tsunamis travel

What causes a tsunami? An ocean scientist explains the physics of these destructive waves

Assistant Professor of Climate Science, Brandeis University

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Sally Warner has received funding from the National Science Foundation and the Office of Naval Research.

Brandeis University provides funding as a member of The Conversation US.

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On Jan. 15, 2022, the Hunga Tonga-Hunga Ha’apai volcano in Tonga erupted, sending a tsunami racing across the Pacific Ocean in all directions.

As word of the eruption spread, government agencies on surrounding islands and in places as far away as New Zealand, Japan and even the U.S. West Coast issued tsunami warnings. Only about 12 hours after the initial eruption, tsunami waves a few feet tall hit California shorelines – more than 5,000 miles away from the eruption.

I’m a physical oceanographer who studies waves and turbulent mixing in the ocean. Tsunamis are one of my favorite topics to teach my students because the physics of how they move through oceans is so simple and elegant.

Waves that are a few feet tall hitting a beach in California might not sound like the destructive waves the term calls to mind, nor what you see in footage of tragic tsunamis from the past . But tsunamis are not normal waves, no matter the size. So how are tsunamis different from other ocean waves? What generates them? How do they travel so fast? And why are they so destructive?

Deep displacement

Most waves are generated by wind as it blows over the ocean’s surface, transferring energy to and displacing the water. This process creates the waves you see at the beach every day.

Tsunamis are created by an entirely different mechanism. When an underwater earthquake, volcanic eruption or landslide displaces a large amount of water, that energy has to go somewhere – so it generates a series of waves. Unlike wind-driven waves where the energy is confined to the upper layer of the ocean, the energy in a series of tsunami waves extends throughout the entire depth of the ocean. Additionally, a lot more water is displaced than in a wind-driven wave.

Imagine the difference in the waves that are created if you were to blow on the surface of a swimming pool compared to the waves that are created when someone jumps in with a big cannonball dive. The cannonball dive displaces a lot more water than blowing on the surface, so it creates a much bigger set of waves.

Earthquakes can easily move huge amounts of water and cause dangerous tsunamis. Same with large undersea landslides. In the case of the Tonga tsunami, the massive explosion of the volcano displaced the water. Some scientists are speculating that the eruption also caused an undersea landslide that contributed to the large amount of displaced water. Future research will help confirm whether this is true or not.

Tsunami waves travel fast

No matter the cause of a tsunami, after the water is displaced, waves propagate outward in all directions – similarly to when a stone is thrown into a serene pond.

Because the energy in tsunami waves reaches all the way to the bottom of the ocean, the depth of the sea floor is the primary factor that determines how fast they move. Calculating the speed of a tsunami is actually quite simple. You just multiply the depth of the ocean – 13,000 feet (4,000 meters) on average – by gravity and take the square root. Doing this, you get an average speed of about 440 miles per hour (700 kilometers per hour). This is much faster than the speed of typical waves, which can range from about 10 to 30 mph (15 to 50 kph).

This equation is what oceanographers use to estimate when a tsunami will reach faraway shores. The tsunami on Jan. 15 hit Santa Cruz, California, 12 hours and 12 minutes after the initial eruption in Tonga. Santa Cruz is 5,280 miles (8,528 kilometers) from Tonga, which means that the tsunami traveled at 433 mph (697 kph) – nearly identical to the speed estimate calculated using the ocean’s average depth.

Destruction on land

Tsunamis are rare compared to ubiquitous wind-driven waves, but they are often much more destructive. The 2004 Indian Ocean tsunami killed 225,000 people. More than 20,000 lost their lives in the 2011 Japan tsunami.

What makes tsunamis so much more destructive than normal waves?

In the open ocean, tsunami waves can be small and may even be undetectable by a boat at the surface. But as the tsunami approaches land, the ocean gets progressively shallower and all the wave energy that extended thousands of feet to the bottom of the deep ocean gets compressed. The displaced water needs to go somewhere. The only place to go is up, so the waves get taller and taller as they approach shore.

When tsunamis get to shore, they often do not crest and break like a typical ocean wave. Instead, they are more like a large wall of water that can inundate land near the coast. It is as if sea level were to suddenly rise by a few feet or more. This can cause flooding and very strong currents that can easily sweep people, cars and buildings away.

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Luckily, tsunamis are rare and not nearly as much of a surprise as they once were. There is now an extensive array of bottom pressure sensors, called DART buoys , that can sense a tsunami wave and allow government agencies to send warnings prior to the arrival of the tsunami.

If you live near a coast – especially on the Pacific Ocean where the vast majority of tsunamis occur – be sure to know your tsunami escape route for getting to higher ground, and listen to tsunami warnings if you receive one.

The eruption of the Hunga Tonga-Hunga Ha’apai volcano severed the main communication cable that connects the people of Tonga to the rest of the world. While the science of tsunamis can be fascinating, these are serious natural disasters. Only a few deaths have been reported so far from Tonga, but many people are missing and the true extent of the damage from the tsunami is still unknown.

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• Hunga Tonga

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What is a tsunami?

A tsunami is a series of waves caused by earthquakes or undersea volcanic eruptions..

On September 29, 2009, a tsunami caused substantial damage and loss of life in American Samoa, Samoa, and Tonga. The tsunami was generated by a large earthquake in the Southern Pacific Ocean.

Did you know?

Tsunamis are giant waves caused by earthquakes or volcanic eruptions under the sea. Out in the depths of the ocean, tsunami waves do not dramatically increase in height. But as the waves travel inland, they build up to higher and higher heights as the depth of the ocean decreases. The speed of tsunami waves depends on ocean depth rather than the distance from the source of the wave. Tsunami waves may travel as fast as jet planes over deep waters, only slowing down when reaching shallow waters. While tsunamis are often referred to as tidal waves, this name is discouraged by oceanographers because tides have little to do with these giant waves.

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Tsunami "Fast Draw" Animation

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National Weather Service TsunamiReady ™

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If you are a disaster survivor, please visit FEMA.gov for up-to-date information on current disaster declarations. If you have questions about your disaster assistance application, you can call (800) 621-3362, visit disasterassistance.gov or use the FEMA mobile app .

A tsunami is a series of enormous ocean waves caused by earthquakes, underwater landslides, volcanic eruptions or asteroids. A tsunami can kill or injure people and damage or destroy buildings and infrastructure as waves come in and go out. Tsunamis can:

Travel 20-30 miles per hour with waves 10-100 feet high.

Cause flooding and disrupt transportation, power, communications and the water supply.

Happen anywhere along U.S. coasts. Coasts that border the Pacific Ocean or Caribbean have the greatest risk.

IF YOU ARE UNDER A TSUNAMI WARNING:

• If caused by an earthquake,  Drop, Cover, then Hold On to protect yourself from the earthquake first.
• Get to high ground as far inland as possible
• Be alert to signs of a tsunami, such as a sudden rise or draining of ocean waters.
• Listen to emergency information and alerts. Always follow the instructions from local emergency managers.
• Evacuate: DO NOT wait! Leave as soon as you see any natural signs of a tsunami or receive an official tsunami warning.
• If you are in a boat, go out to sea.

Prepare NOW

• Learn the signs of a potential tsunami, such as an earthquake, a loud roar from the ocean, or unusual ocean behavior, such as a sudden rise or wall of water or sudden draining of water showing the ocean floor.
• Know and practice community evacuation plans. Some at-risk communities have maps with evacuation zones and routes. Map out your routes from home, work and play. Pick shelters 100 feet or more above sea level, or at least one mile inland.
• Create a family emergency communication plan that has an out-of-state contact. Plan where to meet if you get separated.
• Sign up for your community’s warning system. The Emergency Alert System (EAS) and National Oceanic and Atmospheric Administration (NOAA) Weather Radio also provide emergency alerts.
• Consider earthquake insurance and a flood insurance policy through the National Flood Insurance Program (NFIP). Standard homeowner’s insurance does not cover flood or earthquake damage.

Survive DURING

• If there is an earthquake and you are in a tsunami area, protect yourself from the earthquake first. Drop, Cover, and Hold On. Drop to your hands and knees. Cover your head and neck with your arms. Hold on to any sturdy furniture until the shaking stops. Crawl only if you can reach a better cover, but do not go through an area with more debris.
• When the shaking stops, if there are natural signs or official warnings of a tsunami, move immediately to a safe place as high and as far inland as possible. Listen to the authorities, but do not wait for tsunami warnings and evacuation orders.
• If you are outside of the tsunami hazard zone and receive a warning, stay where you are unless officials tell you otherwise.
• Leave immediately if you are told to do so. Evacuation routes often are marked by a wave with an arrow in the direction of higher ground.
• If you are in the water, then grab onto something that floats, such as a raft or tree trunk.
• If you are in a boat, face the direction of the waves and head out to sea. If you are in a harbor, go inland.

Be Safe AFTER

• Listen to local alerts and authorities for information on areas to avoid and shelter locations.
• Save phone calls for emergencies. Phone systems often are down or busy after a disaster. Use text messages or social media to communicate with family and friends.
• Avoid wading in floodwater, which can contain dangerous debris. Water may be deeper than it appears.
• Be aware of the risk of electrocution. Underground or downed power lines can electrically charge water. Do not touch electrical equipment if it is wet or if you are standing in water.
• Stay away from damaged buildings, roads and bridges.
• If you become injured or sick and need medical attention, contact your healthcare provider and shelter in place, if possible. Call 9-1-1 if you are experiencing a medical emergency.
• Document property damage with photographs. Conduct an inventory and contact your insurance company for assistance.
• Tsunami Safety Graphics
• Tsunami Information Sheet  (PDF)
• Protective Actions Research for Tsunami
• NOAA Tsunami program
• American Red Cross Tsunami Preparedness
• Tsunami Safety (weather.gov)

Last Updated: 03/21/2024

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About Tsunamis

** Get Tsunami Alerts * *

A tsunami is one the most powerful and destructive natural forces. It is a series of waves (not just one) caused by a large and sudden displacement of the ocean. Tsunamis radiate outward in all directions from the disturbance and can move across entire ocean basins. Most tsunamis are caused by large earthquakes below or near the ocean floor, but tsunamis can also be caused by landslides, volcanic activity,  certain types of weather  and near earth objects (e.g., asteroids, comets). Not all earthquakes cause tsunamis.

+ Tsunami Risk

Tsunamis are among the most infrequent of Earth's natural hazards. Each year, there are approximately two tsunamis that cause damage near their source. Tsunamis that cause damage or deaths on distant shores (more than 1,000 kilometers, 620 miles, away) occur about twice per decade.

Tsunamis can strike any U.S. coast , but the risk is greatest for states and territories with Pacific and Caribbean coastlines. Low-lying coastal areas such as beaches, bays, lagoons, harbors and river mouths and areas along rivers and streams that lead to the ocean are the most vulnerable. Tsunamis can wrap around headlands, islands and sand spits, so coasts facing away from the tsunami source may also be at risk.

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.

+ Tsunami Characteristics

Not all tsunamis act the same. And, an individual tsunami may impact coasts differently. A small tsunami in one place may be very large a few miles away.

The speed of a tsunami depends on the depth of the ocean. In the deep ocean, tsunamis are barely noticeable but can move as fast as a jet plane, over 500 mph. As the waves enter shallow water near land, they slow to approximately 20 or 30 mph. That is still faster than a person can run.

As the waves slow down, they can grow in height and currents intensify. Most tsunamis are less than 10 feet high, but in extreme cases, can exceed 100 feet. When a tsunami comes ashore, it will not look like a normal wind wave. It may look like a fast-rising flood or a wall of water. Sometimes, before the water rushes on land, it will drain away suddenly, showing the ocean floor, reefs and fish like a very low, low tide. Tsunamis can travel up rivers and streams that lead to the ocean. A large tsunami can flood low-lying coastal areas more than a mile inland.

The series of waves that flood, drain away and then reflood the land may last for hours. The time between waves ranges from five minutes to two hours. The first wave to reach the shore may not be the largest or the most damaging. It is not possible to predict how long a tsunami will last, how many waves there will be, or how much time there will be between waves.

+ Tsunami Dangers

A tsunami can be very dangerous to coastal life and property. It can produce unusually strong currents, rapidly flood land and cause great destruction. The flow and force of the water and the debris it carries can destroy boats, vehicles, and buildings and other structures; cause injuries; and take lives as the tsunami moves across the land. It only takes six inches of fast-moving water to knock over an adult and two feet of fast-moving water to carry away most vehicles. The water can be just as threatening (if not more so) as it returns to the sea, taking debris and people with it. Flooding and dangerous currents can last for days.

Even small tsunamis can pose a risk. Strong currents can injure and drown swimmers and damage and destroy boats in harbors. And be aware, a tsunami is not surfable. Tsunamis are not like wind waves. They do not have a face, do not curl and break like wind waves and are full of hazardous debris.

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Tsunami Travel Time Maps

NCEI, the World Data Service for Geophysics (including Tsunamis) , and the UNESCO-IOC International Tsunami Information Center , collaborate to provide tsunami travel time maps. Tsunamis are categorized as long waves; therefore, tsunami travel times can be computed with water depth as the only variable ( Murty, T.S., Seismic Sea Waves Tsunamis, Bulletin 198 ). Long waves occur when the distance between wave crests is much greater than the water depth through which the wave is traveling. Wave speed is computed from the square root of the quantity: water depth times the acceleration of gravity. So, tsunami travel times can be computed without any knowledge of the tsunami's height, wavelength, etc.

• Coastal Location Maps
• Tsunami Sources

Coastal Locations Interactive Map

Coastal Locations Interactive Map provides estimated tsunami travel times from any point in the ocean to selected coastal locations.

Launch Coastal Locations Map

Maps do not provide information on the height or the strength of the wave, they only provide record of the arrival times. There are several situations in which the estimated arrival times may not match observed arrival times of the tsunami waves, including but not limited to the following:

• Bathymetry is not accurate in the vicinity of the epicenter.
• Bathymetry is not accurate in the vicinity of the reporting station.
• Epicenter is not well located therefore the origin time is uncertain.
• Epicenter is on land and a pseudo-epicenter off the coast must be selected.
• Nonlinear propagation effects may be important in shallow water.
• Observed travel times do not represent the first wave but instead designate later arrivals.

Tsunami Sources Data Access

Maps generated using Tsunami Travel Time (TTT) software that calculates first-arrival travel times on a grid for a tsunami generated at an earthquake epicenter.

Map contours: 1-hour intervals

• Red: 1-4 hour arrival times
• Yellow: 5-6 hour arrival times
• Green: 7-14 hour arrival times
• Blue: 15-21 hour arrival times

Maps were generated from earthquake epicenters in the NCEI Global Historical Tsunami Database using NCEI 2-Minute Gridded Global Relief Data bathymetry.

NOTE: Maps do not provide the height or the strength of the wave, only the arrival times.

"Event Details," links to runups (observed wave locations). In some cases, runups have observed tsunami arrival times, which can be compared with the map calculated arrival times. Calculated times are usually within 1 hour of the observed travel times, but can differ by more than one hour, particularly when the location is across the Pacific Basin ( why times may be different ).

November 1, 1755 Lisbon, Portugal

A large earthquake, Modified Mercalli Intensity XI, in Lisbon, Portugal, caused damage to the north of Granada, Spain. The earthquake generated a tsunami that affected the coasts of Portugal, Spain, North Africa, and the Caribbean. The tsunami reached Lisbon about 20 minutes after the first destructive shock. It rose to 6 meters at many points along the Portuguese coast and reached 12 meters in some places. It also affected the coast of Morocco where the streets of Safi were flooded. The tsunami reached Antigua about 9.3 hours after the earthquake. Later waves, with estimated runup heights of 7 meters, were observed at Saba, Netherlands, Antilles. The earthquake and tsunami killed between 60,000 and 100,000 people.

Portugal Hazards Tsunami Event Details

October 11, 1918 Puerto Rico

A magnitude 7.3 Mw earthquake on October 11, 1918, in the Mona Passage, west of Puerto Rico, was caused by displacement along four segments of a normal fault, oriented N-S in the Mona Canyon. The earthquake generated a tsunami with runup heights reaching 6 meters, causing extensive damage along the western and northern coasts of Puerto Rico. The earthquake and tsunami caused $29 million damage, 116 people were killed and 100 were reported missing.

Puerto Rico Hazards Tsunami Event Details

November 18, 1929 Grand Banks

On November 18, 1929 a magnitude 7.4 Mw earthquake occurred 250 km south of Newfoundland along the southern edge of the Grand Banks, Canada. The earthquake was felt as far away as New York and Montreal. It triggered a large submarine slump which ruptured 12 transatlantic cables in multiple places and generated a tsunami. The tsunami was recorded along the east coast of Canada and the U.S., as far south as Martinique in the Caribbean, and across the Atlantic Ocean in Portugal. The tsunami caused an estimated $1 million damage and 28 deaths in Newfoundland, Canada.

Grand Banks Hazards Tsunami Event Details

August 27, 1883 Krakatau

The explosion of the Krakatau Volcano in Indonesia on August 27, 1883, generated a 30 meter tsunami in the Sunda Strait which killed 36,000 people. It also caused an atmospheric pressure wave that was recorded on the tide gauges at remote locations including South Georgia Island, Panama, France, England, Alaska, Hawaii, and San Francisco. Due to the shadowing by continents and island groups, a direct tsunami could not have reached most of these locations. Atmospheric gravity waves occurred which may have excited water waves by transferring energy into the ocean. It was recorded with an amplitude of six inches at Sausalito, California.

Krakatau Hazards Tsunami Event Details

December 26, 2004 Sumatra, Indonesia

A magnitude 9.1 Mw earthquake occurred off the west coast of Sumatra, Indonesia, on December 26, 2004. It was the fourth largest earthquake in the world since 1900 and the largest since the 1964 Prince William Sound, Alaska, earthquake. The earthquake generated a tsunami that caused more casualties than any other in recorded history. The tsunami was recorded nearly world-wide on tide gauges in the Indian, Pacific and Atlantic Oceans. In total, more than 227,898 people were killed or missing and 1,126,900 were displaced by the earthquake and subsequent tsunami in 14 countries in South Asia and East Africa.

Sumatra, Indonesia Hazards Tsunami Event Details

February 3, 1923 Kamchatka, Russia

The February 3, 1923 magnitude 8.3 Mw earthquake off the east coast of Kamchatka, Russia, generated an 8-meter tsunami that caused damage in Kamchatka and Hawaii. It was also observed in Japan and California.

Kamchatka, Russia Hazards Tsunami Event Details

December 7, 1944 Kii Peninsula, Japan

The 1944 Pacific-wide tsunami was caused by a magnitude 8.1 Mw earthquake that occurred off the southeast coast of Kii Peninsula, Japan. The earthquake and resulting tsunami caused great destruction and loss of life. About 998 people were killed, 2,135 people were seriously injured, 26,135 homes were totally destroyed, 46,950 homes were partially destroyed, and 3,059 homes were washed away. The tsunami was observed on tide gauges in the Hawaiian and Aleutian Islands.

Peninsula, Japan Hazards Tsunami Event Details

April 1, 1946 Unimak Island, Alaska

Tsunami travel time map for the April 1, 1946 Pacific-wide tsunami caused by an earthquake south of Unimak Island, Alaska.     The April 1, 1946 Pacific-wide tsunami was caused by a magnitude 7.3 Ms earthquake that occurred south of Unimak Island, Alaska. Hawaii experienced the worst damage, with 159 deaths (96 at Hilo) and $26 million in property loss. Total property damage in Alaska was $250,000 while California experienced one death and $10,000 damage from the tsunami. These events led to the development of tsunami travel time charts for the Pacific and the Pacific Tsunami Warning Service.

Unimak Island, Alaska Tsunami Event Details

December 20, 1946 Honshu, Japan

A catastrophic magnitude 8.1 Mw earthquake on December 20, 1946 on the south coast of Honshu, Japan was felt in the central and western parts of the country. The number of homes destroyed directly by the earthquake was 2,598; 1,443 people died. In addition, 1,451 homes were washed away by the ensuing tsunami waves. The tsunami was observed on tide gauges in California, Hawaii and Peru. 

Honshu, Japan Tsunami Event Details

March 4, 1952 Hokkaido, Japan

The magnitude 8.1 Mw earthquake and tsunami on March 4, 1952 off the coast of Hokkaido, Japan, did major damage in Japan. 815 homes were completely destroyed, 1,324 were partially destroyed, 6,395 were slightly damaged, 14 were burned, 91 were washed away, 328 homes and 1,621 non-residential buildings were flooded. Many ships were destroyed, and roads and railway lines were damaged. Twenty-eight people died, 5 people were missing and 287 people were injured in Japan. The tsunami was observed on tide gauges in Hawaii, the west coast of the United States, Alaska, Peru, the Marshall Islands, and Palau.

Hokkaido, Japan Tsunami Event Details

November 4, 1952 Kamchatka, Russia

The magnitude 9.0 Mw earthquake on November 4, 1952 off the east coast of Kamchatka generated a 13-meter wave locally. The waves struck the Hawaiian Islands at 1:00 pm. Property damage from these waves in the Hawaiian Islands was estimated at $800,000 to $1,000,000; however no lives were lost. It also caused damage on the west coast of the United States and was observed on tide gauges throughout the Pacific Basin.

Kamchatka, Russia Tsunami Event Details

March 9, 1957 Aleutian Islands, Alaska

The magnitude 9.1 Mw earthquake on March 9, 1957 south of the Andreanof and Aleutian Islands, generated a tsunami that did severe damage on Adak Island. However, the most damage, approximately $5 million, was in the Hawaiian Islands. There were two indirect fatalities, a reporter and a pilot, and injury to a photographer when their small chartered plane crashed in the ocean near Oahu.

Aleutian Islands, Alaska< Tsunami Event Details

May 22, 1960 Southern Chile  

On May 22, 1960 a Magnitude 9.5 Mw earthquake, the largest earthquake ever instrumentally recorded, occurred in southern Chile. The series of earthquakes that followed ravaged southern Chile. Over a period of days earthquakes ruptured a 1,000 km section of the fault. The number of fatalities associated with both the earthquake and tsunami was estimated to be between 490 and 5,700. Reportedly there were 3,000 injured and initially there were 717 missing in Chile. The main shock generated a tsunami that was not only destructive along the coast of Chile, but was noticed along shorelines throughout the Pacific Ocean area.

Southern Chile Tsunami Event Details

March 28, 1964 Prince William Sound, Alaska

This magnitude 9.2 Mw earthquake and ensuing tsunami caused 125 deaths and $311 million in property loss ($84 million and 106 deaths in Alaska). It was felt over a large area of Alaska and in parts of western Yukon Territory and British Columbia. Its effects were heaviest in south central Alaska. The duration of the shock was estimated at 3 minutes. Vertical displacement occurred over 525,000 sq km. An estimated 20 landslide tsunamis were generated. The tectonic tsunami devastated many towns along the Gulf of Alaska, left serious damage in British Columbia, Hawaii, and along the west coast of the U.S. (15 killed). It was recorded on tide gauges in Cuba and Puerto Rico.

Prince William Sound, Alaska Tsunami Event Details

May 16, 1968 Honshu, Japan

A magnitude 8.2 Mw earthquake on May 16, 1968 off the coast of Honshu Island caused destruction in Japan and generated a tsunami that was observed by tide gauges in Japan and throughout the Pacific Basin. As a result of the earthquake and tsunami, 52 people died and 329 people were injured; 676 homes were completely destroyed and 2,994 homes were partially destroyed; 13 homes burned down and 529 homes were flooded; 97 ships were washed away and 30 were sunk. In addition, roads, bridges and protective dikes were destroyed.

November 29, 1975 Hawaii

On November 29, 1975 a magnitude 7.2 Ms earthquake on the southern coast of the Island of Hawaii generated a locally damaging submarine landslide tsunami that was recorded at tide gauge stations in Alaska, California, Hawaii, Japan, Galapagos Islands, Peru, and Chile. The tsunami caused $1.5 million damage in Hawaii, 2 deaths, and 19 injuries.

1975 Hawaii Tsunami Event Details

One-hour Tsunami Travel Times for Simulated Seismic Zones in the Indian Ocean

1 hour Tsunami travel time map for the Indian Ocean seismic zones; 1 hour Tsunami travel time map for the Makran Coast seismic zone; 1 hour Tsunami travel time map for the Sumatra-Andaman coast seismic zone

These maps were generated by the Pacific Warning Center using Paul Wessel's TTT software. The first image shows one-hour travel times from simulated seismic zones off the Makran and Sumatra-Andaman coasts, Indian Ocean. A 100-kilometer distance around the seismic zones is shown on the map as a dashed line. The other two images show details of the two areas.

Off the coast of central Chile

Tsunami travel time map for a hypothetical earthquake off the coast of central Chile     Travel time map for a hypothetical magnitude 9.2 earthquake off the coast of central Chile at 30oS, 72oW. This scenario was used for the Pacific-wide Tsunami Drill Exercise Pacific Wave that took place May 16-17 2006.

North of the Philippines

Tsunami travel time map for a hypothetical earthquake north of the Philippines     Travel time map for a hypothetical magnitude 8.8 earthquake north of the Philippines at 20oN, 120oE. This scenario was used for the Pacific-wide Tsunami Drill Exercise Pacific Wave '06 that took place 16-17 May 2006.

Tsunami Travel Time Software

Maps were generated using Tsunami Travel Times (TTT) software developed by Paul Wessel, Geoware . TTT software calculates first-arrival travel times on a grid for a tsunami generated at a given earthquake epicenter or coastal location. The technique to compute travel times is an application of Huygen's Principle. It states that all points on a wavefront are point sources for secondary spherical waves. Minimum travel times are computed over the grid starting at the point of interest (e.g. earthquake epicenter). From the starting point, times are computed to all surrounding points. The grid point with minimum time is then taken as the next starting point, and times are computed from there to all surrounding points. The starting point is continually moved to the point with minimum total travel time until all grid points have been evaluated. This technique is explained in Shokin, et al., 1987, Calculations of tsunami travel time charts in the Pacific Ocean, Science of Tsunami Hazards, vol. 5, p. 85-113 .

Maps do not provide information on the height or the strength of the wave, only the arrival times. There are several situations in which the estimated arrival times may not match observed arrival times of the tsunami waves, including but not limited to the following:

• Epicenter is not well located, or the origin time is uncertain.
• Observed travel times do not represent the first wave but instead are later arrivals.

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• Visit a Warning Center

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.

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.

There is no season for tsunamis. A tsunami can happen any time, any season, and during any weather.

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
• Global Historical Tsunami Database
• The TsunamiZone

   2. Causes of 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.

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.
• 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.
• 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.

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. But, 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.

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.

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.

• 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.
• 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.

To learn more about landslides, visit the U.S. Geological Survey’s Landslide Hazards Program .

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.

To learn more about volcanoes, visit the U.S. Geological Survey’s Volcano Hazards Program .

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.

• 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? ”

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 Mexico.

   3. Tsunami Characteristics

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.

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.

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.

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.

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.

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.

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.

   4. Tsunami Detection and Forecasting

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.

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 Mexico, 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).

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.

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 the case of meteotsunamis, NWS Weather Forecast Offices, with decision support from the warning centers, can notify the public of the potential coastal threat given the presence of or potential for certain weather conditions along with observed water-level measurements.

   5. Tsunami Messages

Tsunami messages are issued by the Tsunami Warning Centers to notify emergency managers and other 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 alert level(s), preliminary information about the earthquake, and an evaluation of the threat. If a tsunami is already suspected, the message may also include wave arrival times, recommended life safety actions, and potential impacts. Subsequent messages, both updates and cancellations, are based on additional seismic analysis and results from the tsunami forecast models and may feature more refined, detailed, and targeted information.

See examples of tsunami messages.

A tsunami warning is issued when a tsunami with the potential to generate widespread inundation is imminent, expected, or occurring. Warnings alert the public that dangerous coastal flooding accompanied by powerful currents is possible and may continue for several hours after initial arrival. Warnings alert emergency management officials to take action for the entire tsunami hazard zone. Appropriate actions to be taken by local officials may include the evacuation of low-lying coastal areas and the repositioning of ships to deep waters when there is time to safely do so. Warnings may be updated, adjusted geographically, downgraded, or canceled based on updated information and analysis.

A tsunami advisory is issued when a tsunami with the potential to generate strong currents or waves dangerous to those in or very near the water is imminent, expected, or occurring. The threat may continue for several hours after initial arrival, but significant inundation is not expected for areas under an advisory. Appropriate actions to be taken by local officials may include closing beaches, evacuating harbors and marinas, and the repositioning of ships to deep waters when there is time to safely do so. Advisories may be updated, adjusted geographically, upgraded to a warning, or canceled based on updated information and analysis.

A tsunami watch is issued when a tsunami may later impact the watch area. The watch may be upgraded to a warning or advisory or canceled based on updated information and analysis. Emergency management officials and the public should prepare to take action.

A tsunami information statement is issued when an earthquake or tsunami has occurred of interest to the message recipients. In most cases, information statements are issued to indicate there is no threat of a destructive basin-wide tsunami and to prevent unnecessary evacuations. Information statements for distant events requiring evaluation may be upgraded to a warning, advisory, or watch based on updated information and analysis.

A tsunami threat message is a tsunami message for international partners in the Pacific and Caribbean. The United States does not issue alerts for these partners. The primary purpose of these messages is to help national authorities understand the threat to their coasts so they can determine which alerts to issue for their coastlines, if any. A threat message describes tsunami threats according to the potential hazard and impact to people, structures, and ecosystems on land or in nearshore marine environments. National authorities will determine the appropriate level of alert for each country and may issue additional or more refined information and instructions. A threat message may be updated based on new information, data, and analysis. An information statement may be issued following an earthquake or tsunami of interest to the message recipients if there is little to no threat, but may be upgraded to a tsunami threat message if warranted.

U.S. states and territories in the Pacific and Caribbean (American Samoa, Guam, Hawaii, Northern Mariana Islands, Puerto Rico, U.S. Virgin Islands) should refer *ONLY to the Pacific Tsunami Warning Center messages for their regions. International tsunami threat messages from the Pacific Tsunami Warning Center may mention U.S. states and territories, but they do not reflect the additional modeling and analysis conducted to fine tune forecasts and set alert levels for the United States and should not to be relied on by U.S. states and territories

The Tsunami Warning Centers prepare and issue tsunami messages for their respective designated service areas. Primary recipients of these messages include NWS Weather Forecast Offices, state emergency operations centers, the U.S. Coast Guard, the U.S. military, and designated international authorities. Each of these recipients is responsible for forwarding the message to its own constituents.

The Tsunami Warning Centers issue a cancellation after they determine that a destructive tsunami will not affect an area under a warning, advisory, or watch or that a tsunami has diminished to a level where additional damage is not expected. However, the cancellation of a message does not mean the area is safe. The final decision that an area is safe is up to local and state emergency management officials.

• The National Tsunami Warning Center in Palmer, Alaska, serves the continental United States, Alaska and Canada.
• The Pacific Tsunami Warning Center in Honolulu, Hawaii, directly serves the Hawaiian Islands, the U.S. Pacific and Caribbean territories, and the British Virgin Islands and is the primary international forecast center for the warning systems of the Intergovernmental Oceanographic Commission of the United Nations Educational, Scientific, and Cultural Organization in the Pacific and the Caribbean and Adjacent Regions.

The Tsunami Warning Centers base their initial tsunami messages on the preliminary earthquake information (location, depth, and magnitude) received from seismic networks since that is all the information available within the first few minutes after an earthquake. They use preset criteria ( Atlantic , Pacific ) to decide when to issue a tsunami message and what alert(s) to include. Subsequent messages and alerts are based on impact estimation resulting from additional seismic analysis, water-level measurements, tsunami forecast model results, and historical tsunami information.

Tsunami warnings are typically issued following coastal earthquakes magnitude 6.5 or greater for U.S. and Canadian Atlantic and Gulf coasts, and magnitude 7.1 or greater for all coasts along the Pacific Ocean and Caribbean Sea. Tsunami height also affects alert selection. In general, the warning centers issue a tsunami warning if the forecast or observed tsunami height exceeds 1.0 meter (3.3 feet) or the impact is unknown, and a tsunami advisory if the forecast height is between 0.3 and 1.0 meter (0.98 and 3.3 feet) or the observed height is between 0.5 and 1.0 meter (1.6 and 3.3 feet).

The time it takes for a Tsunami Warning Center to issue a tsunami message depends on the seismic network density and distribution near the earthquake’s point of origin. In regions of high seismic network density, the warning centers can issue messages within five minutes. In areas of lower seismic network density, response time increases to 10-15 minutes.

In the United States, tsunami messages are broadcast through local radio and television, marine radio, wireless emergency alerts , NOAA Weather Radio , NOAA websites (like Tsunami.gov ), and social media accounts (Facebook and Twitter). They may also come through outdoor sirens, local officials, emails and text message alerts from state and local opt-in systems, and telephone notifications. In some cases, like wireless emergency alerts that only broadcast the first tsunami warning, broadcast of a message may depend on the level of alert and/or local circumstances. There may not always be enough time for an official warning, so it is important to understand natural warnings.

Learn how to get alerts from the Tsunami Warning Centers .

   6. Tsunami Safety

A tsunami is one the most powerful and destructive natural forces. It can produce unusually strong currents, rapidly flood land, and devastate coastal communities. Low-lying areas such as beaches, bays, lagoons, harbors, river mouths, and areas along rivers and streams leading to the ocean are the most vulnerable.

Most tsunami damage and destruction is caused by flooding, wave impacts, strong currents, erosion, and debris. The water can be just as dangerous as it returns to the sea, taking debris and people with it. In addition to loss of life and mass injuries, other potential impacts include damage to and destruction of homes and businesses, cultural and natural resources, infrastructure, and critical facilities. Flooding and dangerous currents can last for days. Even small tsunamis can pose a threat. Strong currents can injure and drown swimmers and damage and destroy boats in harbors.

Although tsunamis cannot be prevented, there are things you can do before a tsunami that could save your life and the lives of your family and friends. Importantly, find out if your home, school, workplace, or other places you visit often are in a tsunami hazard zone ( U.S. tsunami maps ).

• Educate yourself about tsunami warnings (official and natural) and ensure you have multiple ways to receive official warnings. In the United States, get a NOAA Weather Radio , sign up for text message alerts from your local government, and verify that your mobile devices receive wireless emergency alerts .
• Make an emergency plan that includes plans for family communication and evacuation.
• Map out routes from home, work, and other places you visit often to safe places on high ground or inland (away from the water) and outside the tsunami hazard zone. Your community may already have identified evacuation routes and assembly areas (if they have not, identify a safe place at least 100 feet above sea level or 1 mile inland). Plan to evacuate on foot if you can; roads may be impassable due to damage, closures, or traffic jams.
• Practice walking your routes, even in darkness and bad weather. This will ease evacuation during an emergency.
• Put together a portable disaster supplies kit with items you and your family (including pets) may need in an emergency. Prepare kits for work and cars, too.
• Be a role model. Share your knowledge and plans with others.

If you have children in school in a tsunami hazard zone, find out the school’s plans for evacuating and keeping children safe. If you are visiting the coast, find out about local tsunami safety. Your hotel or campground should have this information.

Visit the NWS Tsunami Safety and International Tsunami Information Center websites to learn more.

• In the United States, an official tsunami warning will be broadcast through local radio and television, marine radio, wireless emergency alerts , NOAA Weather Radio , and NOAA websites (like Tsunami.gov ) and social media accounts (Facebook and Twitter). It may also come through outdoor sirens, local officials, emails and text message alerts from state and local opt-in systems, and telephone notifications.
• There may not always be time to wait for an official tsunami warning. A natural tsunami warning may be your first, best, or only warning that a tsunami is on its way. Natural tsunami warnings include strong or long earthquakes, a loud roar (like a train or an airplane) from the ocean, and unusual ocean behavior. The ocean could look like a fast- rising flood or a wall of water (bore). Or, it could recede suddenly, showing the ocean floor, reefs, and fish like a very low, low tide. If you experience any of these warnings, even just one, a tsunami could be coming.

How you should respond to a tsunami warning depends on where you are and the type of warning you receive (i.e., official or natural). Be prepared to respond immediately to whatever you hear or see first.

• Stay out of the water and away from beaches and waterways.
• Get more information from radio, television, or your mobile device (text or data) and continue to stay informed throughout the event.
• If officials ask you to evacuate, implement your emergency plan and move quickly to your safe place. If you do not have a safe place or cannot reach it, follow evacuation signs to safety or go as high or as far inland (away from the water) as possible (at least 100 feet above sea level or a mile inland).
• In case of an earthquake, protect yourself. Drop, cover, and hold on. Be prepared for aftershocks. Each time the earth shakes, drop, cover, and hold on.
• As soon as you can move safely, implement your emergency plan and move quickly to your safe place. If you do not have a safe place or cannot reach it, follow evacuation signs to safety or go as high or as far inland (away from the water) as possible (at least 100 feet above sea level or a mile inland).
• If there is earthquake damage, avoid fallen power lines and stay away from weakened structures.
• When you are in a safe place, get more information from radio, television, or your mobile device (text or data).

If you are on the beach or near water and feel an earthquake of any size and length, move quickly to high ground or inland (away from the water) as soon as you can move safely. Get more information from radio, television, or your mobile device (text or data).

If you are outside of the tsunami hazard zone and receive a warning, stay where you are unless officials tell you otherwise.

For your safety and others, always follow instructions from local officials and stay out of the tsunami hazard zone until they tell you it is safe. In the United States, to find out if you are in a tsunami hazard zone, visit the Tsunami Maps web page .

For all U.S. states and territories, evacuation requests/instructions are typically issued and coordinated by local emergency management officials. In the unique case of a tsunami warning issued for local Hawaii earthquakes, the Pacific Tsunami Warning Center decides whether evacuations are necessary. When they are, local and state emergency management officials coordinate them and are responsible for determining when it is safe for people to return to an evacuated area.

Most buildings are not designed to withstand tsunami impacts. However, the upper stories of some strong (e.g., reinforced concrete) and tall buildings may be able to provide protection if no other options are available. If you are concerned that you will not be able to reach a safe place in time, ask your local emergency management office or hotel staff about vertical evacuation. Note, this type of evacuation is not recommended in all areas.

If you are on a boat and you get a tsunami warning, your response will depend largely on where you are.

• If you are in a harbor and get a tsunami warning, you should leave your boat and move quickly to a safe place on land (high ground or inland, away from the water).
• If you are at sea and get a tsunami warning, you should move to a safe depth* and stay away from harbors under warning until officials tell you the threat has passed.

* Safe depths vary by region , but the minimum safe depth is 30 fathoms (180 feet). Your harbor master, port captain, the U.S. Coast Guard, and local and state emergency management offices are the best sources for safe depth and other tsunami safety information and regulations for boaters in your area.

• Make sure you have a way to receive tsunami warnings when you are on the water. The U.S. Coast Guard will issue urgent marine information broadcasts on your marine VHF radio’s channel 16. Additional information will be available from NOAA Weather Radio .
• Make a plan and put together a disaster supplies kit to keep on board your boat. Be aware that shore facilities may be damaged, so if you are at sea during a tsunami, you may not be able to return to the harbor you left. Be prepared to remain at sea for a day or more.
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Interactive map of estimated tsunami travel times to coastal locations

The National Ocean and Atmospheric Administration's tsunami travel times map displays the estimated pre-computed tsunami travel times to select coastal locations from any point in the ocean. The tsunami travel times displayed are in hours and were calculated using TTT software. It is important to note that the travel times shown on the map are estimates and should be used general information only. In the event of an actual tsunami, please visit tsunami.gov for real-time tsunami information.

Click here to use the tsunami travel time map

Source: National Oceanic and Atmospheric Administration

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The Largest Tsunami Ever Recorded Only Claimed 2 Lives

If you're lucky, you've never seen a tsunami in person, but you know that this type of massive wave can cause enormous damage. They can be measured, too, which easily leads to the question: What was the largest tsunami ever recorded ?

Below, we'll take a look at the massive tsunami that occurred in Lituya Bay, Alaska, which is recognized as the largest recorded tsunami in history. Beyond that, we'll also look at the runners up, as well as the science of these giant waves.

What Is a Tsunami?

A tsunami is a giant wave (more accurately, a series of giant waves) that is caused by the rapid displacement of water. Usually, this is due to some kind of sudden geological event, such as an earthquake or volcanic eruption.

Tsunamis can also be caused by landslides or the calving of glaciers — anything that displaces a large enough amount of water to generate a wave that has a far longer wavelength than normal ocean waves. The word comes from Japanese and translates literally to "harbor wave."

Are Tsunami Waves the Same as Tidal Waves?

Despite what you might've seen at the cinema, not all tsunamis appear as enormous, cresting waves that crash on land. Often, they can take the appearance of a rapidly rising tide, which is why they are sometimes called "tidal waves."

Technically, this is incorrect, since the tides are caused by the gravitational pull of the moon. Tsunamis don't need the moon's gravity to form; instead, they are caused by displaced water. The term "tidal waves," while misleading, is still frequently used.

The Largest Tsunami Ever Recorded: Lituya Bay (1,720 Feet)

The Lituya Bay tsunami occurred on July 9, 1958, when an earthquake struck the Alaskan panhandle along the Fairweather fault line. This 7.8-magnitude earthquake shook loose a huge chunk of rock above Lituya Bay, Alaska , causing nearly 40 million cubic yards (30.6 million cubic meters) of the mountainside to crash into the water.

In the way that throwing a rock into the water will cause a splash — and the bigger the rock, the bigger the splash — the displacement of water caused by this landslide generated a wave that strains imagination.

The wave rose 1,720 feet (535 meters) above sea level, altering the landscape around the entirety of the icy bay. For reference, had the Empire State Building been rising out of the bay, the wave would've cleared it.

Luckily, there weren't many people in or around the bay at that time, and only three boats were hit by the world's largest recorded tsunami. This led to just two fatalities, which is still tragic, yet far less devastating than other tsunamis that have hit densely populated areas.

Bigger Than Tsunamis: The Mega-Tsunami

Technically, the Lituya Bay tsunami was a mega-tsunami. This term is used to describe events like the one that occurred in Lituya Bay, where the displaced water is caused by material (usually, rock or ice) that falls into the water. Landslides are a common cause.

Why would that make a difference? It seems that when such a collapse occurs, the waves created are even higher than large waves caused from undersea displacement. This could be due to the suddenness of the water displacement.

5 More of History's Biggest Tsunamis

While destructive tsunamis are relatively rare, such events have occurred throughout history, and it was only in the modern era that precise measurement became possible. Here's a list of the runners up to the Lituya Bay tsunami, in terms of height of the wave.

1. Mount St. Helens Tsunami (853 Feet)

Many people are aware of the Mount St. Helens volcanic eruption, which occurred on May 18, 1980. This eruption also created a tsunami — another mega-tsunami, in fact — when the north flank of the mountain collapsed, creating a landslide that then generated a huge wave.

The wave reached 853 feet (260 meters) high and caused widespread devastation.

2. Vajont Dam Tsunami (820 Feet)

On October 9, 1963, a landslide occurred above the Vajont Dam in Italy, generating a 820-foot (250-meter) wave. This mega-tsunami is notable for being partly human-induced, as the dam created a body of water that was deep enough for a tsunami to be generated.

3. Icy Bay Tsunami (663 Feet)

This mega-tsunami also occurred in Alaska, and was also caused by a massive landslide. On October 17, 2015, a huge volume of rock went crashing into the Taan Fiord, which is a finger of Icy Bay.

The resulting wave had an initial height of approximately 330 feet (101 meters), and had a run up of 633 feet (193 meters). The run up height refers to the elevation of where the wave reached in the surrounding landscape.

4. 1936 Lituya Bay Tsunami (490 Feet)

There's something about Lituya Bay, Alaska. Before the enormous, record setting tsunami of 1958, there was another tsunami in the same bay. This occurred on October 27, 1936, and the cause isn't precisely known (an undersea landslide is a likely candidate), but it did create a massive wave: a run up height of 490 feet (150 meters), and an estimated wave height of between 100 and 200 feet (30 and 76 meters).

5. Karrat Fjord Tsunami (328 Feet)

In Greenland, on June 17, 2007, a landslide caused by melting glacial ice led to a 328-foot (100-meter) giant wave that surged down the fjord and caused wreckage over 62 miles (100 km) away.

Most Destructive: Indian Ocean Tsunami (230,000 Fatalities)

So far, we've been rating the tsunamis on the size of the waves generated, but a taller wave doesn't necessarily mean a more destructive wave. In 2004, an earthquake struck with an epicenter near Sumatra, Indonesia. This led to the most destructive tsunami in history, dubbed the Indian Ocean tsunami .

When the first wave hit the surrounding shorelines, it caused massive destruction in the surrounding 14 countries, from the east coast of India to the west coast of Myanmar. More large waves followed, and the final death toll reached nearly 230,000 people.

Although some of the waves were as high as 100 feet (30 meters), most of the destruction and loss of life was due to the densely populated coastal areas.

Original article: The Largest Tsunami Ever Recorded Only Claimed 2 Lives

Copyright © 2024 HowStuffWorks, a division of InfoSpace Holdings, LLC, a System1 Company

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Tsunamis: Facts About Killer Waves

Get the basics on tsunamis: what they are, what causes them, how they can be avoided, and more.

The Indian Ocean tsunami generated by the most powerful earthquake in decades on December 26 is believed to have killed more than 150,000 people and made millions homeless, making it perhaps the most destructive tsunami in history.

The epicenter of the 9.0 magnitude quake was under the Indian Ocean near the west coast of the Indonesian island of Sumatra, according to the U.S. Geological Survey, which monitors earthquakes worldwide. A violent movement of the Earth's tectonic plates displaced an enormous amount of water, sending powerful shock waves in every direction.

Within hours killer waves radiating from the epicenter slammed into the coastline of 11 Indian Ocean countries, snatching people out to sea, drowning others in their homes or on beaches, and demolishing property from Africa to Thailand.

Tsunamis have been relatively rare in the Indian Ocean. They are most prevalent in the Pacific. But every ocean has generated the scourges. Many countries are at risk.

In the wake of the Christmas weekend tsunami in the Indian Ocean, one of the worst disasters in history, National Geographic News examines the killer waves' causes and warning signs—information that can be a lifesaver in a tsunami zone.

• A tsunami is a series of great sea waves caused by an underwater earthquake, landslide, or volcanic eruption. More rarely, a tsunami can be generated by a giant meteor impact with the ocean.

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Scientists have found traces of an asteroid-collision event that they say would have created a giant tsunami that swept around the Earth several times, inundating everything except the mountains 3.5 billion years ago. The coastline of the continents was changed drastically and almost all life on land was exterminated. ( Read the story )

• Tsunami (pronounced soo-NAH-mee) is a Japanese word. Tsunamis are fairly common in Japan and many thousands of Japanese have been killed by them in recent centuries.

• An earthquake generates a tsunami if it is of sufficient force and there is violent movement of the earth causing substantial and sudden displacement of a massive amount of water.

• A tsunami is not a single wave but a series of waves, also known as a wave train. The first wave in a tsunami is not necessarily the most destructive. Tsunamis are not tidal waves.

• Tsunami waves can be very long (as much as 60 miles, or 100 kilometers) and be as far as one hour apart. They are able to cross entire oceans without great loss of energy. The Indian Ocean tsunami traveled as much as 3,000 miles (nearly 5,000 kilometers) to Africa, arriving with sufficient force to kill people and destroy property.

Scientists say that a great earthquake of magnitude 9 struck the Pacific Northwest in 1700, and created a tsunami that caused flooding and damage on the Pacific coast of Japan. ( Read the story )

As Fast as a Commercial Jet

• When the ocean is deep tsunamis can travel unnoticed on the surface at speeds up to 500 miles per hour (800 kilometers per hour), crossing the entire ocean in a day or less. Scientists are able to calculate arrival times of tsunamis in different parts of the world based on their knowledge of when the event that generated them occurred, water depths, and distances.

• A tsunami may be less than a foot (30 centimeters) in height on the surface of the open ocean, which is why they are not noticed by sailors. But the powerful shock wave of energy travels rapidly through the ocean as fast as a commercial jet. Once a tsunami reaches shallow water near the coast it is slowed down. The top of the wave moves faster than the bottom, causing the sea to rise dramatically.

• Geological features such as reefs, bays, river entrances, and undersea formations may dissipate the energy of a tsunami. In some places a tsunami may cause the sea to rise vertically only a few inches or feet. In other places tsunamis have been known to surge vertically as high as 100 feet (30 meters). Most tsunamis cause the sea to rise no more than 10 feet (3 meters).

The Indian Ocean tsunami caused waves as high as 30 feet (9 meters) in some places, according to news reports. In other places witnesses described a rapid surging of the ocean.

Flooding can extend inland by a thousand feet (300 meters) or more. The enormous energy of a tsunami can lift giant boulders, flip vehicles, and demolish houses. Knowledge of the history of tsunamis in your area is a good indicator of what is likely to happen in a future tsunami event.

• Tsunamis do not necessarily make their final approach to land as a series of giant breaking waves. They may be more like a very rapidly rising tide. This may be accompanied by much underwater turbulence, sucking people under and tossing heavy objects around. Entire beaches have been stripped away by tsunamis.

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• The 2004 Indian Ocean tsunami could rank as the most devastating on record. News reports so far suggest that more than 150,000 people may have lost their lives, many of them washed out to sea.

The most damaging tsunami on record before 2004 was the one that killed an estimated 40,000 people in 1782 following an earthquake in the South China Sea. In 1883 some 36,500 people were killed by tsunamis in the South Java Sea, following the eruption of Indonesia's Krakatoa volcano. In northern Chile more than 25,000 people were killed by a tsunami in 1868.

• The Pacific is by far the most active tsunami zone, according to the U.S. National Oceanic and Atmospheric Administration (NOAA). But tsunamis have been generated in other bodies of water, including the Caribbean and Mediterranean Seas, and the Indian and Atlantic Oceans. North Atlantic tsunamis included the tsunami associated with the 1775 Lisbon earthquake that killed as many as 60,000 people in Portugal, Spain, and North Africa. This quake caused a tsunami as high as 23 feet (7 meters) in the Caribbean.

• The Caribbean has been hit by 37 verified tsunamis since 1498. Some were generated locally and others were the result of events far away, such as the earthquake near Portugal. The combined death toll from these Caribbean tsunamis is about 9,500.

• Large tsunami waves were generated in the Marmara Sea in Turkey after the Izmit earthquake of 1999.

Warning Signs

• An earthquake is a natural tsunami warning. If you feel a strong quake do not stay in a place where you are exposed to a tsunami. If you hear of an earthquake be aware of the possibility of a tsunami and listen to the radio or television for additional information. Remember that an earthquake can trigger killer waves thousands of miles across the ocean many hours after the event generated a tsunami.

• Witnesses have reported that an approaching tsunami is sometimes preceded by a noticeable fall or rise in the water level. If you see the ocean receding unusually rapidly or far it's a good sign that a big wave is on its way. Go to high ground immediately.

Many people were killed by the Indian Ocean tsunami because they went down to the beach to view the retreating ocean exposing the seafloor. Apparently they were unaware that this phenomenon precedes a killer wave. Experts believe that a receding ocean may give people as much as five minutes' warning to evacuate the area.

• Because tsunamis can approach the shore as fast as 100 miles per hour (160 kilometers per hour) it is often too late to get away if you see one. An approaching tsunami is not something to be admired unless you are safely on high ground.

• Remember that a tsunami is a series of waves and that the first wave may not be the most dangerous. The danger from a tsunami can last for several hours after the arrival of the first wave. A tsunami wave train may come as a series of surges that are five minutes to an hour apart. The cycle may be marked by a repeated retreat and advance of the ocean. Stay out of danger until you hear it is safe.

Survivors of the Indian Ocean tsunami reported that the sea surged out as fast and as powerfully as it came ashore. Many people were seen being swept out to sea when the ocean retreated.

• A tsunami surge may be small at one point of the shore and large at another point a short distance away. Do not assume that because there is minimal sign of a tsunami in one place it will be like that everywhere else.

• Tsunamis can travel up rivers and streams that lead to the ocean. Stay away from rivers and streams that lead to the ocean as you would stay away from the beach and ocean if there is a tsunami.

• It's always a good idea to keep a store of emergency supplies that include sufficient medications, water, and other essentials sufficient for at least 72 hours. Tsunami, earthquake, hurricane—an emergency can develop with little or no warning.

Advice for Sailors

• NOAA advises that since tsunami wave activity is imperceptible in the open ocean, vessels should not return to port if they are at sea and a tsunami warning has been issued for the area. Tsunamis can cause rapid changes in water level and unpredictable, dangerous currents in harbors and ports. Boat owners may want to take their vessels out to sea if there is time and they are allowed to do so by port authorities. People should not stay on their boats moored in harbors. Tsunamis often destroy boats and leave them wrecked above the normal waterline.

• Heightened awareness of the potential for a tsunami to inundate the U.S. western coastline has caused NOAA, the U.S. Geological Survey, and the Federal Emergency Management Administration to initiate a program to predict tsunamis more accurately. As a tsunami traverses the ocean, a network of sensitive recorders on the sea floor measures pressure changes in the overhead water, sending the information to sensors on buoys, which, in turn, relay the data to satellites for immediate transmission to warning centers.

• The Tsunami Warning System (TWS) in the Pacific, comprised of 26 member countries, monitors seismological and tidal stations throughout the Pacific region. The system evaluates potentially tsunamigenic earthquakes and issues tsunami warnings. There is no international warning system for tsunamis in the Indian Ocean.

• Use your common sense. If you feel or hear of a strong earthquake do not wait for an official tsunami warning. Tell your family and friends to join you in leaving for high ground.

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