Soon after the destructive tsunami in the Indian Ocean on December 26, 2004, many people asked, could such a tsunami happen in the United States? Information about tsunamis that have struck U.S. coasts in the past, summarized briefly here and in more detail on the Can it happen here in the United States? Web page, provide a foundation for estimating tsunami likelihood in the future.
What Is a Tsunami?
A tsunami is a set of powerful ocean waves most commonly caused by a large earthquake or landslide that occurs 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 tsunami waves approach shore, they behave like a very fast-moving tide that extends far inland. A rule of thumb is that if you see the tsunami, it is already too late to outrun it. Most tsunamis do not "break" like the curling, wind-generated waves popular with surfers. Even "small" tsunamis (for example, 6 ft high) are associated with extremely strong currents, capable of knocking people off their feet. 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, such as the Indian Ocean tsunami of 2004.
Because of complex interactions with the coast, tsunami waves can persist for many hours. If you hear a tsunami warning or if you feel strong shaking at the coast or observe highly unusual wave activity (for example, 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 (generally several hours to days).
For more general information on tsunamis and what to do during a tsunami warning, please visit the Web sites sponsored by the Federal Emergency Management Agency (FEMA, URL http://www.fema.gov/areyouready/tsunamis.shtm), the National Weather Service (URL http://www.nws.noaa.gov/om/brochures/tsunami.htm), State agencies (links at URL http://www.wsspc.org/tsunami/tsunamilinks.html), the National Oceanic and Atmospheric Administration (NOAA, URL http://www.pmel.noaa.gov/tsunami/), and the U.S. Geological Survey (USGS, URL http://walrus.wr.usgs.gov/tsunami/). For tips from the stories of people who have survived tsunamis, please see USGS Circular 1187, "Surviving a TsunamiLessons from Chile, Hawaii, and Japan," (PDF files in English and Spanish can be downloaded from this site).
Data We Can Use to Answer the Question "Could It Happen Here?"
Three primary sources of information can be used to answer the question "Could it happen here?": (1) tsunami catalogs of historical events, (2) the age of geologic deposits left by great earthquakes and tsunamis (see related Sound Waves article "Group Aims to Distinguish Tsunami Deposits from Large-Storm Deposits in the Geologic Record", and (3) computer simulations of tsunamis from potential great earthquakes and landslides around the world. This article focuses on the first source, historical information, taken mainly from the worldwide catalog of historical tsunamis maintained by NOAA's National Geophysical Data Center (NGDC). 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. The term "runup" refers to the vertical height a wave reaches above sea level as it washes ashore, "wave height" to the vertical measurement of the wave before it reaches shore, and "inundation distance" to the horizontal distance a tsunami reaches landward from the shoreline. More information on tsunami measurements can be found on the NGDC Tsunami Introduction Web page.
Hawai'i has a long recorded history of tsunamisboth "teletsunamis" (also called "far-field tsunamis") from earthquakes around the Pacific rim, and "local tsunamis" from earthquakes and landslides near Hawai'i. The Pacific Disaster Center reports that tsunamis have accounted for more lost lives in Hawai'i than the total of all other local disasters. In the 20th century, an estimated 221 people were killed by tsunamis on the islands of Hawai'i. One of the largest and most devastating tsunamis that Hawai'i has experienced was a teletsunami in 1946 from an earthquake along the Aleutian subduction zone. Runup heights reached a maximum of 33 to 55 ft and 159 people were killed. Damage was more than $26 million ($255 million in today's dollars). Other important teletsunamis include one from the 1960 magnitude 9.5 earthquake in southern Chile and one from the 1964 magnitude 9.2 earthquake in the Gulf of Alaska. The May 22, 1960, Chile earthquake generated a 35-ft-high wave, causing 61 deaths and $23 million damage (about $150 million in today's dollars). Hawai'i has also been hit by local tsunamis, primarily from earthquakes and large-scale subsidence along the south flank of Kilauea.
Because Alaska, including the Aleutian Islands, is bordered on the south by a major subduction zone capable of generating large earthquakes, Alaska has sustained many damaging tsunamis. By far, the one that stands out is the tsunami generated by the 1964 magnitude 9.2 earthquake in the Gulf of Alaska. Not only was a Pacific-wide tsunami generated by this great earthquake, but landslides in coastal fiords, such as Port Valdez, also generated localized, extremely damaging waves. The 1964 tsunami caused damage and loss of life across the Pacific. The West Coast & Alaska Tsunami Warning Center in Palmer, AK, indicates that the 1964 tsunami was the most disastrous tsunami to hit the U.S. west coast, causing many fatalities and financial losses.
U.S. West Coast
The historical record of tsunamis along the U.S. west coast consists mainly of teletsunamis generated by large earthquakes around the Pacific Rim. Of the teletsunamis that have struck the West Coast, the 1964 Gulf of Alaska tsunami caused the most extensive damage, particularly in Crescent City, CA, where at least 10 deaths occurred. Potentially tsunamigenic fault structures exist locally offshore the U.S. west coast, most notably the Cascadia subduction zonean offshore fault system about 1,200 km (750 mi) long that extends from northern California to southern Canada and accommodates motion between the Pacific and North American tectonic plates at a rate of about 40 mm/yr (1.6 in./yr). This subduction zone is believed to have most recently ruptured in a magnitude 9.0 earthquake in 1700; the resulting tsunami was recorded in historical accounts in northern Japan. Geologic evidence of submerged vegetation indicates that large or great earthquakes (magnitude 8 to 9) have occurred on average every 500 years along this zone. Great ruptures along this subduction zone would most likely cause local and possibly oceanwide tsunamis that could affect the Western United States.
U.S. Gulf Coast
In historical times, tsunami waves recorded along the Gulf Coast have all been less than 1 m high. Those reported from the 1964 Gulf of Alaska earthquake as felt in Louisiana and Texas are technically termed seichesoscillations of a body of water typically caused by atmospheric disturbances but in this case caused by ground motion from the earthquake. Seiches can also occur in lakes from earthquake movements.
U.S. East Coast
Because the only major subduction zones in the Atlantic Ocean are along the Caribbean Sea, tsunamis have been relatively infrequent here in comparison with the Pacific Ocean. The most famous Atlantic tsunami was the 1755 Lisbon tsunami, generated by an earthquake on a fault offshore Portugal. Teletsunamis from that earthquake were reported in Newfoundland (Canada) and islands in the Caribbean Sea. The most noteworthy local tsunami in North America resulted from the 1929 magnitude 7.3 Grand Banks earthquake near Newfoundland. In this complex event, most, if not all, of the tsunami energy may have been triggered by a submarine landslide. The maximum tsunami runup was 2 to 7 m, which was concentrated on the coast of Newfoundland, although it was recorded as far south as South Carolina. A couple of tsunamis reported from Caribbean earthquakes had runups less than 1 m.
Puerto Rico and the U.S. Virgin Islands
Puerto Rico and the U.S. Virgin Islands are more susceptible than other places in the Eastern United States, because a subduction zone capable of generating large earthquakes lies beneath the Caribbean Sea. Tsunamis have struck Puerto Rico and the Virgin Islands more than six times in recorded history. The tsunami with the greatest amount of damage in Puerto Rico was generated in 1918 by an earthquake off the Mona Passage, east of the island. With a maximum runup of 6 m, the tsunami itself killed 40 people; an additional 76 people were killed by the earthquake. The Caribbean region as a whole has a history of earthquakes that have caused damaging tsunamis. (See Sound Waves article "Workshop Addresses Tsunami Hazard to Puerto Rico, the Virgin Islands, and Other Caribbean Islands".)
Other U.S. Territories
Other territories of the United States are adjacent to large subduction zones. Guam and the Marianas Islands are next to the Marianas Trench. American Samoa is affected by earthquakes about 100 mi away along the Tonga-Kermadec Trench.
in this issue:
Could a Tsunami Happen Here?