technical 

Memorandum OAR PMEL-133, National Tsunami Research Plan, identified six high-priority tsunami 

research areas. The fifth high-priority area is understanding the impact of tsunamis at the coast. This 

includes measuring the tsunami current regime in harbors and at the coast, improving hydrodynamic 

modeling, developing credible fragility models of the interaction of tsunamis with the built and 

natural environment, and validating models through benchmarking. This research is essential for 

a full tsunami risk assessment. The goal of U.S. States and Territories National Tsunami Hazard 

Assessment, a first step toward satisfying the requirement in the Framework, and the fifth-priority 

of the Plan, is to provide a national qualitative assessment of the United States tsunami hazard at a 

regional level by examining the record of historical tsunamis and earthquakes, the predominant cause 

of tsunamis, at the State and territory level.
vi

In this report, two different sources of information are compiled to assess the U.S. tsunami hazard. 

The first involves a careful examination of the NGDC historical tsunami database which resulted 

in a qualitative tsunami assessment based on the distribution of runup heights and the frequency of 

tsunami runups. We characterize the tsunami hazard by first determining the number of individual 

tsunamis reported in each State or territory and then binning the results into five categories of runup 

amplitudes—Undetermined runup height, 0.01 m to 0.5 m, 0.51 m to 1.0 m, 1.01 m to 3.0 m, and 

greater than 3.0 m. Based on the total spread of events, runup amplitudes, and earthquake potential, 

we assigned a subjective hazard from very low to very high. These assessments recognized that 

tsunami runups of a few tens of centimeters have a lower hazard than those with runups of a few to 

many meters.

Our database search reinforces the common understanding that the U.S. Atlantic coast and the Gulf 

Coast States have experienced very few tsunami runups in the last 200 years. In fact, Louisiana, 

Mississippi, Alabama, the Florida Gulf coast, Georgia, Virginia, North Carolina, Pennsylvania, and 

Delaware have no known historic tsunami runup records in the NGDC database. Further, only a 

total of six tsunamis have been recorded anywhere in the other Gulf and East Coast States. Three 

of these tsunamis were generated in the Caribbean, two were related to magnitude 7+ earthquakes 

along the Atlantic coastline, and one reported tsunami in the mid-Atlantic States may be related to an 

underwater explosion or landslide. There is only one documented runup on the Atlantic in the range 

0.51 m to 1.0 m and none in the higher runup ranges.

In contrast, all U.S. coasts in the Pacific Basin as well as Puerto Rico and the U.S. Virgin Islands have 

a “moderate” to “very high” tsunami hazard based on both frequency and known runup amplitudes. 

The sheer number of runups and the large number greater than 3.0 m observed in Alaska and Hawaii 

justified assigning a “very high” hazard for these two States. The Pacific territories including Guam, 

American Samoa, and the Northern Marianas experience many tsunamis, but only one event had an 

amplitude greater than 3.0 m. Accordingly, we assigned a “moderate” hazard to the Pacific island 

territories. Both the frequency of tsunami runups and the amplitudes support a qualitative “high” 

hazard assessment for Washington, Oregon, California, Puerto Rico, and the Virgin Islands. The 

“high” value for Oregon, Washington, and northern California reflects the low frequency (~1 per 

500 years) but the potential for very high runups from magnitude 9 earthquakes on the Cascadia 

subduction zone.

Although tsunami deaths are a measure of risk rather than hazard, we compared the known tsunami 

deaths found in our NGDC database search with our qualitative assessments based on frequency and 

amplitude. There are no known deaths from tsunamis along the U.S. Atlantic coast or Gulf coast and 

only one known death in the Pacific territories (on Guam). In contrast, both Alaska and Hawaii have 

unfortunately experienced several hundred deaths each from tsunamis. Tsunamis in Puerto Rico and 

the U.S. Virgin Islands have caused a total of 172 deaths. The U.S. west coast has had 25 tsunami 

deaths, but due to the lack of written records, there are no estimates of deaths caused by the great 

Cascadia earthquake in 1700. There is evidence from oral traditions and computer simulations that 
vii

this event must have severely impacted native populations.

The NGDC tsunami database contains reported tsunamis and is therefore limited to written records 

existing for an area. The hazard assessment used the USGS National Seismic Hazard Map (NSHM) 

databases to partially extend the time interval. These databases are primarily meant to assess 

earthquakes affecting U.S. possessions and do not include all possible seismogenic tsunami sources 

in the Pacific and Atlantic Basins. However, the databases make it possible to estimate the rate 

of occurrence of larger magnitude earthquakes that could generate a tsunami. The USGS NSHM 

databases are based on tectonic models, and paleoseismic and paleotsunami data. It is important 

to understand that the USGS NSHM databases are used to calculate earthquake, not tsunami, 

occurrence. Along the U.S. Pacific and the Caribbean coastlines, the rate of the largest magnitude 

local subduction zone earthquakes calculated by the USGS is essentially the same as the rate of nearshore tsunami generation, because great (>M7.5 to 8.0) subduction zone earthquakes are the most 

common cause of tsunamis along these coastlines. This is not the case along the Atlantic or Gulf 

coasts, where very limited experience suggests that the occurrence of a large earthquake does not 

necessarily mean a tsunami is generated. 

One difference between most local earthquake sources along the U.S. Pacific and Caribbean coasts 

compared to the U.S. Atlantic coast is that, in the former areas, earthquake displacements directly 

displace huge volumes of water because of large earthquake source dimensions, whereas along the 

Atlantic and Gulf coasts they do not. The extremely limited experience with earthquake sources 

along the Atlantic coast suggests that the earthquake-triggering of underwater landslides is an 

important source for generating tsunamis. The seismic databases obviously do not account for 

earthquake sources removed from the American shorelines or non-earthquake related tsunami sources.

The table (next page) summarizes the results of the NGDC and USGS database searches.

In terms of our goal to provide a national tsunami hazard assessment, better understanding of 

possible sources in the Atlantic Basin and offshore southern California are high priorities. For 

the Atlantic Basin, a better understanding of the cause of the known magnitude 7 events along 

the margin is needed. This should be coupled with an analysis of the potential for these events to 

initiate underwater landslides capable of generating a tsunami similar to that observed after the 1929 

Grand Banks (Newfoundland) earthquake. In addition, the possibility of non-earthquake triggered 

underwater landslides along the Atlantic Basin needs examination. Fortunately, the USGS has begun 

a one-year study of tsunami potential along the Atlantic coast.

An investigation of coastal marshes in search of any evidence of past Atlantic tsunamis should 

support these efforts. In the Caribbean, efforts should be made to better document the catalog 

of tsunamis. Modeling studies should be undertaken for all possible sources, in order to better 

understand earthquake recurrence, tsunami source generation, and propagation issues.
viii

The tsunami threat to southern California from local earthquakes and underwater landslides remains 

highly uncertain. The USGS has begun a two-year study of these sources that, when completed, will 

improve this initial national assessment.

Much uncertainty remains about the variability in size and patterns of deformation of Cascadia 

subduction zone earthquakes. Since this is one of the largest tsunami sources threatening the U.S. 

coast, improvement in our knowledge of this source should have a high national priority. Reducing 

these uncertainties could be accomplished by modeling studies and field investigations designed to 

improve our understanding of the possible ranges of earthquake size and deformation.

We stress that this report is an initial step towards a national tsunami risk assessment. Although this 

National Tsunami Hazard Assessment is focused on the qualitative, relative ranking of American 

coastlines to tsunami hazards, it is instructive to examine how detailed science and emergency 

management practices can greatly improve the country’s ability to prepare for future tsunamis. 

Accordingly, a portion of a more rigorous probabilistic tsunami hazard assessment for Seaside, 

Oregon, is included in Appendix C as an example of products that may be useful in other selected 

communities as tsunami hazard assessment methods mature.

Table A. Qualitative tsunami hazard assessment based on NGDC and USGS databases.

 Region

Hazard based on 

runups

Hazard based on 

frequency

Hazard based on 

local earthquakes

Number of 

reported deaths

U.S. Atlantic coast Very low to low Very low Very low to low None

U.S. Gulf coast Very low Very low Very low None

Puerto Rico and the 

Virgin Islands

High High High 172

U.S. west coast High High High 25

Alaska Very high Very high High 222

Hawaii Very high Very high High 326

U.S. Pacific island

territories

Moderate High High 1
1-1

Section 1. Introduction

Tsunamis are infrequent high impact events that can cause a considerable number of fatalities, inflict 

major damage, and cause significant economic loss to large sections of the U.S. coastlines. Since 

the beginning of the 20th century, tsunami events have caused more than 700 deaths and over $200 

million dollars in damage to the U.S. coastal States and territories. More than 50 percent of the U.S. 

population now live in coastal communities (Crosset et al., 2004) and may be at risk for impacts 

from a destructive tsunami. Had this large population been present in 1700 when the last magnitude 

9 Cascadia earthquake struck, there would have been many times this number of deaths from the 

tsunami impact to the Pacific Northwest. As more people continue to move to the coasts, the risk of 

deaths and damage will continue to climb. Because of the large coastal area affected, it is imperative 

that the United States understand the tsunami threat to its States and territories, and to identify coastal 

areas that face the greatest tsunami risk. 

The tsunami hazard assessment for the United States is presented in Sections 2 and 3. Section 2 

presents an initial qualitative assessment of the tsunami hazard based on the history of tsunamis in the 

United States. Section 3 provides an extension of this assessment based on local earthquake source 

probabilities and their potential to generate tsunami waves that could impact the United States. 

There are several areas where a better characterization of tsunami sources would lead to immediate 

improvement in this initial tsunami hazard assessment. Accordingly, Section 4 highlights some of 

these key gaps in the current knowledge on tsunami sources. For a review of the current state of 

tsunami research, users are urged to read National Tsunami Research Plan (Bernard et al., 2007). 

This document includes state-of-the-science reports on tsunami hazard assessment, warning guidance, 

preparedness, response, and mitigation.

Because there are large parts of the coast with little, if any, familiarity with tsunamis, Appendix A

provides a brief description of the causes, mechanics, and impacts of tsunamis.

An overview of the entire risk management process that includes modules for assessing the 

tsunami hazard, exposure and vulnerability, probable maximum losses to buildings, and mitigation 

is presented in Appendix B. A complete tsunami hazard assessment requires the following key 

elements: collection, analysis, and quality assurance of all data related to U.S. tsunami events; 

assessment of frequency, severity, and uncertainty of tsunami sources; acquisition, quality assurance, 

and archiving of bathymetric and near-shore topographic data; development of tsunami inundation 

forecast tools; and inundation mapping and modeling of all U.S. coastal areas. The next step will be 

to use these data as input for a complete tsunami hazard and risk assessment.
1-2

Finally, Appendix C concludes this report with an example of a more detailed probabilistic tsunami 

hazard assessment that was recently completed for Seaside, Oregon (Tsunami Pilot Study Working 

Group, 2006). 

This report is intended to be an overview of the hazard down to the State level, not a detailed 

description of the tsunami hazard at a particular locality. This document will be updated periodically 

as new research improves our ability to assess the U.S. tsunami hazard.

1.1 References

Bernard, B., Dengler, L., and Yim, S. (editors), 2007, National Tsunami Research Plan: Report of a 

Workshop Sponsored by NSF/NOAA. NOAA Technical Memorandum OAR PMEL–133, 135 

p. http://www.pmel.noaa.gov/pubs/PDF/bern3043/bern3043.pdf

Crosset, K.M., Culliton, T. J., Wiley, P.C., and Goodspeed, T.R., 2004, Population Trends Along the 

Coastal United States: 1980-2008, U.S. Department of Commerce, National Oceanic and 

Atmospheric Administration, National Ocean Service, 47 p.

Tsunami Pilot Study Working Group, 2006, Seaside, Oregon Tsunami Pilot Study—Modernization of 

FEMA Flood Hazard Maps, Joint NOAA/USGS/FEMA Special Report, 94 p., 7 appendices. 

http://pubs.usgs.gov/of/2006/1234/
2-1

Section 2. Known Historical Tsunami Record

NOAA’s National Geophysical Data Center (NGDC) serves as the archive for many kinds of earth 

science information, including global observations of tsunami sources and tsunami runup records. 

Tsunami source information includes the date, location, and source type, plus a summary of 

measurements and effects. The runup database includes the date and location of the observation as 

well as details of the runup measurements and effects at the location. These two databases have been 

significantly improved over the last few years by careful checking of historical entries to flag either 

poorly identified possible tsunami runups or verify a meteorological origin for some entries. 

Historical tsunami data are an important first step for assessing the tsunami hazard of a region. 

Therefore, the NGDC historical tsunami databases were examined to understand the scope of tsunami 

hazards facing U.S. States and territories. Although NGDC is preparing a tsunami deposits database 

that will extend the record into prehistoric times, this section focuses on the known historical tsunami 

record. It is important to note that if there were no people in an area to observe the phenomenon or to 

keep written records, it would not have been reported and included in the historical tsunami database. 

The largest deficit in this regard is the impact to native populations in Oregon, Washington, and 

northern California from the 1700 Cascadia tsunami. 

2.1 Validity of Tsunami Data

A validity score is assigned to each tsunami source event ranging from 0 for erroneous entries to 4 

for definite tsunamis. The validity of tsunami events and runups is based on several factors. For 

example, tsunamis recorded on tide gauges generated by earthquakes recorded on seismographs are 

assigned a high validity of 4 in the database. 

Historical events that occurred before the invention of the seismograph or tide gauge must be 

evaluated differently. If the event caused significant effects such as deaths and damage, or was 

observed in many locations, it is also considered a high validity. For example, a tsunami generated 

by an earthquake in Chile that was observed in Hawaii and California would be assigned a high 

validity. The number of reliable and independent sources that list a historical event also affects the 

validity. Historical tsunami events generated by earthquakes are crosschecked with regional and local 

earthquake catalogs. If the tsunami was reported to have been generated by an earthquake, but there 

are no listings in the earthquake catalogs, the validity is lowered. For example, if a newspaper article 

reports a tsunami wave was generated by an earthquake, but there are no earthquakes that can be 

related, the tsunami event would be assigned a validity of 1. Tsunami events generated by volcanoes 

are crosschecked with volcano catalogs.

Whether the tsunami event occurred before or after the invention of seismographs and tide gauges, a 

high validity of 3 or 4 is considered a confirmed report of a tsunami event, whereas a validity of 0, 1, 

or 2 is considered an unconfirmed report. 
2-2

2.2 NGDC Database Searches

2.2.1 Earliest Historical Accounts in the Pacific

The NGDC tsunami event database was queried to determine the earliest historical accounts of 

tsunamis impacting the U.S. States and territories. The first report was a Hawaiian chant composed 

in the 16th century describing a huge wave that came on the west coast of Molokai and killed the 

inhabitants. A Kamchatka earthquake in 1737 generated the first tsunami observed in Alaska. There 

were a few unconfirmed accounts of tsunamis as early as 1767 in the Pacific islands of Guam, 

American Samoa, and the Northern Mariana Islands; the first confirmed account was in 1837 in 

America Samoa. An earthquake off the coast of southern California in 1812 generated the first 

confirmed tsunami reports on the U.S. west coast and Hawaii. The confirmed report of a destructive 

tsunami striking the Japanese islands in 1700 has been shown by Satake et al. (2003) and Atwater 

et al. (2005) to be the last great Cascadia tsunami that devastated the U.S. west coast, but this 

event is not counted in this section in the present study, since there is no written record for the U.S. 

coast. Although the impact of a Cascadia tsunami is not considered in this section, the results of 

paleoseismic studies are used as input to the USGS earthquake hazard maps. Therefore, research on 

the Cascadia tsunami is included in the results discussed in Section 3.

2.2.2 Earliest Historical Accounts in the Atlantic

Caribbean tsunamis were reported as early as 1498 in Venezuela; the first confirmed observation in 

the U.S. territories was in 1690 in the Virgin Islands. On the east coast of North America there were 

unconfirmed tsunami reports as early as 1688; the first confirmed report on the Canadian east coast 

was from the 1755 Lisbon event. The first confirmed tsunami reports on the U.S. east coast were 

from the 1886 Charleston, South Carolina, tsunami that was observed in South Carolina and Florida. 

In 1918, a Puerto Rico earthquake generated a tsunami that was recorded on a Galveston, Texas, tide 

gauge with a small amplitude.

2.2.3 Runup Counts

To evaluate the impact of tsunamis on each State separately, the NGDC tsunami runup database 

was searched by State. Runups in each State were then divided into tsunami events using the 

tsunami source event date and time. Multiple runups over several hours from great subduction zone 

earthquakes, such as 1960 Chile or 1964 Alaska, observed at multiple sites within a particular State, 

were considered as one event per state. Tsunamis reported on inland waters were not counted, such as 

Lake Erie or Roosevelt Lake, Washington. Tsunamis in Puget Sound and all reported tsunamis in the 

bays of southeastern Alaska were included, including those with local landslide sources. A possible 

submarine landslide in 1964 that generated a tsunami that was recorded in Connecticut, New Jersey, 

Rhode Island, and New York was included in the count. All runups associated with tsunami events 

flagged as either known meteorological events or suspected of being a spurious or unconfirmed entry 

(low validity of 0, 1, or 2) were eliminated from the count. 

The above procedure generated a count of tsunami events recorded in each State (Table 2-1, page 

2-5). The reported runup heights were used to develop additional details of the tsunami runup 
2-3

distribution. For each individual tsunami event, the events were binned based on the maximumrecorded runup height in each State. Measured tsunami runup heights were subdivided into five 

groups:

• Undetermined runup heights

• 0.01 m to 0.5 m 

• 0.51 m to 1.0 m

• 1.01 m to 3.0 m

• greater than 3.0 m 

For example, if a tsunami was recorded in Oregon with two measured runups of 0.5 m and 1.4 m, the 

tsunami was binned into the 1.01 m to 3.0 m group. The same tsunami, if recorded in Washington 

with runups of 0.05 m, 0.15 m, and 0.6 m, would be binned into the 0.51 to 1.0 m group. Some 

tsunamis were observed, but there were no measurements of the runup heights. These events are 

counted as “events with undetermined runup heights.” Finally, the total number of tsunami runups, all 

deaths, and damage in dollars reported as due to tsunamis were summed for each State and Territory. 

2.3 Results

The results were organized into seven broad regions: U.S. Atlantic coast, U.S. Gulf coast, Puerto 

Rico and the Virgin Islands, U.S. west coast, Alaska, Hawaii, and U.S. Pacific island territories. 

Florida’s coast is divided between the Atlantic and Gulf coast regions. The Pacific island territories 

include Guam, Northern Mariana Islands, and American Samoa. It is very clear from Table 2-1 that 

the American experience with tsunamis is greatest in the Pacific Basin based on the total number of 

runup events. Again, we emphasize that the numbers in Table 2-1 do not represent the total number 

of individual tsunamis, but the number of tsunamis per State with reported runups. For instance, the 

1964 Alaska earthquake generated a tsunami that counts as a recorded tsunami event in many States 

(e.g., Washington, Oregon, Alaska, California, Hawaii, etc.). Thus, the numbers in columns three 

through seven (Table 2-1) are essentially State tsunami events with runups within the listed criteria. 

It is important to understand that although a tsunami source event (e.g., the 1964 Alaska tsunami) 

might be counted in several states, it is only counted once for each individual State.

The number of State tsunami events ranges from none in Pennsylvania, Delaware, Virginia, North 

Carolina, Georgia, Alabama, Mississippi, and Louisiana to 114 in Hawaii. The State tsunami events 

include both local sources of all types as well as runups resulting from a distant source. In terms 

of the tsunami events, about 10 percent are in the Atlantic Basin (Atlantic, Gulf, Puerto Rico, and 

the Virgin Islands) and 90 percent are in the Pacific (U.S. west coast, Alaska, Hawaii, and western 

Pacific). Again, one tsunami is often counted in several States.

Of the total 419 tsunami events, there are 376 (262+30+38+46) events with measured runups. The 

remaining events (43) were observed, but there were no runup measurements reported. The totals for 

each maximum runup category show the particular issue with tsunamis. The large total number of 
2-4

tsunami events (262) with runups between 0.01 m and 0.5 m is driven primarily by distant tsunamis. 

For example, American Samoa has 56 recorded tsunami events and 47 of these have measured runup 

amplitudes (40+4+3+0 in columns four through seven). Of the 47 tsunami events with measured 

runups in American Samoa, 40 are 0.01 to 0.5 m; 32 of these 40 events are from distant tsunami 

sources (>1000 km). 

As the measured runup height increases, the total number of tsunami events decreases quickly from 

262 for runups up to 0.5 m, to 30 for runups between 0.51 m and 1.0 m, but increases to 38 between 

1.01 m and 3.0 m, and increases again to 46 for tsunami events with runups more than 3.0 m. These 

numbers reflect the fact that in the subduction zones, local earthquakes generate severe tsunamis with 

amplitudes in excess of 3.0 m.

The Pacific Basin has 43 of the 46 State tsunami events with runup wave heights greater than 3.0 m. 

The number of events with runups greater than 3.0 m for Alaska (16) and Hawaii (18) points out the 

very severe nature of the tsunami threat in those States. The large number of local sources in Alaska 

along both the mainland and Aleutian arc contributes to the tsunami hazard facing Alaska. Both 

significant local tsunami sources and frequent devastating distant tsunamis strike Hawaii. 

In the Atlantic Basin, there are three tsunami events with runups greater than 3.0 m for Puerto Rico 

and the Virgin islands, but there are no measured runups along the U.S. Atlantic coast greater than 3.0 

m. For the U.S. Atlantic coastline, there is only one event with a measured tsunami runup exceeding 

0.5 m. The 0.68 m runup is from the 1929 Grand Banks earthquake and was observed in New Jersey. 

This magnitude 7.3 earthquake caused an underwater landslide that generated the tsunami. This 

generation process is much different from that in subduction zones, where the vertical motion of a 

large area of seafloor generates the initial tsunami, although subsequent landslides can increase later 

wave heights.

The eighth column in Table 2-1 shows the total number of reported tsunami runups per State. The 

number of runups ranges from 0 to more than 400 in Hawaii for the devastating 1946 tsunami that 

struck Hilo and other cities. A total of 1,592 runup reports are available for Hawaii. There are 82 

tsunami runup reports for the Atlantic Basin (U.S. east coast, Gulf coast, Puerto Rico, and the Virgin 

Islands), about 3 percent of the 2,661 total reported runups.

The database search resulted in 746 reported deaths (column 9, Table 2-1) and $207 million damage 

attributed to tsunamis (column 10, Table 2-1). There are no deaths or dollar damages reported for the 

eastern and Gulf coasts. The NGDC database shows that Hawaii, Alaska, Puerto Rico, and the Virgin 

Islands have suffered the largest number of fatalities from tsunamis; Hawaii, Alaska, and the U.S. 

west coast have suffered the largest amount of dollar damages. This is not a surprising result given 

the dangerous subduction zones along the Alaskan and Caribbean coasts and the central location 

of Hawaii—surrounded in the Pacific by many tsunami source regions in addition to local sources 

(Appendix A, Figure A-1). Since the last local Cascadia tsunami occurred in 1700, before significant 
2-5

Table 2-1. 

Tsunami events, total 

number of runups, 

deaths, and dollar 

damage by State 

and region from 

the NOAA/NGDC 

tsunami database. 

Dollars have not been 

adjusted for inflation.

See Section 2.2.3 

for an explanation 

of the counts. For 

more information 

on specific events,

access the online 

database at http://

www.ngdc.noaa.gov/

hazard/tsu_db.shtml.

Maine (1929) 1 1 3

New Hampshire (1929) 1 1 1

Massachusetts (1929) 1 1 2

Rhode Island (1929) 2 1 1 3

Connecticut (1964) 1 1 1

New York (1895) 2 1 1 7

New Jersey (1918) 6 3 2 1 8

Pennsylvania

Delaware

Maryland (1929) 1 1 1

Virginia

North Carolina

South Carolina (1886) 2 1 1 2

Georgia

Florida (1886) 4 3 1 5

Atlantic Coast Totals 21 13 7 1 0 0 33 0 $0

Puerto Rico (1867) 9 1 3 2 2 1 33 142 $4

Virgin Islands (1690) 7 2 1 1 1 2 15 30

PR & VI Totals 16 3 4 3 3 3 48 172 $4

Washington (1891) 21 2 13 1 4 1 64 1 $2

Oregon (1854) 18 1 12 1 2 2 62 5 $1

California (1812) 75 5 48 9 8 5 425 19 $19

West Coast Totals 113 8 73 11 13 8 550 24 $22

Guam (1849) 15 2 10 1 1 1 23 1

Northern Mariana (1990) 1 1 1

American Samoa (1837) 56 9 40 4 3 60

Pacific Is. Totals 72 12 50 5 4 1 84 1

Alaska (1737) Totals 81 6 49 4 6 16 352 222 $122

AMERICAN TOTALS 419 43 262 30 38 46 2661 746 $207

Location (and year of 

first confirmed report) Total number of tsunami events

with any observed runup

Events with undetermined

runup heights

Events with runups 0.01 to 0.5 m

Events with runups 1.01 to 3.0 m

Events with runups > 3.0 m

Total number of runups 

for all tsunami events

Million dollars 

Events with runups 0.51 to 1.0 m

Reported deaths

damage reported

Florida

Alabama

Mississippi

Louisiana

Texas (1918) 1 1 1

Gulf Coast Totals 1 1 0 0 0 0 1 0 $0

Hawaii (1812) Totals 114 0 79 6 11 18 1592 326 $59

U.S. Atlantic Coast U.S. Gulf Coast

Puerto Rico

and

Virgin Islands

U.S. West 

Coast

U.S. 

Pacific Island

Territories
2-6

Figure 2-1. Map showing Cascadia subduction 

zone and tsunami deposit locations in the NOAA/

NGDC tsunami deposit database.

populations lived on the coast and kept written records, there is no estimate of the effect of this 

tsunami on Native American populations. But there are clear references to this event in tribal oral 

traditions (Ludwin et al., 2005), and tsunami sediments have been found at many sites on the west 

coast of Canada and the United States (Figure 2-1). The results in Table 2-1 are summarized on the 

maps in Figure 2-2 for the Pacific and Figure 2-3 for the Atlantic.

In the Pacific Basin, tsunamis generated in one subduction zone are usually recorded at distant 

locations outside of the source region. Based on the NGDC database, that is not necessarily the case 

in the Atlantic. The Atlantic tsunami runups were tabulated by their source (Table 2-2). Known 

historical sources of tsunamis recorded on the U.S. Atlantic and Gulf coasts are few, consisting of:

• two major Atlantic earthquakes greater than magnitude 7: the 1886 Charleston, South Carolina, 

earthquake and the 1929 Grand Banks, Canada, earthquake 

• three Caribbean earthquakes greater than magnitude 7: the 1918 Mona Passage earthquake and 

two Dominican Republic earthquakes in 1946

• one distant earthquake in Sumatra (2004)

• a possible mini-tsunami in 1964, possibly generated by an underwater landslide or explosion

• one local earthquake—the 1895 High Bridge, New Jersey earthquake—that is too small 

(estimated magnitude of 4.3) to directly generate a tsunami but has observations of wave effects 
2-7

Figure 2-2. Map showing total number of tsunami events and total number of events causing 

runup heights from 0.01 m to greater than 3.0 m for U.S. States and territories in the Pacific

Ocean. (Note: Histogram scales in Figures 2-2 and 2-3 are not the same.)
2-8

Figure 2-3. Map showing total number of tsunami events and total number of events causing 

runup heights from 0.01 m to greater than 3.0 m for U.S. States and territories in the Atlantic, 

Gulf Coast, Puerto Rico, and the Virgin Islands. (Note: Histogram scales in Figures 2-2 and 

2-3 are not the same.)
2-9

Table 2-2. Source types for tsunami runups of any size observed on the U.S. Atlantic and Gulf coasts.

Atlantic Coast 

Earthquake

Earthquaketriggered 

Landslide

Local 

Landslide

Caribbean 

Earthquake

Non-Atlantic 

Earthquake

Underwater 

Landslide? Total

Maine 1 1

New Hampshire 1 1

Massachusetts 1 1

Rhode Island 1 1 2

Connecticut 1 1

New York 1 1 2

New Jersey 1 3 1 1 6

Pennsylvania 0

Delaware 0

Maryland 1 1

Virginia 0

North Carolina 0

South Carolina 1 1 2

Georgia 0

Florida 1 2 1 4

Alabama 0

Mississippi 0

Louisiana 0

Texas 1 1

consistent with those expected from a tsunami. The wave effects might reflect an unreported 

(unknown) local landslide or slump, or an unusual sloshing effect from the ground shaking.

The possible mini-tsunami occurred on May 18, 1964, and was recorded on tide gauges in four 

States—Rhode Island, Connecticut, New York, and New Jersey. The waves were characteristic of 

a small tsunami with maximum amplitude of 0.28 m. No seismic source is known for this event. 

Lander and Lockridge (1989) speculated that either an underwater landslide or an explosion caused 

the tsunami.

2.4 Discussion

It is clear from Table 2-1 that States and territories near subduction zones (or surrounded by 

subduction zones in the case of Hawaii) dominate the United States tsunami hazard with respect to the 

known historical record. There are four points to consider. First is the length of the historical record 

of tsunamis documented in the NGDC databases. It is reasonable to assume that most of the Atlantic 

coast tsunami events with runups of 1.0 m or more would likely have been noticed in populated 

places for the past 250 years; on the Gulf coast, for perhaps somewhat less, maybe 200 years. The 

records in Puerto Rico and the Virgin Islands are also likely complete for tsunami events with runups 
2-10

of 1.0 m or more for at least 250 years and 200 or more years for much of California and all of 

Hawaii. The main gap in our knowledge of historic tsunamis is on the Pacific Northwest coast where 

the observational record is about 160 years. We know that a magnitude 9 earthquake and tsunami 

occurred there in 1700, but there is no written record of the local tsunami impact. Thus, aside from 

the Pacific Northwest, the available record throughout most of the United States is sufficient to assign 

an initial qualitative assessment of tsunami hazards.

Second, the tsunami data summarized in Tables 2-1 and 2-2 may still contain some suspicious events, 

such as the 1964 event mentioned above for the four Atlantic States or the 1895 Highbridge, New 

Jersey, earthquake. In the Pacific Basin and the Caribbean there may be a few suspicious events, 

but because of the large number of well-documented tsunamis those events have little overall effect 

on the hazard assessment. This is also not an issue on the Gulf coast, because there are no events 

reported, except for one distant tsunami from the Caribbean with a small amplitude recorded in Texas.

Third, there are striking differences in how tsunamis in the Pacific are recorded compared to tsunamis 

in the Atlantic Basin (Atlantic, Gulf, Puerto Rico, and the Virgin Islands). Although the observations 

in Table 2-1 were not divided into distant and local tsunami sources, the differences in the number 

of recorded tsunami runup events makes this clear. Only three of the Caribbean tsunami events are 

recorded at even a single station on the Atlantic and only one station in Texas on the Gulf coast. 

However, in the Pacific Basin tsunamis tend to get recorded at most States and territories within the 

Basin.

Fourth, this database does not consider explicitly the significant difference in hazard posed by distant 

versus local tsunamis. Local tsunamis arrive at the U.S. coast in minutes whereas distant events 

arrive hours after the causative earthquake. For Hawaii and the lower 48 States, the first arrival of 

distant tsunamis from subduction zones in the Pacific is at least four hours. For example, the level of 

hazard posed by rare magnitude 9 earthquakes in the Gulf of Alaska to the Alaska coast and Cascadia 

subduction zone earthquakes to the Pacific Northwest coast can be far greater than numerous small 

distant tsunamis to these same areas.

2.5 Qualitative Tsunami Hazard Assessment

Table 2-1 provides data for an initial qualitative assessment of tsunami hazards facing American 

States and possessions. The first criterion that can be used is the distribution of runup heights. It is 

clear that the record is sufficient to conclude that there are very few known historical tsunami runups 

on the Atlantic coast and almost none on the Gulf coast. Based solely on the known historical record, 

the qualitative tsunami hazard is labeled as very low to low on the Atlantic coast and very low on the 

Gulf coast. 

It is also clear that Hawaii, Alaska, Puerto Rico, the U.S. Virgin Islands, and the U.S. west coast 

face a much higher tsunami hazard. Tsunami records in Hawaii go back about 200 years. A tsunami 

with a runup greater than 3.0 m, irrespective of source distance, hits Hawaii on average about once 
2-11

Table 2-3. Qualitative tsunami assessment based on NGDC databases.

Region

Hazard based on 

runups

Hazard based on 

frequency

Number of reported 

deaths

U.S. Atlantic coast Very low to low Very low None

U.S. Gulf coast Very low Very low None

Puerto Rico and the 

Virgin Islands

High High 172

U.S. west coast High High 25*

Alaska Very high Very high 222

Hawaii Very high Very high 326

U.S. Pacific island

territories

Moderate High 1

*Does not include any deaths caused by the 1700 Cascadia tsunami on the U.S. west coast

every ten years. Alaska faces about the same frequency of tsunamis with runups greater than 3.0 m. 

Compared to other U.S. States and territories, the tsunami hazard facing these States is very high. 

The U.S. west coast, Puerto Rico and the Virgin Islands clearly have a high tsunami hazard.

Tsunamis frequently strike American possessions in the Pacific, but proportionally many of the events 

record smaller runup heights than elsewhere in the Basin. Based only on the available data from 

NGDC, the qualitative tsunami hazard in American Samoa, Guam, and the Northern Mariana Islands 

is categorized as moderate. Table 2-3 shows the qualitative assessment of tsunami hazards based on a 

consideration of the distribution of recorded runup heights.

A second criterion for assessing tsunami hazard is based on the frequency of runups. The record is 

clear that on the Gulf and Atlantic coasts the frequency is very low. In contrast is the sheer number of 

events striking Alaska and Hawaii; thus, they are qualitatively rated as very high. The U.S. west coast 

and the Pacific island territories are rated high on the basis of frequency, once again showing that 

tsunamis in the Pacific Basin tend to be recorded widely around the rim. Puerto Rico and the Virgin 

Islands are also rated high on the basis of frequency.
2-12

The number of deaths reported in the NGDC database is corroborative evidence supporting our 

qualitative hazard assessments. Strictly speaking, tsunami deaths indicate risk as opposed to hazard, 

but it is worth noting that the number of deaths is consistent with the qualitative assessment. As 

shown in Table 2-1, the number of deaths in Hawaii, Alaska, Puerto Rico, and the Virgin Islands is 

much greater than in the other regions. Accordingly, these areas were assigned a high to very high 

hazard assessment based on the last 200 years of data. The U.S. west coast, with 25 deaths from 

tsunamis, is assessed a high hazard. It is likely that, if estimates of deaths from the 1700 Cascadia 

tsunami existed, the U.S. west coast would rate a very high assessment as well. Despite the large 

number of tsunamis in the western Pacific, few deaths have resulted. A moderate to high hazard was 

assigned here. Finally, with no reported deaths to date on the Gulf or Atlantic coasts, the hazard is 

labeled as very low to low.

2.6 References

Although several hundred references were used to compile the NGDC tsunami data for the U.S. 

States and territories, the following references were the major sources used. A complete list of 

reference citations for each tsunami event is available from the online database: 

http://www.ngdc.noaa.gov/hazard/tsu_db.shtml

Atwater, B.F., Musumi-Rokkaku, S., Satake, K., Tsuji, Y., Ueda, K., and Yamaguchi, D.K., 2005, 

The orphan tsunami of 1700, U.S. Geological Survey Professional Paper 1707, 133 p.

Atwater, B. F., Nelson, A.R., Clague, J.J., Carver, G.A., Yamaguchi, D.K., Bobrowsky, P.T., 

Bourgeois, J., Darienzo, M.E., Grant, W.C., Hemphill-Haley, E., Kelsey, H.M., Jacoby, G.C., 

Nishenko, S.P., Palmer, S.P., Peterson, C.D., Reinhart, M.A, 1995, Summary of coastal 

geologic evidence for past great earthquakes at the Cascadia subduction zone, Earthquake 

Spectra, v. 11, 1–18.

Berninghausen, W.H., 1964, Tsunamis and seismic seiches reported from the eastern Atlantic south of 

the Bay of Biscay, Bulletin of the Seismological Society of America, v. 54, no. 1, p. 439–442.

Berninghausen, W.H., 1968, Tsunamis and seismic seiches reported from the western north and south 

Atlantic and the coastal waters of northwestern Europe, Informal Report No. 68-85, U.S. 

Naval Oceanographic Office, Washington, D.C., 45 p.

Campbell, J.B., 1991, Earthquake History of Puerto Rico, Appendix I, Part A, Aguirre Nuclear Power 

Plant PSAR, Weston Geophysical Research, Inc., 117 p.

Dunbar, P.K., Lockridge, P.S., and Whiteside, L. S., 1992, Catalog of Significant Earthquakes 

2150 B.C.–1991 A.D., Report SE-49, National Geophysical Data Center, 320 p. (Updates: 

http://www.ngdc.noaa.gov/nndc/struts/form?t=101650&s=1&d=1).

Geist, E.L., 2005, Local tsunami hazards in the Pacific Northwest from Cascadia subduction zone 

earthquakes, U.S. Geological Survey Professional Paper 1661B, 17 p.

Heck, N.H., 1947, List of seismic sea waves, Bulletin of the Seismological Society of America, v. 37, 

no. 4, p. 269–286.

Iida, K., 1984, Catalog of Tsunamis in Japan and its Neighboring Countries, Aichi Institute of 

Technology, 52 p.
2-13

Iida, K., Cox, D.C., and Pararas-Carayannis, G., 1967, Preliminary Catalog of Tsunamis Occurring in 

the Pacific Ocean, HIG-67-10, Hawaii Institute of Geophysics, 275 p.

Lander, J.F., 1996, Tsunamis Affecting Alaska 1737–1996, KGRD No. 31, National Geophysical Data 

Center, 155 p. http://www.ngdc.noaa.gov/hazard/data/publications/Kgrd-31.pdf

Lander, J.F., and Lockridge, P., 1989, United States Tsunamis 1690-1988, Publication 41-2, National 

Geophysical Data Center, 265 p. http://www.ngdc.noaa.gov/hazard/data/publications/pub41-2.pdf

Lander, J.F., Lockridge, P.A., and Kozuch, M., 1993, Tsunamis Affecting the West Coast of the United 

States, 1806–1992, KGRD No. 29, National Geophysical Data Center, 242 p. 

http://www.ngdc.noaa.gov/hazard/data/publications/Kgrd-29.pdf

Lander, J.F., and Whiteside, L.S., 2002, The tsunami history of Guam: 1849–1993, Science of 

Tsunami Hazards, the International Journal of the Tsunami Society, v. 20, no. 3, p. 158–174. 

http://epubs.lanl.gov/tsunami/ts203.pdf

Lander, J.F., Whiteside, L.S., and Lockridge, P.A., 2002, A brief history of tsunamis in the Caribbean 

Sea, Science of Tsunami Hazards, the International Journal of the Tsunami Society, v. 20, 

no. 2, p. 57–94. http://epubs.lanl.gov/tsunami/ts202.pdf

Lander, J.F., Whiteside, L.S., and Lockridge, P.A., 2003, Two decades of global tsunamis 1982–2002, 

Science of Tsunami Hazards, the International Journal of the Tsunami Society, v. 21, no. 1, 

p. 3–82. http://library.lanl.gov/tsunami/ts211.pdf

Lockridge, P.A., Whiteside, L.S., and Lander, J.F., 2002, Tsunamis and tsunami-like waves of the 

eastern United States, Science of Tsunami Hazards, the International Journal of the Tsunami 

Society, v. 20, no. 3, p. 120–144. http://epubs.lanl.gov/tsunami/ts203.pdf

Ludwin, R.S., Dennis, R., Carver, D., McMillan, A. D., Losey, R., Clague, J., Jonientz-Trisler, C., 

Bowechop, J., Wray, J., and James, K., 2005, Dating the 1700 Cascadia earthquake: 

Great coastal earthquakes in Native stories, Seismological Research Letters, v. 76, no. 2,

p. 140–148.

Mallet, R., 1853, Catalogue of recorded earthquakes from 1606 B.C. to 1850 A.D., Part I, 1606 B.C. 

to 1755 A.D., Report of the 22nd Meeting of the British Association for the Advancement of 

Science, held at Hull, September 1853, John Murray, London, p. 118–212.

Mallet R. 1854, Catalogue of recorded earthquakes from 1606 B.C. to 1850 A.D., Part II, 1755 A.D. 

to 1784 A.D., Report of the 23rd Meeting of the British Association for the Advancement 

of Science, held at Hull, September 1853, John Murray, London, p. 119–212. 

Mallet R. 1855, Catalogue of recorded earthquakes from 1606 B.C. to 1850 A.D., Part III, 1784 A.D. 

to 1842 A.D., Report of the 24th Meeting of the British Association for the Advancement of 

Science, held at Hull, September 1853, John Murray, London, 326 p.

Milne, J., 1911, Catalogue of destructive earthquakes, Report of the 81st Meeting of the British 

Association for the Advancement of Science, Portsmouth, London, United Kingdom, 

p. 649–740.

National Earthquake Information Center (NEIC), Preliminary Determination of Epicenters (PDE),

1971 to present, a weekly and monthly publication, U.S. Geological Survey.
2-14

O’Loughlin, K.F., and Lander, J.F., 2003, Caribbean Tsunamis: A 500-Year History from 

1498–1998, Kluwer Academic Publishers, 263 p. In cooperation with the National Oceanic 

and Atmospheric Administration (NOAA) and Cooperative Institute for Research in 

Environmental Sciences (CIRES, University of Colorado). 

Priest, G.R., Myers, E., Baptista, A.M., Flueck, P., Wang, K., and Peterson, C.D., 2000, Source 

simulation for tsunamis: Lessons learned from fault rupture modeling of the Cascadia 

subduction zone, Science of Tsunami Hazards, the International Journal of the Tsunami 

Society, v. 18, n. 2, 77–106. http://library.lanl.gov/tsunami/ts182.pdf

Robson, G.R., 1964, An earthquake catalogue for the Eastern Caribbean, 1530–1960, Bulletin of the 

Seismological Society of America, v. 54, no. 2, p. 785–832.

Satake, K., Wang, K., and Atwater, B., 2003, Fault slip and seismic moment of the 1700 Cascadia 

earthquake inferred from Japanese tsunami descriptions, Journal of Geophysical Research,

v. 108, p. ESE 7-1 to 7-17. Abstract at: 

http://www.agu.org/pubs/crossref/2003/2003JB002521.shtml

Singer, A., Rojas, C., and Lugo, M., 1983, Inventario Nacional de Riegos Geologicos, Estado 

Preliminare, Caracas: Fundacion Venezolana de Investigaciones Sismologicas (FUNVISIS), 

Serie Tecnica 03-83, 128 p.

Soloviev, S.L., and Go, C. N., 1974, A Catalogue of Tsunamis on the Western Shore of the Pacific 

Ocean, Academy of Sciences of the USSR, Moscow, 439 p. [Canadian Translation of 

Fisheries and Aquatic Sciences No. 5077, 1984. Translation available from Canada Institute 

for Scientific and Technical Information, National Research Council, Ottawa, Ontario, 

Canada K1A OS2.]

Soloviev, S.L., and Go, C. N., 1975, A Catalogue of Tsunamis on the Eastern Shore of the Pacific 

Ocean, Academy of Sciences of the USSR, 285 p. [Canadian Translation of Fisheries and 

Aquatic Sciences No. 5078, 1984. Translation available from Canada Institute for Scientific 

and Technical Information, National Research Council, Ottawa, Ontario, Canada K1A OS2.]

Soloviev, S.L., Go, C. N., and Kim, K. S., 1992, Catalog of Tsunamis in the Pacific, 1969–1982, 

Academy of Sciences of the USSR, 208 p. [Translated from Russian by Amerind Publishing 

Co. Pvt. Ltd., New Delhi, India, 1988.] http://www.ngdc.noaa.gov/hazard/data/publications/

catalog_tsunamis_pacific_1969_1982.pdf

Stover, C. W., and Coffman, J. L., 1993, Seismicity of the United States, 1568–1989 (Revised), 

U.S. Geological Survey Professional Paper 1527, 418 p.

U.S. Department of the Interior, Geological Survey and U.S. Department of Commerce, National 

Oceanic and Atmospheric Administration, 1928–1983, United States Earthquakes, Annual 

publication, prepared by U.S. Coast and Geodetic Survey, 1928–1968; prepared by NOAA 

1969–1972; prepared by NOAA in cooperation with U.S. Geological Survey 1973–1980, 

prepared by U.S. Geological Survey 1981–1983.
3-1

Section 3. USGS Earthquake Hazards Assessment

The USGS conducts research on earthquake hazards facing all U.S. States and territories. These 

hazard assessments rely on a combination of historical and modern earthquakes, geological 

information, and plate tectonic theories and models. (See Frankel et al., 1996, 2002). These inputs 

allow the earthquake hazard to be determined probabilistically. The USGS earthquake hazard maps 

specify the probability that ground shaking will exceed some level over a given number of years. 

Figure 3-1 is an example of a probabilistic map.

In this section, the results of USGS (NSHM) earthquake database searches are reported for possible 

earthquake sources near U.S. coastlines to extend the NOAA/NGDC tsunami databases. The USGS 

hazard calculations generally do not take into account either earthquake sources more than about 100 

km offshore, or far-field earthquake sources. Ultimately, these sources will need to be considered in 

another matter, particularly for the Atlantic Basin. 

Figure 3-1. Probabilistic earthquake hazard map for the eastern United States within 

100 km offshore. The map shows the peak ground acceleration with a 10 percent 

probability of being exceeded in 50 years. The Charleston, South Carolina, area 

stands out as the only section of coast where significant ground shaking of 10 percent

of the acceleration of gravity or more is expected (Frankel et al., 2002).
3-2

A probabilistic representation for earthquake hazards has been adopted in most building code and 

engineering applications. Engineering and building code officials typically use USGS hazard maps 

that show either a 10 percent chance of exceeding mapped levels over 50 years, or a 2 percent chance 

of exceeding the mapped levels in 50 years. The map showing 10 percent in 50 years gives a 500-

year window for expected ground shaking; the map showing 2 percent in 50 years corresponds to 

2500 years. 

Figure 3-1 (previous page) shows the probabilistic earthquake hazard for the eastern half of the 

United States for a 500-year window. The map shows that ground accelerations with a 10 percent 

probability of exceedance in 50 years vary considerably along the coast. Although ground shaking 

may occasionally be felt along the Gulf and U.S. Atlantic coasts (as occurred in 2006 and 2007), 

moderate to strong shaking is expected primarily near Charleston, South Carolina. 

Extensive databases are used to produce the USGS earthquake probabilistic maps. These 

databases include both historical and instrumental seismicity, earthquake observations such as 

damage patterns, geodetic records, fault history, and fault details. The USGS databases are used to 

calculate earthquake, not tsunami, occurrence. In the Pacific, the rate of the largest subduction zone 

earthquakes calculated by the USGS along U.S. subduction zones is essentially the same as the rate 

of local tsunami generation. For local tsunamis, this is not the case along the U.S. Atlantic or Gulf 

coasts which are not bordered by subduction zones. Thus the occurrence of the largest expected 

earthquakes of magnitude 7 to 7.5 does not necessarily mean a tsunami is generated.

The critical importance of using the USGS NSHM databases together with the NGDC databases is 

clear in Cascadia. Here, the last great subduction earthquake occurred on January 26, 1700. Until the 

USGS launched extensive paleoseismic research in the late 1980s, this event was unknown. 

At numerous coastal marshes along the Pacific Northwest coast, evidence of sudden influxes of 

both saltwater and sand sheets characteristic of tsunami deposits quickly led paleoseismologists 

to conclude that large vertical elevation changes of the marshes were caused by an earthquake. 

These were rapidly followed by inundation from a locally generated tsunami (Atwater, 1992). The 

only source for such an earthquake was the Cascadia subduction zone. Based on paleoseismology 

findings, researchers estimate that this earthquake broke the Cascadia fault along the entire Pacific 

Northwest coast, from northern California to central Vancouver Island (Atwater et al., 1995). The 

fault length, estimated from the distribution of elevation changes observed in coastal marshes, 

supports a magnitude 9+ event similar to the 2004 Sumatra earthquake. 

In the middle of several coastal marshes, geologists found standing dead western red cedar trees 

still rooted in their original growth position. When the marsh elevation dropped suddenly during 

the last Cascadia earthquake, saltwater overtook the marsh, killing the trees. Using tree-ring dating 

techniques, paleoseismologists determined that these trees died during the winter of 1699–1700 

(Yamaguchi et al., 1997). A damaging tsunami was well-recorded in Japan on January 27, 1700, but 
3-3

its source was unknown. Researchers paired the coastal observations in the Pacific Northwest with 

written Japanese records, documenting the tsunami damage (Atwater et al., 2005). This detective 

work established the January 26, 1700, date as the last great Cascadia earthquake.

In addition to the historical Japanese records of the 1700 tsunami (seven runup locations), many 

tsunami deposits have been found along the coasts of California, Oregon, Washington, and British 

Columbia from this event (NGDC tsunami deposit database, Figure 2-1). Although the paleoseismic 

records have considerable variation in the time estimated between great earthquakes, the average 

recurrence time is currently estimated at 500 years for earthquakes of magnitude 9. These great 

earthquakes are recorded by the sudden vertical displacement of numerous coastal marshes, and 

nearly every displaced marsh shows evidence of tsunami deposits (Atwater and Hemphill-Haley, 

1997). These paleoseismic studies underpin the USGS earthquake assessments. 

In subduction zones where tectonic models link these studies to earthquake occurrence, the 200 years 

of runup observations represented by the NGDC database can effectively be extended backward 

in time, extending the historical database. Because the USGS earthquake assessments are cast in 

a probabilistic framework, they represent a proxy for local tsunami hazards in the Alaska, western 

Pacific, Pacific Northwest, and Caribbean subduction zones.

Figure 3-2. Paleoseismic 

and paleotsunami 

record at Willapa Bay, 

Washington. A stack 

of layers dating from 

1700 A.D. back to 900 

B.C. is very distinctive. 

Each layer represents 

the downdropping of 

the marsh during a 

Cascadia subduction 

zone earthquake and 

subsequent depositing of 

locally generated tsunami 

sands. Observations at 

many sites along Cascadia 

have led to the estimated 

recurrence of about 500 

years for great subduction 

earthquakes. (USGS 

photo.)
3-4

3.1 Atlantic and Pacific Basin Differences

A national comparison of tsunami hazards must account for the difference in tectonics between the 

Atlantic and Pacific Ocean Basins. As is well known, much of the Pacific consists of subduction 

zones (Appendix A, Figure A-1)—the Alaska, Cascadia, Kermadac-Tonga, and many others that 

are responsible for producing most of the world’s great earthquakes and tsunamis (Table 2-1). 

The plethora of sources in the Pacific drives the number of tsunami events to the high levels seen 

in Hawaii. Not only are there large tsunami sources local to Hawaii, but the State is also in the 

crosshairs of tsunamis generated around the Pacific from South America to Alaska. To a lesser extent, 

other U.S. States and territories in the Pacific are subject to the dual threat posed by many distant 

sources and significant local sources.

The mid-Atlantic ridge, a spreading system, dominates the Atlantic Basin (Figure A-1). Unlike the 

subduction zone boundaries that dominate the Pacific and the Caribbean, the mid-Atlantic ridge is a 

passive boundary and tends to produce smaller magnitude earthquakes than those in subduction zones 

(Table 2-1). Generally, there is little probability of generating tsunamis along passive boundaries.

Lacking a tectonic plate boundary, the near-shore U.S. Atlantic and Gulf coasts are considered to 

be intraplate. This tectonic setting results in reduced expected earthquake maximum magnitudes 

(compared to the subduction zones in the Pacific) and greatly increased recurrence times. The USGS 

uses a maximum magnitude earthquake of 7.5 for all Gulf and Atlantic coastal areas (Frankel et al., 

1996, 2002). Unlike the Pacific or the Caribbean where earthquakes rupture to the seafloor causing 

large vertical displacements, primary surface faulting and vertical displacement have a very low 

probability of occurrence along the Gulf and Atlantic coasts.

Complicating the assessment in the Atlantic is the uncertainty of the physics responsible for 

producing the largest events. The 1929 magnitude 7.3 Grand Banks, Newfoundland, earthquake 

is one of three known earthquakes with magnitudes of 7 or greater along the eastern coast of 

North America. More importantly, this event generated a tsunami that killed 29 people in Canada. 

Seismological studies show this was a complex earthquake with a largely horizontal slip at a depth 

of about 20 km (Bent, 1995). The seismological parameters rule out vertical displacement on the 

seafloor as the cause of the accompanying tsunami. Instead, an underwater landslide, triggered by the 

earthquake, caused the tsunami. In the NGDC database summarized in Table 2-1, the 1929 tsunami is 

the only runup listed for Maryland, Massachusetts, and New Hampshire, and is one of the two runups 

listed for South Carolina and Rhode Island.

The other two magnitude 7.0 or greater events along the Atlantic coast did not have destructive 

tsunamis. The first was the 1886 Charleston, South Carolina, earthquake. This event had an 

estimated magnitude of 7.3. Based on felt reports, the epicenter was estimated to be just onshore. A 

small, non-destructive tsunami was observed in both Florida and South Carolina. The second event 

occurred near Baffin Bay, Canada, in 1933. Also of magnitude 7.3, this event apparently did not 

cause an underwater landslide, and no tsunami was generated.
3-5

The Geological Survey of Canada has very qualitatively estimated that an earthquake in the 

magnitude range of 6.5 to 7.0 is the minimum required to initiate a significant underwater landslide 

in offshore Atlantic conditions (John Adams, Geological Survey of Canada, personal communication, 

2006). Because events along and near the Atlantic coast are not expected to rupture faults to the 

surface and thus directly displace a volume of water, the simple occurrence of an event of this 

magnitude does not necessarily generate a tsunami. In areas along the Grand Banks where detailed 

bathymetric data exists, the number of observed underwater landslides is much less than the rate 

predicted from seismicity data. There is considerable uncertainty in relying on seismicity data alone.

The Geological Survey of Canada’s estimate of magnitude 6.5 was used to assess the likelihood of 

an earthquake initiating underwater landsliding along the Gulf and Atlantic coasts. But it must be 

understood that as on the Canadian coast, the rate of underwater landsliding will be less than the 

earthquake rate. How many of those landslides may generate tsunamis is yet another unknown. 

Considerable research is needed on offshore earthquake rates and magnitudes, the stability of offshore 

sediments along the margin, shaking levels required to initiate underwater mass movement, and the 

likelihood of tsunami generation from such movement.

In subduction zones offshore U.S. States and territories, including the Caribbean, Cascadia, Alaska, 

American Samoa, and Guam/Northern Mariana, the rate of the largest magnitude earthquakes 

estimated in USGS hazard assessments is essentially the same as the generation of local tsunamis. 

The maximum magnitude and the recurrence time of subduction zone earthquakes differ from area 

to area, but the relation to tsunami generation remains more or less the same. In some subduction 

zones such as Cascadia, the great events that occur on the fault interface between the subducting and 

overriding plates give rise to the slip on the ocean floor that generates severe tsunamis. In others, 

such as the Caribbean, faults within the overriding plate (upper plate) generate severe local tsunamis. 

Although the details of how earthquakes generate tsunamis are ultimately important for forecasting 

local wave behavior (whether these events occur on the long subducting plate interface or in the upper 

plate), these details are of lesser importance in the national overview. Accordingly, this report focuses 

on the recurrence of local earthquakes in subduction zones that exceed a local magnitude. A local 

magnitude of 7.5 is used for Puerto Rico and the Virgin Islands, and a local magnitude of 8.1 was 

used for Cascadia (Frankel et al., 2002).

3.2 Earthquake Probabilities determined from the USGS NSHM Databases

3.2.1 Non-Subduction Zones: Atlantic and Gulf Coasts

For the U.S. Atlantic and Gulf coasts, the USGS NSHM databases were searched for any known 

earthquakes with estimated magnitudes of 6.5 or greater located within 50 km of the shoreline. The 

probability of recurrence of earthquakes greater than magnitude 6.5 over 500 and 5000 years was 

then determined (Table 3-1, next page). Unlike the subduction zones where the largest earthquakes 

can be associated confidently with tsunami generation, along the U.S.Atlantic and Gulf coasts that is 

not possible. In some cases, such as the 1886 Charleston earthquake, the event is onshore, but serves 
3-6

Table 3-1. Local earthquake sources—USGS probability of earthquake occurrence for 

non-subduction zones. (Table last updated in 2007.)

State/Territory

Earthquake with Magnitude 

> 6.5 in 500 years within 50 

km of coast

Earthquake with Magnitude 

> 6.5 in 5000 years within 50 

km of coast

Historical maximum 

magnitude observed near 

shore or offshore Comment

U.S. Atlantic Coast

Maine <3% <30% <6

New Hampshire <3% <30% <6

Massachusetts <3% <25% <6

Rhode Island <2% <15% <6

Connecticut <2% <30% <6

New York <4% <30% <6

New Jersey <4% <30% <6

Pennsylvania <3% <15% <6

Delaware <3% <15% <6

Maryland <2% <15% <6

Virginia <1% <4% <6

North Carolina <1 to 5% <5% <6

South Carolina <35% 100% 7.3 1886 Charleston, 

non-destructive 

tsunami

Georgia <1% <10% <6

Florida <1% <3% <6

U.S. Gulf Coast

Florida <1% <3% <6

Alabama <1% <4% <6

Mississippi <1% <5% <6

Louisiana <1% <5% <6

Texas <1% <4% <6

to drive the probability of an event greater than magnitude 6.5 along the South Carolina and Georgia 

coasts considerably higher than in other areas along the Atlantic. 

In Figure 3-3 the return time for a magnitude 6.5 or greater earthquake provides an alternate view 

of how infrequently earthquakes of this magnitude occur along these coasts. Recurrence times are 

typically thousands of years everywhere except along the South Carolina coast. Additional research 

is needed to determine if the South Carolina area is unique in terms of the rate of magnitude 7 

events along the U.S.Atlantic coast. If earthquake magnitudes closer to 7 are required to initiate 

underwater landslides, the mean recurrence times lengthen even more. Figure 3-3 also shows that 

the USGS earthquake hazard assessment is centered on estimating ground shaking as opposed to 

tsunami occurrence. The gray areas offshore in Figure 3-3 are outside of the areas considered in the 

earthquake assessment.
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3.2.2 Subduction Zones: Pacific and Caribbean

The search results for subduction zones are summarized in Table 3-2 (next page). For comparison 

with the Atlantic and Gulf coasts, the probability of events greater than magnitude 6.5 over 500 years 

was included. This probability does not have the simple interpretation available for the Atlantic and 

Gulf coasts. 

Subduction zones have three distinct earthquake source zones, and all three contribute to the 

earthquake hazard and two are sources for severe tsunamis. The interface between the Juan de 

Fuca and North American plates (Figure 3-4) is an example of a subduction zone. The long sloping 

interface between the downgoing plate (Juan de Fuca) and overriding plate (North American) is the 

subduction interface (fault). These types of plate boundaries are where most of the world’s great 

earthquakes of magnitude 8 and larger occur. When the 

interface slips, sometimes tens of meters, the ocean floor 

moves as well. Since subduction zones have lengths of 

hundreds if not more than 1000 km and widths of 100 km 

or more, the slip between the plates is able to displace 

hundreds of km3

 of water. 

The other two earthquake sources are in the downgoing 

and the upper plates. Earthquakes in the downgoing plate 

(Juan de Fuca), at depths of about 40 km and deeper and 

with magnitudes sometimes as large as 8 or greater, can 

Figure 3-3. Return time for earthquakes with 

magnitude greater than 6.5 and within 50 km 

of a given location. (Figure courtesy of Steve 

Harmson, USGS.)

Figure 3-4. Diagram showing subduction 

zone. (USGS figure.)
3-8

Table 3-2. Local earthquake sources—USGS probability of earthquake occurrence for subduction zones. The 

magnitude of a subduction zone earthquake is referred to as “msubduct.”

State/Territory

Non-subduction 

earthquake with 

magnitude > 6.5 in 

500 years within 50 

km of coast

Subduction zone 

event with magnitude 

>msubduct in 500 years 

within 150 km of coast

Maximum magnitude 

observed or estimated 

for near shore or 

offshore Comment

 Puerto Rico and the Virgin Islands, msubduct = 7.5

 Puerto Rico 100% ~100% 7.5 1918 Mona Passage, severe tsunami**

 Virgin Islands 100% ~100% 7.5 1867 Virgin Islands, severe tsunami

Pacific Coast--Cascadia, msubduct = 8.1

 Washington 30% to 90% ~100% 9+ 1700 Cascadia, severe tsunami

 Oregon 10% to 100% ~100% 9+ 1700 Cascadia, severe tsunami

 California 100% ~100% 9+ 1700 Cascadia, severe tsunami

Pacific Coast--Alaska, msubduct = 7.5

 Alaska 100%* ~100% 9.2 1964 Alaska, severe tsunami

Pacific Island Territories, msubduct = 7.8

 Guam N/A ~100% 7.8 1993 Guam, non-destructive tsunami

 Northern Mariana N/A ~100% 7.8 1993 Guam, non-destructive tsunami

 American Samoa N/A ~100% 8.5 1917 Northern Tonga trench, moderate tsunami

* Alaska calculation for magnitude >6.5 includes subduction interface events

** Events as large as magnitude ~8 are estimated in the Puerto Rico trench

cause significant ground shaking. Their depth limits their potential to directly generate tsunamis, as 

little, if any, surface displacement occurs on the ocean floor. However, the ground shaking from these 

events could cause underwater landslides that might generate tsunamis. Events in the upper plate 

(North American), again sometimes of magnitude 8, can cause strong ground shaking and generate 

severe tsunamis. The destructive 1918 Mona Passage earthquake and tsunami in Puerto Rico 

occurred on an upper plate fault within the Caribbean subduction zone system.

The search results for the subduction zones are considerably different than those for the Atlantic and 

Gulf coasts. First, for those subduction zones where data is available, the probability of an event 

greater than magnitude 6.5 over 500 years is extremely high everywhere. In the Caribbean and 

Cascadia, the probabilities do not include earthquakes on the subduction interface, whereas for Alaska 

the probability does. Generally, the events represented in the second column are not expected to 

generate tsunamis both because of their magnitude and the location of the causative faults. There are 

numerous earthquakes of magnitude 6.5+ off the California coast at depths of a few kilometers that 

did not generate either underwater landslides or significant tsunamis. 

The third column of Table 3-2 shows the probability of a subduction zone event capable of generating 

a tsunami. In the Caribbean, the calculation includes expected upper plate fault sources, whereas in 
3-9

Cascadia the calculation is only for a great interface event. Despite these differences, the conclusion 

is obvious—these coasts will almost certainly be hit by severe local tsunamis over 500 years. 

3.2.3 Hawaii

Hawaii has a long history of damaging tsunamis (Table 2-1). Although not near a subduction zone, 

Hawaii’s tsunami record includes numerous events resulting from large local earthquakes, as well 

as those tsunamis generated in the Pacific subduction zones. The most recent example of a local 

earthquake that generated a destructive tsunami is the 1975 Kalapana magnitude 7.5 event. This 

earthquake occurred on the south flank of Kilauea, moving the flank into the ocean and causing a 

tsunami that killed two people camped on the beach in Hawaii Volcanoes National Park.

The earthquake database search criteria were adjusted to meet Hawaii’s tectonic setting. Table 3-

3 shows that the probability of a magnitude 6.5 or greater earthquake and that of a magnitude 7.5 

or greater event is about 100 percent over 500 years. The probability that these events will cause 

landslides is high, as there are examples in the recent historical and geological record.

3.2.4 Southern California

The offshore area of southern California has a series of crustal faults that have the potential to 

produce underwater earthquakes and generate local tsunamis. The 1927 Lompoc magnitude 7.1 

earthquake produced a tsunami recorded at numerous sites in southern California, with the largest 

reported runup being 1.8 m in Surf, California. Earthquake fault parameters have been used to 

reproduce the tsunami runup pattern observed along the California coast (Satake and Sommerville, 

1992). These suggest that primary faulting was responsible for generating the tsunami. 

How frequently such events may generate tsunamis is uncertain, but the nearly 100 percent certainty 

of repeat events greater than magnitude 6.5 and 7.5 in offshore California indicates that this is an 

area of needed research. The likelihood that these events could generate underwater landslides and 

Table 3-3. Local earthquake sources—USGS probability of earthquake occurrence for

Hawaii, southern California, and the Arctic coast of Alaska.

 State/area

Magnitude > 6.5 in 

500 years within 50 

km of coast

Magnitude > 7.5 in 

500 years within 50 

km of coast

Maximum magnitude 

observed or estimated for 

near shore or offshore Comment

Hawaii and Southern California

 Hawaii ~100% ~100% 7.9 1868 Ka’u district, severe tsunami

 Southern California ~100% ~100% 7.1 1927 Lompoc, moderate tsunami

Arctic Coast—Alaska

 Alaska <1% N/A <6 Arctic coast rated no tsunami risk by Alaska
3-10

a tsunami requires study. The possibility of landslides generating tsunamis in southern California is 

also discussed in Section 4.

3.2.5 Alaska Arctic Coast

The Arctic coast of Alaska is part of the large intraplate area of the North American plate. As 

such, it has no near-shore tectonic plate boundary. The USGS estimates very low probabilities for 

earthquakes greater than magnitude 6.5. The State of Alaska considers a risk of a tsunami here to be 

none to very low.

3.3 Discussion

The USGS NSHM databases reinforce the conclusions from the NGDC historical tsunami runup 

database (Table 2-1). Because U.S. States and territories in the Pacific Basin and the Caribbean are 

located at plate boundaries with geologically short recurrence times, the tsunami hazard for these 

areas is much higher than along the non-plate boundary areas of the Gulf and Atlantic coasts. On 

the Gulf and Atlantic coasts, the probability of magnitude 6.5 and greater earthquakes is low almost 

everywhere with the notable exception of South Carolina. 

The earthquake data was used to extend the initial assessment of tsunami hazards based on the NGDC 

database shown in Table 2-3. Because subduction zone events can also generate local tsunamis, a 

reliable estimate for tsunami recurrence in those zones is possible. In most Pacific U.S. States and 

territories, a local tsunami, often with severe runups, is expected at least once every 500 years. In 

Alaska, portions of the subduction zone have an even higher expected frequency of both earthquakes 

and tsunamis. This contrasts markedly with the assessment on the Gulf and Atlantic coasts, where the 

rate of even the minimum magnitude event thought necessary to generate a tsunami is very small over 

500 years. And, as noted above, even the occurrence of a magnitude 6.5 or greater event carries no 

certainty that it will result in producing a landslide-driven tsunami.

Table 3-4 summarizes the qualitative tsunami hazard assessment based on the USGS NSHM 

databases.
3-11

Table 3-4. Qualitative assessment of tsunami hazards based on USGS NSHM databases.

Region

Probability that an 

earthquake generates a 

local tsunami in 500 years 

by seafloor displacement 

U.S. Atlantic coast Very low to low

U.S. Gulf coast Very low

Puerto Rico and the Virgin 

Islands

High 

U.S. west coast High 

Alaska High 

Hawaii High

U.S. Pacific island

territories

High 

3.4 References

Adams, J., Geological Survey of Canada, personal communication, 2006. 

Atwater, B.F., 1992, Geologic evidence for earthquakes during the past 2000 years along the Copalis 

River, southern coastal Washington, Journal of Geophysical Research, v. 97, no. B2, 

p. 1901–1919.

Atwater, B. F., Nelson, A.R., Clague, J.J., Carver, G.A., Yamaguchi, D.K., Bobrowsky, P.T., 

Bourgeois, J., Darienzo, M.E., Grant, W.C., Hemphill-Haley, E., Kelsey, H.M., Jacoby, G.C., 

Nishenko, S.P., Palmer, S.P., Peterson, C.D., Reinhart, M.A, 1995, Summary of coastal 

geologic evidence for past great earthquakes at the Cascadia subduction zone, Earthquake 

Spectra, v. 11, 1–18.

Atwater, B.F., and Hemphill-Haley, E., 1997, Recurrence intervals for great earthquakes of the past 

3,500 years at northeastern Willapa Bay, Washington, U.S. Geological Survey Professional 

Paper 1576, 108 p.

Atwater, B.F., Musumi-Rokkaku, S., Satake, K., Tsuji, Y., Ueda, K., and Yamaguchi, D.K., 2005, 

The orphan tsunami of 1700, U.S. Geological Survey Professional Paper 1707, 133 p.

Bent, A.L., 1995, A complex double-couple source mechanism for the MS

 7.2 1929 Grand Banks 

earthquake, Bulletin of the Seismological Society of America, v. 85, p. 1003–1020.
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Frankel, A.D., Mueller, C.S., Barnhard, D., Perkins, D., Leyendecker, E. Hanson, S., and Hopper, M., 

1996, National seismic-hazard maps: Documentation June 1996, U.S. Geological Survey 

Open-File Report 96-532, 110 p.

Frankel, A.D., Peterson, M.D., Mueller, C.S., Haller, K.M., Wheeler, R.L., Leyendecker, E.V., 

Wesson, R.L., Harmsen, S.C., Cramer, C.H., Perkins, D.M., and Rukstales, K.S., 2002, 

Documentation for the 2002 update of the National Seismic Hazards Maps, U.S. Geological 

Survey Open-File Report 02-240, 33 p.

Satake, K., and Sommerville, P.G., 1992, Location and size of the 1927 Lompoc, California, 

earthquake from tsunami data, Bulletin of the Seismological Society of America, v. 82, 

p. 1710–1725.

U.S. Geological Survey, 2006, Historic Earthquakes, Charleston, South Carolina, 1886 September 01 

02:51 UTC (local August 31). http://earthquake.usgs.gov/regional/states/events/1886_09_01.php

Yamaguchi, D.K., Atwater, B.F., Bunker, D.E., Benson, B.E., and Reid, M.S., 1997, Tree-ring 

dating the 1700 Cascadia earthquake, Nature, v. 389, p. 922–923; editors’ correction 

in v. 390, p. 352.
4-1

Section 4. Gaps in Knowledge of Tsunami Sources

The NGDC tsunami database and the USGS NSHM databases characterize a significant part of the 

tsunami hazard facing the United States. Since the frequency of tsunamis generated by subduction 

zone earthquakes far exceeds the frequency of other sources, such as volcanic eruptions or underwater 

landsliding, further research on these subduction zone sources is critical for hazard assessments of 

nearby coastlines, but will have little effect on the national tsunami hazard assessment. However, 

non-earthquake sources, such as a volcano collapse in the mid-Atlantic and slope failure along 

the Atlantic coastal shelf, are possible tsunami sources that could affect the hazard assessment. 

Further, in the Atlantic Basin, the available databases may not fully characterize earthquake sources, 

particularly along the Azores–Gibraltar fracture zone. In this section, we highlight several key 

tsunami source studies that would narrow the uncertainty for a tsunami hazard assessment of U.S. 

coastlines. 

4.1 Atlantic Basin

4.1.1 Earthquake Sources

Earthquakes along the eastern seaboard are poorly understood. It is clear that a better understanding 

of the generation of magnitude 7+ earthquakes similar to the 1929 Grand Banks earthquake is 

necessary. Unfortunately, such source studies are apt to be difficult, as there are very few available 

earthquakes that might provide some clues of the stresses responsible for generating these events. 

Again, the mere occurrence of such events does not guarantee a local tsunami.

Outside of the Caribbean, the Azores–Gibraltar fracture zone represents the clearest distant threat 

to the U.S Atlantic, Caribbean, and possibly Gulf coasts. It forms a portion of the plate boundary 

between the African and the Eurasian plates. The 1755 Lisbon earthquake generated a severe tsunami 

that devastated the City of Lisbon. Wave heights of 30 m were recorded along the European coast 

(Kozak and James, 1998). A highly stylized illustration, done 100 years after the actual event, shows 

how this earthquake and tsunami resonated in Europe (Figure 4-1).

Figure 4-1. Published in 

the Illustrated London News, 

March 30, 1850, as part of a 

story on the Lisbon earthquake 

(Kozak and James, 1998).
4-2

Figure 4-2. Map details of African-Eurasian plate boundary near 

the Iberian Peninsula. The plate boundary is complex here, 

with evidence of vertical motion indicated by earthquake focal 

mechanisms. (Figure courtesy of Dr. Eulalia Gràcia, Centre 

Mediterrani d’Investigacions Marines i Ambientals, Unidad de 

Tecnologia Marina (CSIC), Barcelona, Spain. Modified from Gràcia

et al., 2003a.)

The 1755 tsunami was reported in the West Indies, where an initial sea level rise of 1.0 m was 

followed by large waves (Kozak and James, 1998). There was a report in Cape Bonavista, Canada, of 

the harbor being drained then refilling and overflowing parts of the community. No observations on 

the U.S. Atlantic seaboard have been found for this event, although one numerical model suggests 

2.0 m waves offshore with onshore runups of 3.0 m (Mader, 2001). 

The plate boundary between the 

Eurasian and African plates is 

thought to be the source for the 1755 

earthquake (Gràcia et al., 2003a; 

Thiebot and Gutscher, 2006). For 

much of its length eastward from the 

mid-Atlantic ridge, this boundary is 

either a passive spreading center or 

supports largely horizontal slip (small 

map insert, Figure 4-2). However, 

near the Iberian Peninsula, the plate 

boundary becomes complex. Sea 

floor bathymetry shows evidence 

of significant vertical motions (near 

Gorringe Bank) and there are large 

areas in deep water that could 

generate a major tsunami similar to 

the 1755 event. 

Three lines of research examining 

the 1755 event would be useful. 

First is developing a better understanding of earthquake occurrence and possible tsunami generation 

near the Iberian Peninsula. Fortunately, European researchers have an aggressive program underway 

to investigate both earthquake and tsunami generation (Gràcia et al., 2003a; Gràcia et al., 2003b; 

Gutscher et al., 2006). Second, possible evidence of the 1755 tsunami on the Caribbean and Atlantic 

coast should be pursued. Such a study needs to take a multi-disciplinary approach, from examining 

local newspapers to sifting through tidal marshes for possible tsunami deposits. Third, modeling 

efforts are needed to understand any propagation or source effects that might control the severity of 

the tsunami recorded in the Caribbean and North America for a source near the Iberian Peninsula. 

A second source area, considerably less likely to generate a major tsunami, is the northern edge of 

the Caribbean plate west of Hispaniola (Figure 4-3). The northern boundary consists of a broad 

strike-slip zone that is dominated by horizontal motion, but features along the boundary indicate 

some vertical motion is accommodated over time. The installation of a Deep-ocean Assessment 

and Reporting of Tsunamis (DARTTM) station in the Gulf of Mexico (25.409° N, 86.800° W) is 
4-3

geographically located in part to provide a warning from poorly studied sources such as this plate 

boundary. 

4.1.2 Landslide and Volcano Sources

Because there are many fewer subduction zones in the Atlantic than the Pacific, there are fewer 

earthquake source regions to generate tsunamis. Underwater landslides are increasingly recognized 

as a potentially deadly tsunami source that need further study. Earthquakes (such as the 1929 Grand 

Banks event), volcanic activity, or gravity slumping could all generate underwater landsliding 

sufficient to cause tsunamis. The deadly Grand Banks tsunami, generated by an earthquake-induced 

underwater landslide, was recognized in 1952 as the first documented turbidity current (Lockridge 

et al., 2002). Recently, Tuttle et al. (2004) found sand layers deposited by the 1929 tsunami on 

Newfoundland (Figure 4-4). These researchers were able to distinguish the tsunami deposits from 

storm-related deposits, demonstrating that tsunami deposit studies can be successfully conducted on 

the Atlantic coast.

Figure 4-3. Caribbean plate boundaries. The area west of Hispaniola may have some potential to generate 

a tsunami, but this potential is poorly studied. (Figure courtesy of Uri ten Brink, USGS, Woods Hole, 

Massachusetts.)
4-4

Figure 4-4. Tsunami deposits at Taylor’s Bay on 

Newfoundland’s southern coast from the 1929 

Grand Banks earthquake and tsunami. The 

photograph shows three sandy units deposited by 

consecutive waves. (Tuttle et al., 2004.)

Figure 4-5. Slope failures and submarine 

canyons along a portion of the Atlantic 

coast. (Figure courtesy of Dr. David Twichell, 

Woods Hole Science Center, USGS.)

Since the Grand Banks tsunami, many underwater landslides have been found in the Atlantic Basin. 

Along the Atlantic coast, a number of underwater landslides are known in the deep canyons (Figure 

4-5). Like landslides on land, underwater landslides show a distribution with many small slides 

incapable of generating a significant tsunami, to a few very large slides that could possibly generate 

catastrophic tsunamis.

The Albemarle–Currituck underwater landslide, offshore of the North Carolina–Virginia coast, is an 

example of the landslide problem for the Atlantic States. Driscoll et al. (2000) determined the age of 

the landslide as about 18,000 years and estimated the volume as about 140 km3

. Because of the large 

volume of displaced material, it is possible that this event generated a tsunami even though no record 

is known.

Side-scan sonar imagery of the landslide 

shows a series of three en-echelon 

(steplike) cracks to the north of the 

existing slide (Figure 4-6, next page). 

Driscoll et al. (2000) suggested that 

these cracks could be a prelude to a 

future landslide, with the potential to 

generate a tsunami. These authors 

estimated that such a tsunami might 

have runups of several meters. An 

improved assessment of tsunami 

hazards along the Atlantic coast requires 
4-5

additional study of the underwater 

landslide record and tsunami deposit 

studies onshore. Fortunately, the USGS 

has begun a one-year investigation 

of tsunami sources along the Atlantic 

coast with funding from the Nuclear 

Regulatory Commission. This study has 

the advantage of looking at sources over 

15,000 years that might affect nuclear 

facilities along the U.S. east coast. Those 

findings will be incorporated into a future 

version of this national assessment.

Failure of slopes on volcanic islands 

may also generate tsunamis. Soufriere 

Hills, Montserrat, in the Caribbean, 

erupted in 1997, 1999, and 2003. Each 

eruption resulted in a slope failure which generated a local tsunami. To date, no volcanic eruption 

in the Caribbean has generated a significant tsunami recorded along the Atlantic or Gulf coasts, 

but historically damaging tsunamis related to volcanic eruptions are responsible for much damage 

and loss of life in the Caribbean. More distant from the United States, the volcanic systems on the 

Canary Islands are viewed by some researchers as a potentially disastrous tsunami source. Ward 

and Day (2001) suggested that a massive volcanic eruption could trigger a massive landslide, with a 

block 15 to 20 km wide and 15 to 25 km long, dropping at high speeds into the deep Atlantic Ocean. 

According to their calculations, massive waves would result, and the United States coastline could see 

wave heights of as much as 10 to 25 m. 

There is considerable controversy surrounding this estimate. Some researchers question whether 

such a catastrophic landslide is possible and suggest instead that an eruption would lead to a series of 

much smaller landslides (Wynn and Masson, 2003) and other researchers estimate far smaller waves 

reaching American shores, on the order of 3 m (Mader, 2001). Considerable research is needed to 

narrow the wide uncertainty surrounding such proposed catastrophic tsunamis in the Atlantic. It 

should be stressed that these very catastrophic sources are admitted, even by proponents, to be very 

rare compared with the 500 or so year recurrence time typical in Pacific and Caribbean subduction 

zones.

4.2 Pacific Basin

In principle, the Pacific could expect to see tsunamis generated by underwater landslides and volcanic 

activity causing sector collapses. However, given the recurrence time of 500 or less years in virtually 

all subduction zones, other sources will add relatively little to short-term hazard assessments. For 

Figure 4-6. Side-scan sonar image of the Albemarle-Currituck 

landslide off the Virginia and North Carolina coasts. The 

scarps to the north of the slide are marked a, b, and c and are 

discussed in the text. (Driscoll et al., 2000.)
4-6

Figure 4-7. One of four megalandslides along the Cascadia subduction zone margin (Goldfinger et al.,

2000). The Heceta Slide is ~80x50 km, and 2 km deep.The chaotic seafloor outlined by the dashed line

marks the debris lobe of the slide, only about half of which is visible due to burial beneath the smooth abyssal 

plain in the foreground. Long-traveled buried debris indicates a catastrophic, not slow, failure. Numerous 

smaller slides ~10–15 km in width dot the continental slope. One young example is seen in the left 

foreground. Such slides are infrequent, but exceptionally large tsunami sources. The Oregon coast and 

Cascade Range is in the background.

example, Goldfinger et al. (2006) report that a number of exceptionally large, and many smaller, 

submarine landslides have been identified along the Cascadia margin that are thought to have 

repeat times on the order of 75,000 years for smaller slides, to 500,000 years for the largest slides. 

Goldfinger et al. (2006) estimate that the initial tsunami height of the largest of these slides may have 

been ~60 m, with a shallow water wave height of 10.0 m–20.0 m, and initial heights for the other 

largest slides were likely ~40 m (Figure 4-7). The smaller landslides most likely generated tsunamis 

with initial heights of ~1.0 m–5.0 m.

Similarly, upper plate faults connected to the slipping master subduction zone fault could play a 

significant role in generating very large tsunamis. The details of whether faults immediately off the 

northern California, Oregon, and Washington coasts might have significant vertical displacement 

during a Cascadia earthquake will have large repercussions for local tsunami hazard calculations. 

However, these details will do little to change the relative ranking of the very high and frequent 

hazard facing the Pacific States and territories, versus the very low and much less frequent hazard for 

Atlantic coastal States.

Improving the assessment of southern California is a high priority area. The Lompoc earthquake 

generated a tsunami with 1.8 m runups at several locations. Work is needed to assess the threat 
4-7

of both earthquake-generated tsunamis by underwater surface displacement and to improve 

understanding of the potential for large underwater landslides that could generate tsunamis. In 

southern California, there is a very high rate of magnitude 7.0 and greater offshore earthquakes. A 

clearer understanding of possible locally-generated tsunamis would improve the national assessment.

Fortunately, the USGS has begun a two-year assessment of the tsunami potential of southern 

California as part of a new Federal Hazards Initiative. This assessment will look at both landslide 

and earthquake sources in southern California. As with the USGS east coast study, the results 

from southern California will significantly improve the overall national assessment and will be 

incorporated into future versions.

4.3 Discussion

The understanding of possible tsunami sources is clearly incomplete, as is the severity of tsunamis 

expected from underwater landslides or island sector collapse. It is clear that the lessons learned by 

Tuttle et al. (2004) for the 1929 tsunami need to be applied to marsh sites along the Atlantic, Gulf, 

and southern California coasts in an effort to begin to document the tsunami record. The 1755 event 

is a particularly attractive target for research, as the date and location of the event is well known. 

Ample evidence exists that the 1755 tsunami was observed in the West Indies, but determining 

whether runups occurred further north would help prioritize possible tsunami source region studies in 

the Atlantic—particularly the Azores–Gibraltar zone.

Also clear is the difficult nature of designing mitigation strategies for very rare, but possibly large, 

tsunamis from landslides or volcanic sector collapse in the Atlantic. From the view of warning, the 

deployment of DARTTM stations in the Atlantic is designed to provide warning to American shores 

from distant sources like those noted here. Until the severity of waves possible on the Atlantic is 

better understood, the true nature of the tsunami hazard will remain uncertain. 

It is also clear that lacking firm constraints on some tsunami sources, the Tsunami Warning Centers 

and other wave modeling researchers will have to continue to generate suites of theoretical waves 

that could affect American coastlines in the Atlantic. A successful research program on documenting 

tsunamis and understanding potential sources will allow updating and improving models used to 

provide warning guidance along the Atlantic and Gulf coasts.

4.4 National Tsunami Research Plan

The results of a review of the current state of tsunami research and a strategic plan for tsunami 

research in the United States are presented in the National Tsunami Research Plan (Bernard et al., 

2007). This document includes state-of-the-science reports on hazard assessment, warning guidance, 

preparedness, response, and mitigation. Generally, the specific recommendations outlined here are 

reflected in this broader review of tsunami research needs. 
4-8

4.5 References

Bernard, B., Dengler, L., and Yim, S. (editors), 2007, National Tsunami Research Plan: Report of a 

Workshop Sponsored by NSF/NOAA. NOAA Technical Memorandum OAR PMEL–133, 

135 p. http://www.pmel.noaa.gov/pubs/PDF/bern3043/bern3043.pdf

Driscoll, N.W., Weissel, J.K., and Goff, J.A., 2000, Potential for large-scale submarine slope failure 

and tsunami generation along the U.S. mid-Atlantic coast, Geology, v. 28, no. 5, p. 407–410.

Gràcia, E., Daňobeita, J., Verges, J., and Bartolome, R., 2003a, Crustal architecture and tectonic 

evolution of the Gulf of Cadiz (SW Iberian margin) at the convergence of the Eurasian and 

African plates, Tectonics, v. 22, no. 4, p. 7-1–7-19.

Gràcia, E., Daňobeita, J., Verges, J., and PARSIFAL Team, 2003b, Mapping active faults offshore 

Portugal (36°N–38°N): Implications for seismic hazard assessment along the southwest 

Iberian Margin, Geology, v. 31, no. 1, p. 83–86.

Goldfinger, C., Kulm, L.D. Kulm, and Menill, L., 2000, Super-scale slumping of the southern Oregon 

continental margin: B. Keating and C. Waythomas, editors, Pure and Applied Geophysics 

Special Volume on Landslides, v. 157, p. 1189–1226.

Gutscher, M.A., Baptista, M.A., and Miranda, J.M., 2006, The Gibraltar Arc seismogenic zone (part 

2): Constraints on a shallow east dipping fault plane source for the 1755 Lisbon earthquake 

provided by tsunami modeling and seismic intensity, Tectonophysics, v. 426, no. 1-2, p 153–

166.

Kozak, J.T., and James, C.D., 1998, Historical Depictions of the 1755 Lisbon Earthquake, 

http://nisee.berkeley.edu/lisbon/. Figure 4-1 from the Kozak Collection of historical 

earthquake images, National Information Service for Earthquake Engineering, 

http://nisee.berkeley.edu/kozak/.

Lockridge, P.A., Whiteside, L.S., and Lander, J.F., 2002, Tsunamis and tsunami-like waves of the 

eastern United States, Science of Tsunami Hazards, the International Journal of the Tsunami 

Society, v. 20, no. 3, p. 120–144. http://epubs.lanl.gov/tsunami/ts203.pdf

Mader, C.L, 2001, Modeling the 1755 Lisbon tsunami, Science of Tsunami Hazards, the International 

Journal of the Tsunami Society, v. 19, no. 2, p. 93–98. http://library.lanl.gov/tsunami/ts192.pdf

Thiebot, E. and Gutscher, M.A., 2006, The Gibraltar Arc seismogenic zone (part 1): Constraints on a 

shallow east dipping fault plane source for the 1755 Lisbon earthquake provided by seismic 

data, gravity and thermal modeling, Tectonophyiscs, v. 426, no. 1-2, p. 135–152.

Tuttle, M.P., Ruffman, A., Anderson, T., and Jeter, H., 2004, Distinguishing tsunami from storm 

deposits in eastern North America: The 1929 Grand Banks tsunami versus the 1991 

Halloween storm, Seismological Research Letters, v. 75, p. 117–131.

Ward, S.N. and Day, S., 2001, Cumbre Vieja volcano—Potential collapse and tsunami at La Palma, 

Canary Islands, Geophysical Research Letters, v. 28, no. 17, p. 3397–3400.

Wynn, R.M., and Masson, D.G., 2003, Canary Islands landslides and tsunami generation: Can we 

use turbidite deposits to interpret landslide processes?, in Locat, Jacques, and Jurgen Mienert 

(editors), Proceedings, 1st International Symposium on Submarine Mass Movements and 

Their Consequences, Kluwer, Dordrecht, p. 325–332. 
5-1

Section 5. Next Steps

This report is the first national tsunami hazard assessment of the United States and its territories. 

The core of the report consists of an evaluation of the NGDC historical tsunami databases and a 

consideration of probable local earthquake sources along U.S. coastlines capable of generating 

tsunamis. The results were summarized in tables that identified a qualitative tsunami hazard 

assessment of “very low” to “very high.” These assessments recognized that tsunami runups of a few 

tens of centimeters have a lower hazard than those with runups of a few to many meters.

Hazard assessments are by their nature dynamic and new understanding of tsunami sources should 

trigger a review to ensure that the relative rankings of America’s coastal tsunami threat reported here 

remain appropriate. As noted, two new studies of tsunami sources by the USGS, one off the U.S. east 

coast and one in southern California, when completed, will significantly improve the completeness of 

the national assessment presented here. The need to improve the characterization of tsunami sources 

was also one of six high-priority research areas identified in the National Tsunami Research Plan 

(Bernard et al., 2007). This plan was the result of a review in July 2006 by a group of tsunami experts 

on the current state of knowledge in areas essential to tsunami risk reduction. The six high-priority 

research areas are:

1. Enhance and sustain tsunami education

2. Improve tsunami warnings

3. Understand the impacts of tsunami at the coasts

4. Develop effective mitigation and recovery tools

5. Improve characterization of tsunami sources

6. Develop a tsunami data acquisition, archival, and retrieval system

The Plan also described the need to develop a probabilistic framework for characterizing tsunami 

sources that includes thousands of years of recurrence. Moving to a probabilistic framework, 

however, depends on high-resolution elevation models that currently do not exist for most coastal 

communities and the availability of resources for conducting numerous inundation and propagation 

simulations. A clear benefit of these inundation and propagation studies is the ability to provide 

estimates of currents in inundation areas. In some situations, relatively small runup amplitudes might 

have considerable currents that could cause significant damage in harbors and on beaches.

Recent examples of probabilistic tsunami hazard assessments illustrate the challenge of conducting 

this type of analysis. A probabilistic assessment of Western Australia (Burbridge et al., 2007) 

considers only tsunami sources from earthquakes along the Sunda Arc in the Indian Ocean. Offshore 

wave heights, not inundation or runup heights used in this report, form the basis of the tsunami hazard 

assessment. The goal of the report is noteworthy, however, as it represents an attempt to identify 
5-2

areas of Western Australia with the highest tsunami hazard from modeling studies and use these 

results to guide subsequent inundation modeling.

Two additional examples of probabilistic tsunami hazard assessments that include inundation 

modeling and runup height estimates are the Seaside, Oregon, study (Tsunami Pilot Study Working 

Group, 2006) described in Appendix C and a recent study of tsunami runup hazards in California at 

marine oil terminal sites (Borrero et al., 2006). The California study estimated tsunami runup heights 

for both 100– and 500–year return times. These runup heights for 100–year return times range 

from 1.0 m (3.3 ft) in the western Carquinez Strait in San Francisco Bay, to 3.3 m (11.0 ft) at Port 

Hueneme on the southern California coast. For a 500–year return period, the heights increase at these 

two locations to 1.2 m (4.0 ft) and 6.4 m (21.0 ft) respectively. A better understanding of inundation 

characteristics for the highly varied U.S. coastal environments is an obvious critical missing 

ingredient to a national hazard assessment.

As the tsunami assessment matures, the next step is to determine what is at risk from tsunamis. 

A complete tsunami hazard and risk assessment, called for in the NSTC Framework report, 

brings together the hazard assessment, the exposure of people and structures, and their combined 

vulnerability. The National Tsunami Research Plan (Bernard et al., 2007) identified another 

research area that is directly related to conducting this type of assessment. The Plan identified the 

need to understand the impacts of tsunamis at the coast, including measuring the tsunami current 

regime in harbors and at the coast, improving hydrodynamic modeling, developing credible fragility 

models of the interaction of tsunamis with the built and natural environment, and validating models 

through benchmarking. The complexity inherent in a tsunami risk assessment demands a full and 

collaborative partnership of the NTHMP.

5.1 References

Bernard, B., Dengler, L., and Yim, S. (editors), 2007, National Tsunami Research Plan: Report of a 

Workshop Sponsored by NSF/NOAA. NOAA Technical Memorandum OAR PMEL–133, 

135 p. http://www.pmel.noaa.gov/pubs/PDF/bern3043/bern3043.pdf

Borrero, J., Dengler, L., Uslu, B., and Synolakis, C., 2006, Numerical Modeling of Tsunami Effects at 

Marine Oil Terminals in San Francisco Bay, Report prepared for Marine Facilities Division 

of The California State Lands Commission, 38 p., 6 appendices. http://www.slc.ca.gov/

division_pages/MFD/MOTEMS/SFTsunamiReport/sf_mot_final_report.doc

Burbridge, D., Cummins, P., and Mleczko, R., 2007, A Probabilistic Tsunami Hazard Assessment for 

Western Australia, Report to the Fire and Emergency Services Authority of Western Australia, 

Earthquake and Tsunami Hazard Project, Geoscience Australia, 58 p.

Tsunami Pilot Study Working Group, 2006, Seaside, Oregon Tsunami Pilot Study—Modernization of 

FEMA Flood Hazard Maps, Joint NOAA/USGS/FEMA Special Report, 94 p., 7 appendices. 

http://pubs.usgs.gov/of/2006/1234/
6-1

Section 6. Conclusion

The first action called for in the joint report by the sub-committee on Disaster Reduction and the U.S. 

Group on Earth Observations entitled Tsunami Risk Reduction for the United States: A Framework 

for Action, is to develop standardized and coordinated tsunami hazard and risk assessments for all 

coastal regions of the United States and its territories. In response to this report, at the request of the 

National Tsunami Hazard Mitigation Program (NTHMP), NOAA’s National Geophysical Data Center 

(NGDC) and the United States Geological Survey (USGS) collaborated to conduct the first tsunami 

hazard assessment of the United States and its territories.

This report provides a national qualitative assessment of the United States tsunami hazard at a 

regional level by examining the record of historical tsunamis and earthquakes, the predominant cause 

of tsunamis, at the State and territory level. Two different sources of information were compiled to 

assess the United States tsunami hazard. The NGDC historical tsunami database was first examined 

and resulted in a qualitative tsunami hazard assessment based on the distribution of runup heights 

and the frequency of tsunami runups. The NGDC tsunami database contains reported tsunamis and 

is therefore limited to written records existing for an area. The hazard assessment also used the 

USGS National Seismic Hazard Map (NSHM) databases to partially extend the time interval. These 

databases made it possible to estimate the rate of occurrence of larger magnitude earthquakes that 

could generate a tsunami. Based on the total spread of events, runup amplitudes, and earthquake 

potential, we assigned a subjective hazard from very low to very high. These assessments recognize 

that tsunami runups of a few tens of centimeters have a lower hazard than those with runups of a few 

to many meters.

This hazard assessment reinforces the common understanding that the U.S. Atlantic coast and the 

Gulf Coast States have experienced very few tsunami runups. In contrast, all U.S. coasts in the 

Pacific Basin as well as Puerto Rico and the U.S. Virgin Islands have a “moderate” to “very high” 

tsunami hazard based on both frequency and known runup amplitudes. The total number and the 

large number of runups greater than 3.0 m observed in Alaska and Hawaii justified assigning a “very 

high” hazard for these two States. We assigned a “moderate” hazard to the Pacific island territories 

due to the rare occurrence of events with amplitudes greater than 3.0 m. Both the frequency and 

the amplitudes of tsunami runups support a qualitative “high” hazard assessment for Washington, 

Oregon, California, Puerto Rico, and the Virgin Islands. The “high” value for Oregon, Washington, 

and northern California reflects the low frequency (~1 per 500 years) but the potential for very high 

runups from magnitude 9 earthquakes on the Cascadia subduction zone. Although tsunami deaths 

are a measure of risk rather than hazard, we compared the known tsunami deaths found in our NGDC 

database search with our qualitative assessments based on frequency and amplitude.

The table (next page) summarizes the results of the NGDC and USGS database searches.
6-2

Table 6-1. Qualitative tsunami hazard assessment based on NGDC and USGS databases.

 Region

Hazard based on 

runups

Hazard based on 

frequency

Hazard based on 

local earthquakes

Number of 

reported deaths

U.S. Atlantic coast Very low to low Very low Very low to low None

U.S. Gulf coast Very low Very low Very low None

Puerto Rico and the 

Virgin Islands

High High High 172

U.S. west coast High High High 25

Alaska Very high Very high High 222

Hawaii Very high Very high High 326

U.S. Pacific island

territories

Moderate High High 1

As our understanding of the tsunami hazard facing the United States improves, this document will 

be updated. Updates will be required as additional knowledge is obtained of possible sources in 

the Atlantic Basin and offshore southern California. Updates will also be required as knowledge 

increases of the Cascadia subduction zone earthquakes. 

A complete tsunami hazard and risk assessment, called for in the Framework report, brings together 

the hazard assessment, the exposure of people and structures, and their combined vulnerability. 

The next step is to determine what is at risk from tsunamis. NGDC, in partnership with States and 

territories and other federal offices, plans to examine the exposure, vulnerability, and, finally, the risk 

of people and structures to tsunamis on the coasts of the United States.
A-1

Appendix A. What is a Tsunami?

adapted from the International Tsunami Information Center Web site 

http://ioc3.unesco.org/itic/

The word tsunami is composed of the Japanese words “tsu” (which means harbor) and “nami” (which 

means “wave”). A tsunami is a series of large waves of extremely long wavelength and period. They 

are usually generated by a violent, impulsive undersea disturbance or activity near the coast or in the 

ocean. When a sudden displacement of a large volume of water occurs, or if the seafloor is suddenly 

raised or dropped by an earthquake, tsunami waves can be formed by forces of gravity. The waves 

travel out of the area of origin and can be extremely dangerous and damaging when they reach the 

shore.

The most destructive tsunamis are generated from large, shallow earthquakes with an epicenter or 

fault line near or on the ocean floor. These usually occur in regions of the earth characterized by 

tectonic subduction along tectonic plate boundaries (Figure A-1). 

Figure A-1. The earth’s surface is broken into seven large and many small moving plates. Three types of 

movement are recognized at the plate boundaries: convergent, divergent, and transform-fault. At convergent 

boundaries, plates move toward each other and collide. Subduction occurs where an oceanic plate collides with 

a continental plate and the oceanic plate slides beneath the continental plate forming a deep ocean trench. This 

is the type responsible for most tsunamis (e.g., Gulf of Alaska Trench, Aleutian Trench, Chile Trench, Tonga 

Trench, and Puerto Rico Trench). At divergent boundaries, plates move away from each other (e.g., mid-Atlantic 

ridge). At transform-fault boundaries, plates move horizontally past each other (e.g., San Andreas fault, Cayman 

trough north of Puerto Rico).
A-2

When these plates collide or move past each other, they cause large earthquakes, which tilt, offset, 

or displace large areas of the ocean floor along a length of a few kilometers to as much as 1,000 

km or more. The sudden vertical displacement of several meters over such large areas disturbs the 

ocean’s surface, displaces water, and generates destructive tsunami waves. The waves can travel 

great distances from the source region, spreading destruction along their path. It should be noted 

that not all earthquakes generate tsunamis. Usually, it takes an earthquake with a Richter magnitude 

exceeding 7.5 to produce a destructive tsunami.

Although relatively infrequent, violent volcanic eruptions also represent impulsive disturbances 

that can displace a great volume of water and generate extremely destructive tsunami waves in 

the immediate source area. According to this mechanism, waves may be generated by the sudden 

displacement of water caused by a volcanic explosion, by a volcano’s slope failure, or more likely by 

an explosion and collapse of the volcanic magmatic chambers.

Less frequently, tsunami waves can be generated from displacement of water resulting from rock 

falls, icefalls, and sudden submarine landslides or slumps. Such events may be caused impulsively 

from the instability and sudden failure of submarine slopes, which are sometimes triggered by the 

ground motions of a strong earthquake. Major earthquakes are suspected to cause many underwater 

landslides, which may contribute significantly to tsunami generation. In general, the energy of 

tsunami waves generated from landslides or rock falls is rapidly dissipated as they travel away from 

the source and across the ocean, or within an enclosed or semi-enclosed body of water—such as a 

lake or a fjord. 

All oceanic regions of the world can experience tsunamis, but in the Pacific Ocean and its marginal 

seas, there is a much more frequent occurrence of large, destructive tsunamis because of the many 

large earthquakes along the margins of the Pacific Ocean.

Once a tsunami has been generated, its energy is distributed throughout the water column, regardless 

of the ocean’s depth. The waves travel outward on the surface of the ocean in all directions away 

from the source area, much like the ripples caused by throwing a rock into a pond, except that the 

origin is not necessarily a point but can be along the entire length of ground disruption.

In the deep ocean, the height of the tsunami may be only a few centimeters to a meter or more. 

Tsunami waves in the deep ocean can travel at high speeds for long periods of time for distances of 

thousands of kilometers and lose very little energy in the process. The deeper the water, the greater 

the speed of the tsunami waves will be. At the deepest ocean depths, the speed of the tsunami wave 

will be as much as 800 km/h. Therefore, a tsunami generated in the Aleutian Islands may reach 

Hawaii in less than four and a half hours.
A-3

Several measurements are used to describe tsunamis. Inundation is the maximum horizontal distance 

inland that a tsunami penetrates. Runup is the maximum height above mean sea level the wave 

reaches at the maximum inundation. Water height is the average height of a tsunami measured 

from the trough to the crest after removing the tidal variation. Any tsunami runup over a meter is 

dangerous to people and property. 

There are three direct factors of destruction from tsunamis: inundation, wave impact on structures, 

and erosion. Strong, tsunami-induced currents lead to the erosion of foundations and the collapse 

of bridges and seawalls. Flotation and drag forces move houses and overturn railroad cars. 

Considerable damage is caused by the resultant floating debris, including boats and cars that become 

dangerous projectiles that may crash into buildings, break power lines, and may start fires. Fires from 

damaged ships in ports, or from ruptured coastal oil storage tanks and refinery facilities, can cause 

damage greater than that inflicted directly by the tsunami. Of increasing concern is the potential effect 

of tsunami draw down, when receding waters uncover cooling water intakes of nuclear power plants.

Tsunami waves arrive at a coastline as a series of successive crests (high water levels) and troughs 

(low water levels). As they enter the shallow waters of the coastline, their speed decreases to about 

50–60 km/h. For example, in 15 m of water the speed of a tsunami will be only 45 km/h. However, 

100 km or more away, another tsunami wave travels in deep water towards the same shore at a 

much greater speed, and still behind it there is another wave, traveling at even greater speed. As the 

tsunami waves become compressed near the coast, the wavelength is shortened and the wave energy 

is directed upward—thus increasing their heights considerably.

Even if a tsunami wave may have been 1 m or less in the deep ocean, it may grow into a huge 

30–35 m wave when it sweeps over the shore. Thus, tsunami waves may smash into the shore like 

a wall of water or move in as a fast-moving flood or tide—carrying everything in their path. If the 

tsunami waves arrive at high tide or during a storm, the effects will be cumulative and the inundation 

and destruction even greater. A locally-generated tsunami may reach a nearby shore in less than ten 

minutes. For people living near the coast, the shaking of the ground is a warning that a tsunami may 

be imminent. For tsunamis from more distant sources, however, accurate warnings of when a tsunami 

might arrive are possible because tsunamis travel at a known speed. 

Tsunami warning systems have been established to protect life and property from the tsunami hazard 

by providing timely, accurate, reliable, and effective tsunami products to coastal populations and 

emergency management within the area-of-responsibility, as well as by advancing other aspects of 

tsunami hazard mitigation. The primary operational objectives of a tsunami warning system are to 

rapidly locate, size, and otherwise characterize major earthquakes, determine their tsunamigenic 

potential, predict tsunami arrival times, predict coastal runup when possible, and disseminate 

appropriate warning and informational products based on this information. The United States has 
A-4

established two tsunami warning centers: the Pacific Tsunami Warning Center (PTWC) in Honolulu, 

Hawaii, and the West Coast/Alaska Tsunami Warning Center (WC/ATWC) in Palmer, Alaska. 

PTWC serves as the operational headquarters for the Tsunami Warning System in the Pacific. PTWC 

works closely with other regional and national centers in monitoring seismological and sea level 

stations and instruments around the Pacific Ocean to evaluate potentially tsunamigenic earthquakes. 

The system disseminates tsunami information and warning messages to more than 100 points across 

the Pacific. Regional tsunami warning centers operated by the United States (WC/ATWC), France, 

and Japan provide regional warnings to the U.S. and Canadian coastlines, French Polynesia, and the 

northwest Pacific, respectively. 
B-1

Appendix B. Risk Management Process

by Dale Dominey-Howes and Paula Dunbar

In this section the risk management process is discussed. This includes the identification and 

characterization of the hazard, identification of the vulnerability of people and assets that are exposed 

to the hazard, estimates of losses, and identification of appropriate mitigation strategies. This process 

is outlined in Figure B-1 (next page). The process is also described as it applies to tsunamis.

B.1 Hazard Assessment Module

A hazard assessment involves the process of identification and evaluation of a potentially-damaging 

phenomenon in a given area (Blanchard, 2006). In the hazard assessment module two activities are 

undertaken: (1) the identification and characterization of the hazard sources; and, (2) the calculation 

of the probability of occurrence. For hazards that occur frequently (e.g., floods), historical data 

give a reasonably accurate picture of the long-term hazard in terms of recurrence and the expected 

magnitude of events. For hazards that occur infrequently, such as earthquakes or tsunamis, historical 

data are insufficient to accurately assess the long-term hazard. Geologic data can be used to extend 

the record significantly for earthquakes and tsunamis (Atwater et al., 2005; Dominey-Howes, 2002). 

Two approaches are often used to characterize infrequent hazards. The first approach, deterministic, 

uses discrete, single-valued events to arrive at a scenario-like description of the hazard. The second 

approach, probabilistic, allows the use of multi-valued or continuous events and models incorporating 

the magnitude and frequencies of all events that could impact a site. In other words, the inputs to a 

probabilistic analysis are the same as those used in a deterministic analysis, plus the assessment of the 

frequency of occurrence of the phenomenon. 

Probabilistic tsunami hazard analysis (PTHA) was derived from probabilistic seismic hazard analysis 

(PSHA), developed by Cornell (1968). PSHA was modified to develop a PTHA that calculates wave 

heights and more recently to produce a probabilistic tsunami inundation map (Tsunami Pilot Study 

Working Group, 2006). 

A probabilistic tsunami hazard analysis requires historical and prehistorical tsunami data, quantitative 

probabilistic models of local and far-field tsunami sources (earthquake, landslide, volcano), highresolution digital elevation models (topography, bathymetry, tidal information), and numerous 

inundation and propagation simulations for the potential local and far-field tsunami sources. The 

result of this type of analysis, probabilistic tsunami inundation maps, was published in the Seaside, 

Oregon Tsunami Pilot Study (Tsunami Pilot Study Working Group, 2006), and described in 

Appendix C.
B-2

Figure B-1. Risk management process. HAZUS-MH is a Federal Emergency Management Agency software 

program that estimates potential losses from earthquakes, hurricane winds, and floods. HAZUS-MH includes

national databases such as elevation, census tract, and building stock inventory data.

B.2 Exposure and Vulnerability Assessment Module

After the tsunami hazard has been characterized, the next step is to determine who and what is 

actually exposed to tsunami inundation and runup. This requires an assessment of the people, homes, 

commerce, industry, natural resources, etc., that are in the tsunami inundation zones for a given 

event. However, exposure does not equal vulnerability—the susceptibility to harm or damage during 

tsunami inundation. The vulnerability of a physical structure would be influenced by factors such as 
B-3

structural design, material, condition of structure, and distance from shoreline. The factors that would 

control the vulnerability of a person would include age, gender, education, mobility, and physical 

health. Therefore, the next step is to quantify the vulnerability of the people and structures exposed to 

tsunami inundation. 

B.3 Loss Assessment Module

The next step is to combine the hazard, exposure, and vulnerability assessment modules, to calculate 

probable maximum losses (including number of casualties, direct economic losses, and indirect 

economic losses due to business interruption, etc.). From these probable maximum losses, it is 

possible to determine appropriate hazard mitigation strategies and disaster management plans. An 

examination of the literature indicates that the only validated tsunami vulnerability assessment and 

loss estimation methodology is that referred to as the Papathoma Tsunami Vulnerability Assessment 

(PTVA) model (Papathoma, 2003; Papathoma et al, 2003; Papathoma and Dominey-Howes, 2003; 

Dominey-Howes and Papathoma, 2006).

B.4 Mitigation Module

Once the probable maximum losses for tsunami have been determined, it is up to local government 

and emergency management agencies to develop appropriate mitigation and disaster management 

strategies. These would include such measures as land use zoning, building codes and regulations, 

outreach and awareness, etc. This completes the risk management process. 

B.5 References

Atwater, B.F., Musumi-Rokkaku, S., Satake, K., Tsuji, Y., Ueda, K., and Yamaguchi, D.K., 2005, The 

orphan tsunami of 1700, U.S. Geological Survey Professional Paper 1707, 133 p.

Blanchard, B.W., 2006, Theory, Principles and Fundamentals of Hazards, Disasters and U.S. 

Emergency Management, Department of Homeland Security. Glossary at: http://www.csc.

noaa.gov/vata/glossary.html.

Cornell, C.A., 1968, Engineering seismic risk analysis, Bulletin of the Seismological Society of 

America, v. 58, p. 1583–1606.

Dominey-Howes, D., 2002, Documentary and geological records of tsunamis in the Aegean Sea 

region of Greece and their potential value to risk assessment and disaster management, 

Natural Hazards, v. 25, p. 195–224.

Dominey-Howes, D. and Papathoma, M., 2006, Validating a tsunami vulnerability assessment model 

(the PTVA Model) using field data from the 2004 Indian Ocean tsunami, Natural Hazards, v. 

40, p. 113–136.

Papathoma, M., 2003, Tsunami vulnerability assessment using a geographic information system with 

special reference to Greece, unpublished PhD Thesis, Coventry University, UK, 289 p.

Papathoma, M. and Dominey-Howes, D., 2003, Tsunami vulnerability assessment and its implications 

for coastal hazard analysis and disaster management planning, Gulf of Corinth, Greece, 

Natural Hazards and Earth System Sciences, v. 3, p. 733–744.
B-4

Papathoma, M., Dominey-Howes, D., Zong, Y. and Smith, D., 2003, Assessing tsunami vulnerability, 

an example from Herakleio, Crete, Natural Hazards and Earth System Sciences, v. 3, p. 

377–389.

Tsunami Pilot Study Working Group, 2006, Seaside, Oregon Tsunami Pilot Study—Modernization of 

FEMA Flood Hazard Maps, Joint NOAA/USGS/FEMA Special Report, 94 p., 7 appendices. 

http://pubs.usgs.gov/of/2006/1234/
C-1

Appendix C. Probabilistic Tsunami Hazard Assessment—

Seaside, Oregon Tsunami Pilot Study—Modernization of 

FEMA Flood Hazard Maps

adapted from Seaside, Oregon Tsunami Pilot Study, Tsunami Pilot Study Working Group, 2006

A Tsunami Pilot Study was carried out for Seaside/Gearhart, Oregon, to develop an improved 

Probabilistic Tsunami Hazard Assessment (PTHA) methodology and to provide recommendations for 

improved tsunami hazard assessment guidelines. The study was part of FEMA’s Map Modernization 

Program. It was an interagency effort by FEMA, the U.S. Geological Survey, and the National 

Oceanic and Atmospheric Administration, in collaboration with the University of Southern California, 

Middle East Technical University, Portland State University, Horning Geoscience, Northwest 

Hydraulics Consultants, and the Oregon Department of Geological and Mineral Industries.

The study consisted of a number of components including tsunami source specification; data 

acquisition of historical records, paleotsunami deposits and eyewitness reports; high-resolution digital 

elevation model development; tsunami propagation and inundation model development; probabilistic 

computations; and development of a GIS database.

C.1 Results

The final products of this study were two tsunami inundation maps. The first is a 100–year tsunami 

map, or maximum tsunami wave heights with a 1 percent annual probability of exceedance (Figure 

C-1, next page). The second is a 500–year tsunami map, or maximum tsunami wave heights with 0.2 

percent annual probability of exceedance (Figure C-2). For a complete description of the process, it 

is suggested that readers obtain a copy of the complete report referenced below.

C.2 References

Tsunami Pilot Study Working Group, 2006, Seaside, Oregon, Tsunami Pilot Study—Modernization of 

FEMA Flood Hazard Maps, Joint NOAA/USGS/FEMA Special Report, 94 p., 7 appendices. 

http://pubs.usgs.gov/of/2006/1234/
C-2

Figure C-1. Figure from the Seaside Tsunami Pilot Study (2006), showing a 100–year tsunami map for the 

Seaside-Gearhart, Oregon, area.
C-3

Figure C-2. Figure from the Seaside Tsunami Pilot Study (2006), showing a 500–year tsunami map for the 

Seaside-Gearhart, Oregon, area.