Could It Happen Here?

The Question

Soon after the devastating tsunami in the Indian Ocean on December 26, 2004 many people have asked, "Could such a tsunami happen in the United States?" This web page summarizes what we know about tsunamis that have struck the U.S. in the past, providing a foundation for estimating tsunami likelihood in the future. Below, we outline the sources of data that can help answer the question, and then indicate when and how large tsunamis have been for specific regions of the U.S.

What are Tsunamis?

Tsunamis are ocean waves caused by large earthquakes and landslides that occur near or under the ocean. Scientists do not use the term "tidal wave" because these waves are not caused by tides. Tsunami waves are unlike typical ocean waves generated by wind and storms. When tsunamis approach shore, the behave like a very fast moving tide that extends far inland. A rule of thumb is that if you see the tsunami, it is too late to out run it. Most tsunamis do not "break" like the curling, wind-generated waves popular with surfers. Even "small" tsunamis (for example, 6 feet in height) are associated with extremely strong currents, capable of knocking someone off their feet. Because of complex interactions with the coast, tsunami waves can persist for many hours. As with many natural phenomena, tsunamis can range in size from micro-tsunamis detectable only by sensitive instruments on the ocean floor to mega-tsunamis that can affect the coastlines of entire oceans, as with the Indian Ocean tsunami of 2004. If you hear a tsunami warning or if you feel strong shaking at the coast or very unusual wave activity (e.g., the sea withdrawing far from shore), it is important to move to high ground and stay away from the coast until wave activity has subsided (usually several hours to days). For more general information on tsunamis and what to do during a tsunami warning, please visit sites sponsored by FEMA, the National Weather Service, state agencies, NOAA, and the USGS.

Data We Can Use to Answer the Question

There are three primary sources of information we can use to answer the question Could it Happen Here? (1) tsunami catalogs of historic events, (2) the age of geologic deposits left by great earthquakes and tsunamis, and (3) computer simulations of tsunamis from potential great earthquake and landslides around the world. In this web page, we will focus mainly on the historic information (1). The National Geophysical Data Center (part of NOAA), maintains a worldwide catalog of histroric tsunamis. This catalog includes two types of measurements: runup observations from eyewitness accounts and wave height readings from tide gauge stations, most often located in harbors. In scientific terms, runup refers to the vertical height a wave reaches above a reference sea level as it washes ashore. Wave height is the vertical measurement of the wave before it reaches shore. Inundation distance is the horizontal distance a tsunami reaches landward from the shoreline. More information on tsunami measurements can be found at the NGDC Tsunami Introduction Page.

Hawai‘i

Hawai'i has a long recorded history of tsunamis. Tsunamis have come from both earthquakes around the Pacific rim or "Ring of Fire", termed teletsunamis or far-field tsunamis, and from earthquakes and landslides near Hawai'i, termed local tsunamis. One of the largest and most devastating tsunamis Hawai'i has experience was a teletsunami in 1946 from an earthquake along the Aleutian subduction zone. Runup heights reached a maximum of 33-55 feet and 159 people were killed. Other important teletsunamis include one from the 1960 M=9.5 earthquake in southern Chile and one from the 1964 M=9.2 earthquake in the Gulf of Alaska. Local tsunamis have also hit Hawai'i, primarily from earthquakes and large-scale subsidence along the south flank of Kiluea. The largest of these were in 1868 that killed 81 people and in 1975. Overall, approximately 32 tsunamis with runup > 1 meter have occured in Hawai'i since 1811.

Alaska

Because Alaska, including the Aleutian Islands, is bordered to the south by a major subduction zone capable of generating large earthquakes, Alaska has experienced a number of damaging tsunamis. By far, the one that stands out is the tsunami generated from the 1964 M=9.2 earthquake that occurred in the Gulf of Alaska. Not only was a Pacific-wide tsunami generated from this great earthquake, but landslides in the coastal fjords such as Valdez also generated localized, but extremely damaging waves. Alaska's famous fjords are also the source for another type of "tsunami": one in which landslides perched on the steep walls of fjords catastrophically fail and splash into the water, generating extreme wave heights, such as the 1958 Lituya Bay landslide. Again this is a localized phenomenon that does not produce teletsunamis as with tsunamis produced by great earthquakes. Overall, approximately 16 tsunamis of all sources with runup > 1 meter have occurred in Alaska since 1853.

U.S. West Coast

The historic record of tsunamis along the U.S. west coast includes mainly teletsunamis, generated from large earthquakes around the Pacific Rim. However, using detailed Japanese historic accounts, scientists have determine that a tsunami was generated n January 26, 1700 by a local earthquake close to magintude 9, offshore the Pacific Northwest (the Cascadia subduction zone). Analysis of geologic deposits indicates that a number of earthquakes, possibly of magnitude 8-9, have occured in the past, indicating that future tsunamis from the Cascadia subduciton zone are possible. Of the teletsunamis that have struck the West Coast, the 1964 Gulf of Alaska tsunami caused the most extensive damage, particularly in Crescent City, California. Overall, approximately 28 tsunamis with runup > 1 m have occurred along the U.S. West Coast since 1812.

U.S. Gulf Coast

In historic times, tsunami waves recorded along the Gulf Coast have all been less than 1 meter. Some of the reports are from the 1964 Gulf of Alaska earthquake recorded in Loisiana and Texas and are technically termed a seiche. A seiche is an oscillation of a body of water, typically caused by atmospheric distrurbances, but in this case caused by the ground motion from the earthquake. Seiches can also occur in lakes from earthquake movements. There are a couple of early 20th-centutry reports of tsunami waves from Caribbean earthquakes along the Gulf Coast that are difficult to evaluate, but the wave heights all appear to be less than 1 meter.

U.S. East Coast

Because there are no major subduction zones in the Atlantic Ocean, except for where it borders the Caribbean Sea, there has been a relatively low frequency of tsunamis compared to the Pacific Ocean. The most famous Atlantic tsunami is the 1755 Lisbon tsunami, that was generated by an earthquake on a fault offshore Portugal. The most noteworthy North America local tsunami is the 1929 M=7.2 Grand Banks earthquake near Newfoundland, Canada. This is a complex event; most, if not all, of the tsunami energy may have been triggered by a submarine landslide. The maximum tsunami runup from this event was 2-7 meters concentrated on the coast of Newfoundland, though it was recorded as far south as South Carolina. Like the Gulf Coast, there a couple of reports of small tsunamis from Caribbean earthquakes, all less than 1 m.

Puerto Rico/U.S. Virgin Islands

Puerto Rico and the U.S. Virgin Islands are more susceptible than other locations in the eastern U.S., because of a subudction zone that lies beneath the Caribbean Sea, capable of generating large earthquakes. The web site of the Puert Rico Tsunami Warning and Mitigation Program also asks whether a tsunami similar to the one in the Indian Ocean could hit the Caribbean region. An event in 1867 off the Virgin Islands is thought to have generated waves 12 meters high. The tsunami with the greatest amount of damage in Puerto Rico was in 1918 from an earthquake off the Mona Passage. With a maximum runup of 6 meters, the tsunami itself killed 40 people with an additional 76 people killed by the earthquake. The Caribbean region as a whole has a history of other earthquakes that have caused damaging tsunamis.

Other U.S. Territories in the western Pacific, like Guam, also have a history of tsunamis.

Acknowledgments

These historic reports are largely based on the tsunami catalog maintained by the NGDC. Paper copies of the tsunami catalogs can be ordered from the publications web site. (See Copyright notice.)