Tsunamis are powerful natural phenomena that can travel vast distances across oceans. Curious about the reach of these seismic sea waves? travels.edu.vn offers a comprehensive guide to understanding how far tsunamis can travel, their causes, and what to do if you’re in a potential impact zone. Learn about tsunami wave propagation and protect yourself with our expert advice.
1. Understanding Tsunamis: A Force of Nature
1.1. Defining a Tsunami
A tsunami is a sequence of exceptionally long waves triggered by a significant and abrupt disturbance of the ocean. The crests of these waves can be separated by distances ranging from tens to hundreds of miles. Tsunamis radiate outward in every direction from their origin point, capable of traversing entire ocean basins. Upon reaching the coastline, they can induce extensive coastal flooding and generate potent currents that persist for hours or even days.
1.2. Origin of the Term “Tsunami”
The term “tsunami” originates from the Japanese language, combining the characters for “harbor” (“tsu”) and “wave” (“nami”).
1.3. Tsunami vs. Seismic Sea Wave or Tidal Wave
A tsunami can be referred to as a seismic sea wave if it is the result of an earthquake, as “seismic” refers to earthquake-related phenomena. However, tsunamis can also be caused by other disturbances unrelated to seismic activity. Therefore, the term “tsunami” is internationally recognized as encompassing waves generated by any substantial and sudden displacement of the ocean. It is important to note that tsunamis are not associated with tides, which are caused by the gravitational forces exerted by the Sun and Moon on Earth’s oceans. Therefore, referring to a tsunami as a tidal wave is inaccurate.
1.4. Can Tsunamis Be Predicted?
While it’s impossible to predict exactly when and where the next tsunami will strike, similar to the unpredictability of the earthquakes that often cause them, Tsunami Warning Centers closely monitor seismic activity. They identify earthquakes likely to generate tsunamis and issue warnings when a potential threat exists. Once a tsunami is detected, these centers utilize forecast models to predict wave height, arrival times, potential flooding areas, and the duration of the event. In cases where the tsunami’s source is close to the coast, there may not be sufficient time for detailed forecasts, emphasizing the importance of recognizing natural warning signs and being prepared to react swiftly.
1.5. Frequency of Tsunamis
According to the Global Historical Tsunami Database, tsunamis that inflict damage or cause fatalities near their source occur approximately twice annually. Tsunamis causing damage or deaths on distant shores, defined as more than 1,000 kilometers (620 miles) away, occur roughly twice per decade.
1.6. Global Distribution of Tsunamis
Tsunamis can be generated in any of the world’s oceans, inland seas, and large bodies of water. They have caused devastation and loss of life in coastal regions worldwide. However, certain areas are more susceptible to tsunamis due to their proximity to tsunami sources, the bathymetry (depth and shape of the ocean floor near the coast), and the topography (coastal elevation and features). The Global Historical Tsunami Database records 754 confirmed events between 1900 and 2015, with approximately 78% occurring in the Pacific Ocean, around the geologically active “Ring of Fire.” The Atlantic Ocean and Caribbean Sea account for 8%, the Mediterranean Sea 6%, the Indian Ocean 5%, and other seas 1%.
Since 1900, the highest percentage of tsunamis originated off the coast of Japan (21%), followed by Russia (8%) and Indonesia (8%). While most tsunamis are minor and either nondestructive or only affect nearby coasts, some can inflict damage and fatalities on distant shores, defined as more than 1,000 kilometers (620 miles) away. Since 1900, the most significant distant tsunamis originated off the coasts of Alaska, Chile, Japan, Indonesia, Pakistan, and Russia.
You can explore the locations of past tsunamis and learn more through the Natural Hazards Interactive Map.
1.7. Tsunami Risk Areas in the United States
An assessment of tsunami hazards in the United States indicates that any U.S. coast can be struck by a tsunami, but the level of risk varies. These risk levels are determined based on historical data (up to 2014), geological evidence, and proximity to tsunami sources, offering insight into potential future events.
| Region | Hazard Level |
|---|---|
| U.S. West Coast | High to Very High |
| Alaska (Southern Coast) | High to Very High |
| Alaska Arctic Coast (incl. West) | Very Low |
| Hawaii | High to Very High |
| American Samoa | High |
| Guam and Northern Mariana Islands | High |
| Puerto Rico/U.S. Virgin Islands | High |
| U.S. Atlantic Coast | Very Low to Low |
| U.S. Gulf Coast | Very Low |
Distant tsunamis pose a threat to all U.S. coasts, but coastlines near subduction zones face the greatest risk. Large earthquakes and landslides in these areas can generate damaging waves that impact both nearby and distant coasts, such as those around the Pacific and Caribbean. The U.S. East and Gulf Coasts are not near subduction zones, and earthquakes are less frequent and of smaller magnitude. The most likely tsunami sources on these coasts are underwater landslides and meteotsunamis.
1.7.1. Tsunami Risk in Anchorage and Upper Cook Inlet, Alaska
Compared to the Southern Coast of Alaska, Anchorage and the upper Cook Inlet have a very low tsunami risk. Tsunamis entering the upper Cook Inlet from the Gulf of Alaska or the lower Cook Inlet are weakened by the relatively shallow waters, reducing their potential danger.
1.8. Significant Tsunamis Affecting the United States
According to the Global Historical Tsunami Database, as of January 2018, 30 reported tsunamis have affected U.S. states and territories, each causing at least one fatality or $1 million in damage (adjusted to 2017 dollars).
| Region | Local Tsunami* | Distant Tsunami* |
|---|---|---|
| U.S. West Coast | 1820, 1878, 1894, 1930 | 1946, 1952, 1957, 1960, 1964, 1975, 2006, 2010, 2011 |
| Alaska | 1788, 1845, 1853, 1900, 1917, 1946, 1957, 1958, 1964, 1994 | 1960 |
| Hawaii | 1868, 1975 | 1837, 1868, 1877, 1923, 1946, 1952, 1957, 1960, 1964, 2011, 2012 |
| American Samoa | 2009 | 1946, 1960 |
| Guam and Northern Mariana Is. | 1849 | — |
| Puerto Rico/U.S. Virgin Is. | 1867, 1918 | — |
*See the definition of local and distant tsunamis in the question below. For more details, consult the Historic Tsunami Calendar.
1.9. When Can Tsunamis Occur?
Tsunamis are not seasonal events. They can occur at any time of the year, during any season, and in any type of weather.
1.10. Resources for More Information
Several online resources offer further information about tsunamis, including:
- The COMET Program’s Tsunami Distance Learning Course
- National Weather Service’s JetStream Online Weather School
- National Weather Service’s Tsunami Safety website
- International Tsunami Information Center
- Global Historical Tsunami Database
- The TsunamiZone
2. Unveiling the Causes of Tsunamis
2.1. The Primary Trigger of Tsunamis
A tsunami is initiated by a significant and sudden disruption of the ocean’s equilibrium. While large earthquakes occurring beneath or near the ocean floor are the most prevalent cause, tsunamis can also be triggered by landslides, volcanic eruptions, specific weather conditions, and even near-Earth objects such as asteroids or comets. A review of the Global Historical Tsunami Database reveals that the majority (88%) of tsunamis are caused by earthquakes or earthquake-induced landslides.
2.2. How Earthquakes Generate Tsunamis
Earthquakes release the energy needed to generate tsunamis through abrupt movements in the water column. The key earthquake characteristics that contribute to tsunami formation are location, magnitude, and depth. Most tsunamis are caused by earthquakes with a magnitude exceeding 7.0. These typically occur under or very near the ocean, usually at or near subduction zones where oceanic and continental plates converge, and at depths of less than 100 kilometers (62 miles) below the Earth’s surface. Generally, an earthquake must exceed magnitude 8.0 to generate a dangerous distant tsunami.
For an earthquake to trigger a tsunami, it must be sufficiently large and close to the ocean floor to cause vertical displacement. When the ocean floor rises or falls, the water above it follows suit. As the water seeks to regain its balance, the tsunami propagates outward in all directions. The extent of the ocean floor’s movement, the area over which this movement occurs, reflected in the earthquake’s duration, and the water’s depth at the source are all significant factors in determining the size of the resulting tsunami. Earthquakes can also induce landslides, which can subsequently generate tsunamis.
An earthquake causes vertical displacement of the ocean floor, generating a tsunami. Alt text: Diagram illustrating how an underwater earthquake causes a tsunami with radiating waves.
Examples of earthquake-generated tsunamis:
- March 11, 2011, Honshu Island, Japan (video) – A magnitude 9.1 earthquake generated a tsunami that caused immense local devastation and was observed throughout the Pacific. In Japan, the earthquake and tsunami displaced over 500,000 people, caused approximately $236 billion (2016 dollars) in damage, and resulted in a nuclear accident. The tsunami was responsible for most of the 18,457 deaths. Along the Japanese coast, tsunami waves reached heights of 128 feet and traveled nearly five miles inland. Outside Japan, minimal loss of life occurred due to warnings and evacuations, but the United States experienced over $91 million in damage and one fatality. This event remains the most expensive natural disaster in history.
- December 26, 2004, Northern Sumatra, Indonesia (animation) – A magnitude 9.1 earthquake generated the deadliest tsunami ever recorded. The tsunami’s impact was observed in 15 countries in Southeastern and Southern Asia, as well as Eastern and Southern Africa. The event resulted in approximately 230,000 deaths, displacement of 1.7 million people, and economic losses of roughly $13 billion (2016 dollars). On the northern coast of Sumatra, waves reached heights of up to 167 feet and traveled as far as three miles inland. The extent of the losses can be partly attributed to the lack of an official tsunami warning system in the Indian Ocean at the time and limited knowledge about tsunamis.
- March 27, 1964, Prince William Sound, Alaska (animation) – A magnitude 9.2 earthquake, the largest recorded in U.S. history, generated multiple tsunamis that devastated coastal communities in Alaska. Waves reached heights of up to 167 feet, with a splash mark reaching 220 feet, and caused damage along the west coasts of the United States and Canada, as well as in Hawaii. Tsunami damage was approximately $1 billion (2016 dollars), and the event caused about 124 deaths. NOAA’s National Tsunami Warning Center was established in response to this tsunami.
- April 1, 1946, Aleutian Islands, Alaska (animation) – A magnitude 8.6 earthquake generated a tsunami that caused destruction across the Pacific. The majority of the 167 lives lost and $322 million (2016 dollars) in damage occurred in Hawaii, where waves reached heights of up to 55 feet. NOAA’s Pacific Tsunami Warning Center was established in response to this tsunami.
- November 1, 1755, Lisbon, Portugal (animation) – An estimated magnitude 8.5 earthquake in the Atlantic Ocean generated a tsunami that impacted the coasts of Portugal, Spain, North Africa, and the Caribbean. The earthquake and tsunami resulted in an estimated 50,000 deaths and widespread destruction.
- January 26, 1700, Cascadia Subduction Zone (animation) – An estimated magnitude 9.0 earthquake generated a tsunami that inundated the coasts of Cascadia, encompassing northern California, Oregon, Washington, and southern British Columbia, as well as coastal villages in Japan on the other side of the Pacific Ocean. Today, the Cascadia Subduction Zone is considered one of the largest U.S. tsunami threats.
Source: Global Historical Tsunami Database
For more information about earthquakes, visit the U.S. Geological Survey’s Earthquake Hazards Program.
2.2.1. Earthquake Types That Generate Tsunamis
Most earthquakes that generate tsunamis occur on thrust or reverse faults, typically found in subduction zones where tectonic plates converge. However, 10-15 percent of damaging tsunamis are caused by strike-slip earthquakes, characterized by horizontal movement of the Earth. These tsunamis are often associated with landslides, sloping ocean floors, or seamounts (underwater mountains that act like paddles, pushing water horizontally). Tsunamis caused by strike-slip earthquakes generally affect regions near the source only.
To learn more about faults, visit the visual glossary from the U.S. Geology Survey.
2.2.2. The Largest Earthquake Ever Recorded
The largest earthquake ever recorded was a magnitude 9.5 earthquake off the coast of Southern Chile on May 22, 1960. This earthquake and the second largest earthquake, the 1964 magnitude 9.2 in Prince William Sound, Alaska, both generated devastating tsunamis.
Learn more about past earthquakes from the U.S. Geological Survey.
2.3. Landslides: Another Trigger for Tsunamis
In the context of tsunami generation, “landslide” is a broad term encompassing various types of ground movement, including rock falls, slope failures, debris flows, slumps, ice falls/avalanches, and glacial calving (the breaking off of large pieces of ice from a glacier). Tsunamis can be generated when a landslide enters the water and displaces it from above (subaerial) or when water is displaced in front of and behind an underwater (submarine) landslide. The amount of landslide material displacing the water, its speed, and the depth it reaches all influence tsunami generation. Landslide-generated tsunamis can be larger than seismic tsunamis near their source and can impact coastlines within minutes with little to no warning. However, they typically lose energy quickly and rarely affect distant coastlines.
Most landslides that generate tsunamis are triggered by earthquakes, but other forces, like gravity, wind, and increased precipitation, can cause unstable slopes to fail suddenly. Earthquakes that are not large enough to directly generate a tsunami may still cause a landslide that subsequently generates a tsunami. A landslide-generated tsunami can occur independently or in conjunction with a tsunami directly generated by an earthquake, complicating the warning process and compounding the losses.
Examples of landslide-generated tsunamis:
- July 17, 1998, Papua New Guinea – A moderate magnitude 7.0 earthquake triggered a large underwater landslide that generated a deadly tsunami. Three waves, the highest measuring roughly 49 feet high, struck the coast within 20 minutes of the earthquake, destroying entire villages. Approximately 2,200 lives were lost, and more than 10,000 people were displaced.
- July 10, 1958, Southeast Alaska – A magnitude 7.8 earthquake triggered a number of submarine landslides, rock falls, and ice falls that generated tsunamis, resulting in five fatalities. A rock fall into Lituya Bay caused water to surge over the opposite shore, clearing trees around the bay up to a maximum height of 1,720 feet. This is considered the largest tsunami ever recorded.
- November 18, 1929, Grand Banks, Newfoundland, Canada – A magnitude 7.3 earthquake in the Atlantic Ocean triggered a submarine landslide that generated a tsunami. Waves up to 43 feet high were responsible for 28 deaths and $14 million (2016 dollars) in damage along the coast of Newfoundland.
Source: Global Historical Tsunami Database
To learn more about landslides, visit the U.S. Geological Survey’s Landslide Hazards Program.
2.4. Volcanic Activity and Tsunamis
Tsunamis generated by volcanoes, both above and below water, are less frequent, but several types of volcanic activity can displace enough water to generate destructive tsunamis. These include:
- Pyroclastic flows (flowing mixtures of rock fragments, gas, and ash)
- Submarine explosions relatively near the ocean surface
- Caldera formation (volcanic collapse)
- Landslides (e.g., flank collapse, debris flows)
- Lateral blasts (sideways eruptions)
Like other nonseismic tsunamis, volcanic tsunamis typically lose energy quickly and rarely affect distant coastlines.
Examples of volcano-generated tsunamis:
- August 27, 1883, Indonesia – The volcano Krakatau (Krakatoa) exploded and collapsed, generating one of the largest and most destructive tsunamis ever recorded. Waves reaching 135 feet high destroyed coastal towns and villages along the coasts of Java and Sumatra, killing more than 34,000 people.
- May 21, 1792, Kyushu Island, Japan – At the end of the four-month eruption of the Unzen volcano, a flank collapse generated a tsunami with waves reaching 180 feet high, causing destruction around the Ariake Sea and resulting in more than 14,000 deaths.
- ~1610 BC, Greece – The volcano Santorini (Thera) erupted, generating a tsunami that swept the shores of nearby islands and contributed to the end of the Minoan culture on the nearby island of Crete.
Source: Global Historical Tsunami Database
To learn more about volcanoes, visit the U.S. Geological Survey’s Volcano Hazards Program.
2.5. How Weather Creates Tsunamis
Air pressure disturbances often associated with fast-moving weather systems, like squall lines, can generate tsunamis, known as “meteotsunamis”. These are similar to tsunamis generated by earthquakes. Their development depends on the intensity, direction, and speed of the air pressure disturbance as it travels over the ocean, as well as the ocean’s depth. Meteotsunamis are regional, and certain parts of the world are prone to them due to a combination of factors, such as local weather patterns and the shape and features of the surface of the Earth, both above and below the ocean.
Examples of meteotsunamis:
- June 13, 2013, Northeastern United States (animation) – Tsunami-like waves crashed upon the New Jersey and southern Massachusetts coasts, despite clear skies and calm weather. In Barnegat Inlet, New Jersey, three people were injured when a six-foot wave swept them off a jetty and into the water. After ruling out other sources, scientists determined the waves had been generated by a derecho (a high-speed windstorm associated with a strong band of thunderstorms) that had passed through the area hours earlier.
- June 21, 1978, Vela Luka, Croatia – Without warning and during relatively nice weather, flooding waves inundated the port town of Vela Luka. Scientists ultimately identified the source as atmospheric and deemed it the strongest meteotsunami on record. This event featured 19.5-foot waves, lasted several hours, and caused millions of dollars in damage.
To learn more about meteotsunamis, read “What Is a Meteotsunami?“
2.6. Tsunamis Caused by Near Earth Objects
It is exceedingly rare for a near-Earth object like an asteroid or comet to reach the Earth, and there is still considerable uncertainty regarding their potential to generate tsunamis, as well as the size and reach of such tsunamis if they were to occur. Scientists believe that near-Earth objects could generate a tsunami in two ways. Large objects (approximately 1,000 meters, 0.62 miles, or more in diameter) that survive the passage through Earth’s atmosphere could impact the ocean, displacing water and generating an “impact” tsunami. Smaller objects tend to burn up in the atmosphere, exploding before reaching the Earth’s surface. If this occurs above the ocean, the explosion could release energy into the ocean, generating an “airburst” tsunami.
Example of a near-Earth object tsunami: Evidence suggests that the Chicxulub impact on Mexico’s Yucatán Peninsula, which likely caused a mass extinction at the end of Cretaceous period 65 million years ago, may have generated a tsunami that reached hundreds of miles inland around the Gulf of America.
3. Key Characteristics of a Tsunami
3.1. The Wave Train Nature of Tsunamis
A tsunami consists of a series of waves, not just a single wave. These waves are often referred to as the tsunami wave train. A large tsunami can persist for days in some locations.
3.2. Tsunami Speed: How Fast Do They Travel?
The speed of a tsunami is directly related to the depth of the water through which it travels. Deeper water allows for faster tsunami speeds. In the deep ocean, tsunamis can travel at speeds exceeding 500 mph, comparable to that of a jet plane, and can cross entire oceans in less than a day. As they approach shallower waters near land, their speed decreases to around 20 or 30 mph, similar to the speed of a car.
Tsunami speed can be calculated by taking the square root of the product of the water depth and the acceleration of gravity (32.2 feet per second squared). In 15,000 feet of water, this equates to approximately 475 miles per hour. At these speeds, a tsunami can travel from the Aleutian Islands to Hawaii in about five hours, or from the coast of Portugal to North Carolina in approximately eight and a half hours.
The relationship between tsunami speed and ocean depth is depicted in a graph. Alt text: Graph showing how tsunami speed decreases as ocean depth decreases, from deep ocean to shallow coastal waters.
3.3. Tsunami Size: Wave Height and Inundation
In the deep ocean, a tsunami’s wavelength (the distance between waves) can be hundreds of miles, but the wave height is often minimal, rarely exceeding three feet. Mariners at sea typically do not notice tsunamis as they pass beneath their vessels. As tsunamis enter shallow water near land and decelerate, their wavelengths decrease, their wave heights increase, and currents intensify. While most tsunamis are less than 10 feet high when they strike land, extreme cases can exceed 100 feet near their source. The first wave may not be the largest or the last. A substantial tsunami can inundate low-lying coastal areas by more than a mile inland.
Tsunami impacts can vary significantly depending on offshore and coastal features. Reefs, bays, river entrances, undersea features, and the slope of the beach can all influence the size, appearance, and impact of tsunamis. A relatively small and nondestructive tsunami in one location can become very large and violent just a few miles away.
3.4. Visual Appearance of a Tsunami at the Coast
As a tsunami reaches the coast, it may appear as a rapidly rising flood or a wall of water known as a bore. Its appearance can vary along the coastline. It will not resemble a typical wind wave. Tsunamis rarely form great towering breaking waves. Before the water rushes inland, it may suddenly recede, exposing the ocean floor, reefs, and marine life, resembling an extremely low tide.
3.5. How Long Does a Tsunami Last?
Large tsunamis can persist for days in certain locations, often reaching their peak a few hours after arrival and gradually subsiding afterward. The period between tsunami crests (the tsunami’s period) ranges from approximately five minutes to two hours. Dangerous tsunami currents can persist for days.
3.6. Local vs. Distant Tsunamis
Tsunamis are often categorized as local or distant, depending on the location of the tsunami source and its potential impact area. Local tsunamis originate close to the coast and can arrive in less than one hour, presenting the greatest danger due to limited warning time. Distant tsunamis originate far from the coast, allowing more time for warnings and response.
3.7. Tsunamis vs. Normal Ocean Waves
Most ocean waves are generated by wind, whereas tsunamis have different causes. Tsunamis affect the entire water column, from the surface to the ocean floor, while wind waves only affect the surface.
Waves are further characterized by their wavelength (horizontal distance between wave crests), period (time between wave crests), and speed. These characteristics highlight additional differences between tsunamis and wind waves. Wavelengths are measured in miles for tsunamis and in feet for wind waves. Periods are measured in minutes for tsunamis and in seconds for wind waves. Tsunamis are also faster than wind waves and, while smaller in height in the deep ocean, can grow to much greater heights and cause far more destruction at the coast.
| Feature | Tsunami | Wind Wave |
|---|---|---|
| Source | Earthquakes, landslides, volcanic activity, weather, near-Earth objects | Winds blowing across the ocean surface |
| Energy Location | Entire water column | Ocean surface |
| Wavelength | 60-300 miles | 300-600 feet |
| Wave Period | 5 minutes – 2 hours | 5-20 seconds |
| Wave Speed | 500-600 mph (deep water), 20-30 mph (near shore) | 5-60 mph |
4. Tsunami Detection and Forecasting Technologies
4.1. Responsibilities of Tsunami Warning Centers
The National Weather Service (NWS) operates two Tsunami Warning Centers staffed around the clock. Their primary mission is to protect life and property from tsunamis. They monitor observational networks, analyze earthquakes, evaluate water-level data, issue tsunami messages, conduct public outreach, and collaborate with the National Tsunami Hazard Mitigation Program and other organizations to continuously improve their operations.
4.2. How Are Tsunamis Detected?
Tsunami Warning Centers rely on a global observation system, including seismic and water-level networks, to determine when and where to issue tsunami messages. These networks are critical for providing timely and accurate information:
- Seismic Networks: These networks provide data on earthquake location, depth, magnitude, and other characteristics. Warning centers analyze this data to assess if an earthquake could have generated a tsunami and if a tsunami message is necessary.
- Water-Level Networks: If an earthquake meets certain criteria, warning centers examine water-level data for changes that could indicate a tsunami’s existence and size. Primary data sources include the Deep-ocean Assessment and Reporting of Tsunami (DART) systems and an extensive array of coastal water-level stations.
4.3. The Role of DART Systems
DART (Deep-ocean Assessment and Reporting of Tsunami) systems were developed by NOAA for early detection, measurement, and real-time reporting of tsunamis in the open ocean. The NWS’s National Data Buoy Center operates and maintains the U.S. network of DART systems, which is part of a larger international network. The U.S. network consists of 39 systems (as of 2016) strategically located throughout the Pacific and Atlantic Oceans, the Gulf of America, and the Caribbean Sea.
Each system includes a bottom pressure recorder (BPR) anchored on the ocean floor and a companion surface buoy. When a tsunami passes over a BPR, it detects and records changes in water pressure. This information is transmitted acoustically to the surface buoy, which then relays it via satellite to the warning centers for incorporation into tsunami forecast models.
See how a DART system works (video).
Diagram of a Deep-ocean Assessment and Reporting of Tsunamis (DART) buoy system. Alt text: Illustration showing how a DART buoy detects pressure changes caused by a tsunami on the ocean floor and transmits data via satellite.
4.4. Coastal Water-Level Stations
Coastal water-level stations collect vital data on ocean height at specific coastal locations. Their primary purpose is to monitor tides for navigation, so they are located on the coast, generally on piers in harbors. Data from these stations is relayed via satellite to the warning centers, confirming tsunami arrival time and height and being incorporated into forecast models. These stations are owned and operated by various national and international organizations. In the United States, most of the tsunami-capable coastal water-level stations are operated and maintained by NOAA’s Center for Operational Oceanographic Products and Services as part of the National Water Level Observation Network.
4.5. The Process of Tsunami Forecasting
In most cases, an earthquake is the first sign of a potential tsunami. Because seismic waves travel about 100 times faster than tsunamis, earthquake information is available before any tsunami data. Three key earthquake parameters – location, depth, and magnitude – help Tsunami Warning Centers determine if a tsunami is possible. This preliminary seismic data helps determine if a tsunami message should be issued and at what alert level.
Once a message is issued, warning centers conduct additional seismic analysis and run tsunami forecast models using available seismic and water-level data. These numerical models use real-time data and pre-established scenarios to simulate tsunami movement across the ocean and estimate coastal impacts, including wave height and arrival times, flooding location and extent, and event duration. The forecasts, combined with historical tsunami data and additional seismic analysis, help warning centers decide if they should issue an updated or cancellation message.
Forecasting nonseismic tsunamis, such as those caused by landslides, volcanoes, and weather, is more difficult because they can arrive with little to no warning. Even if detected by a DART system or coastal water-level station, there may not be time for a detailed forecast. In the case of meteotsunamis, NWS Weather Forecast Offices, with decision support from the warning centers, can notify the public of potential coastal threats based on weather conditions and observed water-level measurements.
5. Understanding Tsunami Alert Messages
5.1. The Purpose of Tsunami Messages
Tsunami messages are issued by Tsunami Warning Centers to inform emergency managers, local officials, the public, and other partners about the potential for a tsunami following a possible tsunami-generating event. For the United States, Canada, and the British Virgin Islands, these messages include alerts. There are four levels of tsunami alerts: warning, advisory, watch, and information statement.
Initial tsunami messages include the alert level(s), preliminary earthquake information, and a threat assessment. If a tsunami is suspected, the message may also include wave arrival times, recommended safety actions, and potential impacts. Subsequent messages, including updates and cancellations, are based on additional seismic analysis and tsunami forecast model results, featuring more refined, detailed, and targeted information.
See examples of tsunami messages.
5.1.1. Tsunami Warning: A Call to Action
A tsunami warning is issued when a tsunami with the potential to generate widespread inundation is imminent, expected, or occurring. Warnings alert the public to the possibility of dangerous coastal flooding accompanied by powerful currents that can persist for several hours after the initial arrival. These warnings prompt emergency management officials to take action for the entire tsunami hazard zone, including evacuating low-lying coastal areas and repositioning ships to deep waters, when time permits. Warnings can be updated, geographically adjusted, downgraded, or canceled based on updated information and analysis.
5.1.2. Tsunami Advisory: Exercise Caution
A tsunami advisory is issued when a tsunami with the potential
