Tsunamis, also called seismic sea waves, are a force of nature with devastating potential. How Far Can Tsunamis Travel across the ocean? This question is critical for understanding the risks and preparing for these natural disasters. travels.edu.vn is committed to providing you with the essential knowledge for tsunami awareness and safety. This comprehensive guide explores the causes, characteristics, detection, and safety measures associated with tsunamis, ensuring you’re well-informed and prepared. Explore distant tsunami events and tsunami travel times.
1. General Tsunami Information
Understanding the basics of tsunamis is essential for anyone living in or visiting coastal areas. This section answers common questions about these powerful waves.
1.1 What is a Tsunami?
A tsunami is a series of extremely long waves caused by a large and sudden displacement of the ocean. The distance between the crests of these waves can range from tens to hundreds of miles. Tsunamis radiate outward in all directions from the point of origin and can move across entire ocean basins. When they reach the coast, they can cause dangerous coastal flooding and powerful currents that can last for several hours or even days. Tsunami preparedness can save lives.
1.2 What is the Origin of the Word “Tsunami”?
The word “tsunami” comes from the Japanese characters for harbor (“tsu”) and wave (“nami”). This reflects the fact that tsunamis often cause significant damage in coastal harbors.
1.3 Is a Tsunami the Same as a Seismic Sea Wave or a Tidal Wave?
A tsunami is a seismic sea wave if it is generated by an earthquake (“seismic” means relating to an earthquake). However, tsunamis can also be generated by nonseismic disturbances, such as landslides, volcanic eruptions, and even meteor impacts. The term “tsunami” has been internationally adopted to refer to waves caused by any large and sudden displacement of the ocean.
Tsunamis are not related to tides, which are caused by the gravitational pull of the Sun and Moon on Earth’s oceans. Therefore, it is incorrect to call a tsunami a tidal wave.
1.4 Can Tsunamis Be Predicted?
While scientists cannot predict when and where the next earthquake or other tsunami-generating event will occur, Tsunami Warning Centers can issue tsunami messages when they think a tsunami is possible. These centers monitor seismic activity and use tsunami forecast models to predict wave height, arrival times, and the extent of flooding.
Once a tsunami is detected, warning centers use these models to forecast wave height and arrival times, location and amount of flooding, and how long the tsunami will last. In some cases, when the source of a tsunami is close to a coast, there may not be time for the warning centers to issue a detailed forecast for all at-risk coastal areas. Therefore, people should recognize natural warnings and be prepared to respond to them.
1.5 How Often Do Tsunamis Happen?
According to the Global Historical Tsunami Database, tsunamis that cause damage or deaths near their source occur approximately twice per year. Tsunamis that cause damage or deaths on distant shores (more than 1,000 kilometers, 620 miles, away) occur about twice per decade.
1.6 Where Do Tsunamis Happen?
Tsunamis can be generated in all of the world’s oceans, inland seas, and in any large body of water. They have caused damage and deaths in coastal areas all around the world. However, certain areas are particularly prone to tsunamis due to their proximity to tsunami sources, the depth and shape of the ocean floor near the coast (bathymetry), and coastal elevation and features (topography).
Of the 754 confirmed events in the Global Historical Tsunami Database between 1900 and 2015, about 78% occurred in the Pacific Ocean (around the geologically active “Ring of Fire”), 8% in the Atlantic Ocean and Caribbean Sea, 6% in the Mediterranean Sea, 5% in the Indian Ocean, and 1% in other seas.
Since 1900, the highest percentage of tsunamis was generated off Japan (21%) followed by Russia (8%) and Indonesia (8%). Most tsunamis are small and nondestructive or only affect coasts near their source, but some tsunamis can cause damage and deaths on distant shores (more than 1,000 kilometers, 620 miles, away). The most significant distant tsunamis since 1900 originated off Alaska, Chile, Japan, Indonesia, Pakistan, and Russia.
To see where tsunamis have happened and learn more about them, visit the Natural Hazards Interactive Map.
1.7 Where Can Tsunamis Happen in the United States?
An assessment of the tsunami hazard in the United States shows that a tsunami can strike any U.S. coast, but the hazard level varies. These hazard levels are based largely on the historical record (through 2014), geological evidence, and location relative to tsunami sources, all of which provide clues to what might happen in the future.
| Region | Hazard Level |
|---|---|
| U.S. West Coast | High to Very High |
| Alaska (Southern Coast) | High to Very High |
| Alaska Arctic Coast (includes Western Coast) | 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 |
While distant tsunamis pose a threat to all U.S. coasts, the hazard is greatest for coastlines near subduction zones, where large earthquakes and associated landslides can produce damaging waves that threaten nearby and distant coasts, like those around the Pacific and Caribbean. The U.S. East and Gulf Coasts are not near subduction zones, and earthquakes are not as large or as frequent as in other regions. The most likely sources of tsunamis on these coasts are underwater landslides and meteotsunamis.
1.7.1 What is the Tsunami Hazard Level for Anchorage and the Upper Cook Inlet in Alaska?
The tsunami hazard level for Anchorage and the upper Cook Inlet is very low when compared to the Southern Coast of Alaska. When tsunamis enter the upper Cook Inlet from the Gulf of Alaska or the lower Cook Inlet, they are weakened by the relatively shallow water of the upper Cook Inlet to a point that they are no longer dangerous.
1.8 What Are Some of the Most Damaging Tsunamis to Affect the United States?
According to the Global Historical Tsunami Database, as of January 2018, 30 reported tsunamis that caused at least one death or $1 million in damage (2017 dollars) have affected U.S. states and territories.
| 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 Islands | 1849 | — |
| Puerto Rico/U.S. Virgin Islands | 1867, 1918 | — |
* See question below: “What is the difference between a local and a distant tsunami?” To learn more, see the Historic Tsunami Calendar.
1.9 When Do Tsunamis Happen?
There is no season for tsunamis. A tsunami can happen any time, any season, and during any weather.
1.10 Where Can I Learn More About Tsunamis?
There are a number of online resources that can provide more information about tsunamis. Key resources include the following:
- The COMET Program’s Tsunami Distance Learning Course (six independent modules)
- National Weather Service’s JetStream Online Weather School (tsunami module)
- National Weather Service’s Tsunami Safety website
- International Tsunami Information Center
- Global Historical Tsunami Database
- The TsunamiZone
2. Causes of a Tsunami
Understanding what causes a tsunami can help in assessing the potential risk and preparing for these events.
2.1 What Causes a Tsunami?
A tsunami is caused by a large and sudden displacement of the ocean. Large earthquakes below or near the ocean floor are the most common cause, but landslides, volcanic activity, certain types of weather, and near earth objects (e.g., asteroids, comets) can also cause tsunamis. Most of the tsunamis (88%) in the Global Historical Tsunami Database were generated by earthquakes or landslides caused by earthquakes.
2.2 How Do Earthquakes Generate Tsunamis?
Earthquakes provide the energy to generate tsunamis through sudden movements to the water column. Key earthquake characteristics that contribute to tsunami generation are location, magnitude, and depth. Most tsunamis are generated by earthquakes with magnitudes over 7.0 that occur under or very near the ocean (usually at or near subduction zones, where oceanic and continental plates collide) and 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.
An earthquake must be big enough and close enough to the ocean floor to cause the vertical movement of the ocean floor that typically sets a tsunami in motion. As the ocean floor rises or drops, so too does the water above it. As the water moves up and down, seeking to regain its balance, the tsunami radiates out in all directions. The amount of movement of the ocean floor, the size of the area over which it occurs (which may be reflected in how long the earthquake lasts), and the depth of the water at its source are all important factors in the size of a resulting tsunami. Earthquakes can also cause landslides that generate tsunamis.
Examples of earthquake-generated tsunamis:
- March 11, 2011 Honshu Island, Japan (video)—A magnitude 9.1 earthquake generated a tsunami that caused tremendous devastation locally and was observed all over the Pacific. In Japan, the earthquake and tsunami displaced more than 500,000 people, caused approximately $236 billion (2016 dollars) in damage and resulted in a nuclear accident. Most of the 18,457 deaths were due to the tsunami. Along the coast of Japan, tsunamis reached 128 feet high and almost five miles inland. Outside Japan there was very little loss of life due to warnings and evacuations, but in the United States there was more than $91 million in damage and one death. This event is the most expensive natural disaster in history.
Alt text: Devastation caused by the Honshu Island tsunami, illustrating the impact of distant tsunami events.
- December 26, 2004 Northern Sumatra, Indonesia (animation)—A magnitude 9.1 earthquake generated the deadliest tsunami in history. The tsunami was responsible for the majority of the impacts, which were observed in 15 countries in Southeastern and Southern Asia and Eastern and Southern Africa. Impacts included approximately 230,000 deaths, the displacement of 1.7 million people, and roughly $13 billion (2016 dollars) in economic losses. On the northern coast of Sumatra, waves reached up to 167 feet high and traveled as far as three miles inland. The extent of the losses can be attributed in part to the lack of an official tsunami warning system in the Indian Ocean at the time and limited knowledge about tsunamis.
Alt text: Animation depicting the devastating reach of the Sumatra tsunami, emphasizing tsunami travel times across the Indian Ocean.
- March 27, 1964 Prince William Sound, Alaska (animation)—A magnitude 9.2 earthquake (the largest recorded in U.S. history) generated a number of tsunamis that devastated coastal communities in Alaska with waves as high as 167 feet (and a 220 foot splash mark) and caused damage along the west coasts of the United States and Canada and in Hawaii. Tsunami damage was approximately $1 billion (2016 dollars). About 124 deaths were caused by the tsunami. 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 was destructive across the Pacific. Most of the 167 lives lost and $322 million (2016 dollars) in damage were in Hawaii where waves reached as high as 55 feet. NOAA’s Pacific Tsunami Warning Center was established in response to this tsunami.
- November 1, 1755 Lisbon, Portugal (animation)—A magnitude 8.5 (estimated) earthquake in the Atlantic Ocean generated a tsunami that affected the coasts of Portugal, Spain, North Africa, and the Caribbean. The earthquake and tsunami killed an estimated 50,000 people and caused widespread destruction.
- January 26, 1700 Cascadia Subduction Zone (animation)—A magnitude 9.0 (estimated) earthquake generated a tsunami that inundated the coasts of Cascadia (a region that includes northern California, Oregon, Washington, and southern British Columbia) as well as coastal villages on the other side of the Pacific Ocean in Japan. Today, the Cascadia Subduction Zone is considered one of the largest U.S. tsunami threats.
Source: Global Historical Tsunami Database
To learn more about earthquakes, visit the U.S. Geological Survey’s Earthquake Hazards Program.
2.2.1 What Types of Earthquakes Generate Tsunamis?
Most of the earthquakes that generate tsunamis occur on thrust or reverse faults. These earthquakes originate mainly where tectonic plates move toward each other in subduction zones. However, 10-15 percent of damaging tsunamis are generated by strike-slip earthquakes, where the movement of the earth is horizontal. These tsunamis are likely generated by associated landslides, movement of a sloping ocean floor, or the presence of seamounts, which are underwater mountains (that can act like paddles and push the water horizontally). Tsunamis generated by strike-slip earthquakes normally affect regions near the source only.
To learn more about faults, visit the visual glossary from the U.S. Geology Survey.
2.2.2 What Was 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 How Do Landslides Generate Tsunamis?
Relating to tsunami generation, “landslide” is a general term that incorporates a number of 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 ahead of and behind an underwater (submarine) landslide. Tsunami generation depends on the amount of landslide material that displaces the water, the speed it is moving, and the depth it moves to. Landslide-generated tsunamis may be larger than seismic tsunamis near their source and can impact coastlines within minutes with little to no warning, but they usually lose energy quickly and rarely affect distant coastlines.
Most landslides that generate tsunamis are caused by earthquakes, but other forces (like gravity, wind, and increased precipitation) can cause overly steep and otherwise unstable slopes to suddenly fail. Earthquakes that are not large enough to directly generate a tsunami may be large enough to cause a landslide that in turn can generate a tsunami. A landslide-generated tsunami may occur independently or along with a tsunami directly generated by an earthquake, which can complicate the warning process and compound the losses.
Examples of landslide-generated tsunamis:
- July 17, 1998 Papua New Guinea—A moderately sized 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 that killed five people. A rock fall into Lituya Bay sent water surging over the opposite shore, clearing trees around the bay up to a maximum height of 1,720 feet. It is considered the largest tsunami ever recorded.
Alt text: Aerial view of Lituya Bay, illustrating the extreme height reached by the tsunami caused by a landslide.
- 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 How Do Volcanoes Generate Tsunamis?
Tsunamis generated by volcanoes, both above and below water, are infrequent, 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, such as those generated by landslides, volcanic tsunamis usually 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 and killed 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 that caused destruction around the Ariake Sea and 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 Does Weather Generate Tsunamis?
Air pressure disturbances often associated with fast moving weather systems, like squall lines, can generate tsunamis. These “meteotsunamis” 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 Can Near Earth Objects Generate Tsunamis?
It is very rare for a near earth object like an asteroid or comet to reach the earth, and there is still a lot of uncertainty about their potential to generate tsunamis and the size and reach of those tsunamis if they do. Scientists believe there are two ways near earth objects could generate a tsunami. Large objects (approximately 1,000 meters, 0.62 mile, or more in diameter) that make it through Earth’s atmosphere without burning up could hit the ocean, displacing water and generating an “impact” tsunami. Smaller objects tend to burn up in the atmosphere, exploding before they reach the Earth’s surface. If this happens above the ocean, the explosion could release energy into the ocean and generate 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 the Cretaceous period 65 million years ago, may have generated a tsunami that reached hundreds of miles inland around the Gulf of America.
3. Tsunami Characteristics
Understanding the characteristics of a tsunami, such as its speed, size, and behavior, is vital for effective disaster preparedness and mitigation.
3.1 How Many Waves Are There in a Tsunami?
A tsunami is a series of waves, not just one. These waves are often referred to as the tsunami wave train. A large tsunami may continue for days in some locations.
3.2 How Fast Does a Tsunami Travel?
The speed of a tsunami depends on the depth of the water it is traveling through. The deeper the water, the faster the tsunami. In the deep ocean, tsunamis can move as fast as a jet plane, over 500 mph, and can cross entire oceans in less than a day. As the waves enter shallow water near land, they slow to the speed of a car, approximately 20 or 30 mph.
Tsunami speed can be computed 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 works out to about 475 miles per hour. At rates like this, a tsunami will travel from the Aleutian Islands to Hawaii in about five hours; or from the Portugal coast to North Carolina in eight and a half hours. Tsunami travel times can vary based on ocean depth.
3.3 How Big is a Tsunami?
In the deep ocean, the wavelength of a tsunami (the distance between waves) may be hundreds of miles, but its waves may be barely noticeable and are rarely more than three feet high. Mariners at sea will not normally notice tsunamis as they pass beneath their hulls. As the waves enter shallow water near land and slow down, their wavelengths decrease, they grow in height, and currents intensify. When they strike land, most tsunamis are less than 10 feet high, but in extreme cases, can exceed 100 feet when they strike near their source. The first wave may not be the last or the largest. A large tsunami can flood low-lying coastal areas more than a mile inland.
Not all tsunamis act the same, and an individual tsunami may affect coasts differently due to offshore and coastal features. Reefs, bays, entrances to rivers, undersea features, and the slope of the beach can all influence the size, appearance, and impact of tsunamis when they strike the coast. A small nondestructive tsunami in one place may be very large and violent a few miles away.
3.4 What Does a Tsunami Look Like When It Reaches the Coast?
When a tsunami reaches the coast, it may look like a fast-rising flood, or a wall of water (bore). Its appearance may differ at different points along a coast. It will not look like a normal wind wave. Tsunamis rarely become great towering breaking waves. Sometimes, before the water rushes on land, it will suddenly recede, showing the ocean floor, reefs, and fish like a very low, low tide.
3.5 How Long Does a Tsunami Last?
Large tsunamis may continue for days in some locations, reaching their peak often a couple of hours after arrival and gradually tapering off after that. The time between tsunami crests (the tsunami’s period) ranges from approximately five minutes to two hours. Dangerous tsunami currents can last for days.
3.6 What is the Difference Between a Local and a Distant Tsunami?
Tsunamis are often referred to as local or distant. The type of tsunami depends on the location of the source of the tsunami and where it may strike land. The source of a local tsunami is close to the coast and may arrive in less than one hour. The danger is greatest for local tsunamis because warning time is limited. A distant tsunami is generated far away from a coast, so there is more time to issue and respond to warnings.
3.7 How Are Tsunamis Different From Normal Ocean Waves?
Most ocean waves are generated by wind. Tsunamis are not the same as wind waves. First of all, they have different sources. In addition, tsunamis move through the entire water column, from the ocean surface to the ocean floor, while wind waves only affect the ocean surface.
Waves can also be described based on 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 although they may be smaller in height in the deep ocean, tsunamis can grow to much greater heights and cause much more destruction than wind waves at the coast.
| Feature | Tsunami | Wind Wave |
|---|---|---|
| Source | Earthquakes, landslides, volcanic activity, certain types of weather, near earth objects | Winds that blow across the surface of the ocean |
| Location of energy | Entire water column, from the ocean surface to the ocean floor | Ocean surface |
| Wavelength | 60-300 miles | 300-600 feet |
| Wave Period | 5 minutes – 2 hours | 5-20 seconds |
| Wave Speed | 500-600 miles per hour (in deep water) 20-30 miles per hour (near shore) | 5-60 miles per hour |
4. Tsunami Detection and Forecasting
Early detection and accurate forecasting are crucial for minimizing the impact of tsunamis.
4.1 What Are the Responsibilities of the Tsunami Warning Centers?
The NWS operates two Tsunami Warning Centers, which are staffed 24 hours a day, 7 days a week. The main mission of the warning centers is to help protect life and property from tsunamis. To do this, they monitor observational networks, analyze earthquakes, evaluate water-level information, issue tsunami messages, conduct public outreach, and coordinate with the National Tsunami Hazard Mitigation Program and government, academic, and international organizations to continually improve their operations.
4.2 How Are Tsunamis Detected?
The Tsunami Warning Centers depend on an observation system that includes seismic and water-level networks from around the world to help them determine when and where to issue tsunami messages. These networks are critical to the warning centers’ ability to provide timely and accurate messages:
- Seismic Networks—When an earthquake occurs, seismic networks provide information about an earthquake’s location, depth, magnitude, and other source characteristics. The warning centers analyze this information to determine if the earthquake could have generated a tsunami and if a tsunami message is necessary.
- Water-Level Networks—If an earthquake meets certain criteria, the warning centers turn to water-level information, looking for changes in water-level height that could indicate the existence and size of a tsunami. The primary sources of information about water-level change are a network of Deep-ocean Assessment and Reporting of Tsunami (DART) systems and an extensive array of coastal water-level stations.
4.3 What is a DART System?
DART (Deep-ocean Assessment and Reporting of Tsunami) systems were developed by NOAA for the 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 is composed of 39 systems (as of 2016) strategically located throughout the Pacific and Atlantic Oceans, the Gulf of America, and the Caribbean Sea.
Each system consists of a bottom pressure recorder (BPR) anchored on the ocean floor and a separately moored companion surface buoy. When a tsunami passes over a BPR, the instrument detects and records the changes in the overlying water pressure. An acoustic link transmits information from the BPR to the surface buoy, which then relays it via satellite to the warning centers where the information is incorporated into tsunami forecast models.
See how a DART system works (video).
4.4 What is a Coastal Water-Level Station?
Coastal water-level stations collect important information about the height of the ocean at specific coastal locations. Their primary purpose is to monitor tides for navigation purposes, thus they are located on the coast (in contrast to the DART systems, which are in deep water), generally on piers in harbors. Information from these stations is relayed via satellite to the warning centers where it is used to confirm tsunami arrival time and height and is incorporated into tsunami forecast models. Coastal water-level stations are owned and operated by a number of national and international organizations. In the United States, most of the tsunami-capable coastal water-level stations (i.e., data is available in one-minute intervals) 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 How Are Tsunamis Forecast?
In most cases, the first sign of a potential tsunami is an earthquake. Seismic waves travel about 100 times faster than tsunamis, so information about an earthquake is available before information about any tsunami it may have generated. Three key pieces of information about an earthquake help the Tsunami Warning Centers determine if it was capable of generating a tsunami: location, depth, and magnitude. The warning centers use this preliminary seismic information to decide if they should issue a tsunami message and at what alert level(s).
Once a message is issued, the warning centers conduct additional seismic analysis and run tsunami forecast models using information from the seismic and water-level networks as it becomes available. These numerical models use the real-time information and pre-established scenarios to simulate tsunami movement across the ocean and estimate coastal impacts, including wave height and arrival times, the location and extent of coastal flooding, and event duration. The resulting forecasts, combined with historic tsunami information and additional seismic analysis, help the warning centers decide if they should issue an updated or cancellation message.
It is more difficult to forecast nonseismic tsunamis (like landslide and volcanic tsunamis and meteotsunamis), which can arrive with little to no warning. Even if a nonseismic tsunami is detected by a DART system or coastal water-level station, there may not be time to develop a detailed forecast. In
