Tsunamis radiate outward in all directions from their source and can move across entire ocean basins, around islands and into bays, sounds and up rivers. When they reach the coast, they can cause dangerous coastal flooding and powerful currents that impact marine operation and navigation, and can last for several hours or days.

Tsunamis are infrequent but can pose a serious threat to life and property when they occur. Tsunamis have claimed hundreds of thousands of lives and caused hundreds of billions of dollars in damage around the world.

In deep ocean water, tsunami waves may go unnoticed. But as the waves travel closer to the shore, they build in height as the water becomes more shallow. The speed of tsunami waves depends on ocean depth: The deeper the water, the faster the wave travels. Tsunami waves may travel as fast as jet planes through 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 destructive waves — though a tsunami at high tide may cause more damage and flooding. 

What causes a tsunami?

About 80% of all known tsunamis are triggered by earthquakes. These seismic events move Earth's surface, displacing the water above and generating waves that rapidly travel in all directions across the ocean or body of water. 

Not all earthquakes create tsunamis. 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 does the water above it. As the water moves up and down, seeking to regain its balance, the tsunami radiates 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 critical factors in the size of a resulting tsunami. Earthquakes can also cause landslides that generate tsunamis.

How landslides generate tsunamis

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, which can in turn 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.

What is a meteotsunami?

Large storms over the ocean or large water bodies can cause meteotsunamis, which, like tsunamis, can also be destructive. 

Meteotsunamis are driven by air-pressure disturbances often associated with fast-moving weather events, such as severe thunderstorms, squalls and other storm fronts. The storm generates a wave that moves towards the shore, and is amplified by a shallow continental shelf and inlet, bay or other coastal feature.

Meteotsunamis have been observed to reach heights of 6 feet (1.82 meters) or more. They occur in many places around the world, including the Great Lakes, Gulf of Mexico, Atlantic Coast and the Mediterranean and Adriatic Seas.

Identifying a meteotsunami is a challenge because its characteristics are almost indistinguishable from a seismic tsunami. Meteotsunamis can also be confused with wind-driven storm surge or a seiche. These uncertainties make it difficult to predict a meteotsunami and warn the public of a potential event. However, NOAA scientists have identified atmospheric conditions that are likely to generate meteotsunamis and continue to research them. 

Where do tsunamis occur?

Although tsunamis occur relatively infrequently, they can be a serious threat to life and property. Tsunamis have claimed hundreds of thousands of lives and caused hundreds of billions of dollars in damage around the world.

A tsunami can strike any coast (ocean, lake or even river) at any time. There is no season for tsunamis. This is quite evident when we look at records of past tsunamis. Tsunamis typically happen where there are large faults that can generate large earthquakes. Most of these large faults are around the Pacific Ring of Fire. However, any active fault that crosses water may be capable of generating a tsunami. As mentioned above, other causes of tsunamis include landslides and volcanic eruptions which can happen far away from any ocean coastline. 

Tsunamis are sometimes referred to as "tidal waves," but this is very misleading as they are not related to tides.

The events were gathered from scientific and scholarly sources, regional and worldwide catalogs, tide gauge data, deep ocean sensor data, individual event reports and unpublished works. One example, published on the USGS website, demonstrates how old records in Japan helped unravel what happened during the “orphan” tsunami of 1700. 

Of the approximately 2,800 events in the NCEI Global Historical Tsunami Database, more than 1,400 are confirmed tsunamis. Over 270 confirmed deadly tsunamis have resulted in over 544,000 known (or confirmed) deaths. It is important to note that this number may include deaths from the generating event (e.g., earthquake) as it is not always possible to separate deaths from the different causes. It is likely that these figures should be much higher, but in many events the actual number of fatalities is not known. Tsunamis have not always been reported the same way throughout history, and the way that deaths resulting from them has not always been counted in the same manner. The tsunamis prior to the 1900s were especially recorded in ways very different from modern methods.

For example, many Indigenous communities have oral histories of tsunamis that impacted coastlines and damaged or destroyed their communities. By listening to these histories and documenting where similar stories exist along stretches of coastline, researchers are able to better understand and unravel details about tsunamis that happened before there was monitoring equipment or written record of such events.  

Between 1900 and 2015, a total of 754 confirmed tsunamis have occurred, with 78% of those events in the Pacific Ocean and 5% in the Indian Ocean, according to the Global Historical Tsunami Database. The highest percentage of tsunamis generated since 1900 are off of Japan (21%) and Indonesia (8%). In the U.S., tsunamis occur about twice a decade. The risk is greatest for states and territories with Pacific and Caribbean coastlines.

How does NOAA get information about ancient tsunamis?

The events and observations in the NCEI Global Historical Tsunami Database were gathered from the NOAA Tsunami Warning Centers, NOAA National Data Buoy Center, NOAA National Ocean Service, UNESCO/IOC-NOAA International Tsunami Information Center, NOAA Pacific Marine Environmental Laboratory, U.S. Geological Survey, national and government databases and reports, tsunami catalogs, post-event reconnaissance reports, journal articles, newspapers, internet sources, email and other written documents. 

A historical tsunami means it was documented in the written records, or by scientific instrumentation, near the time of the event.

NCEI has archived over 8,000 documents that describe damage and effects from tsunami, earthquake and volcano events. The NCEI historical tsunami database is focused on documented historical tsunamis, as such it does not include sources only inferred from the study of tsunami deposits or other geologic and archeological evidence.

Tsunami deposits are the physical evidence left behind when a tsunami impacts a shoreline or affects submarine sediments. NCEI maintains a list of publications that characterize global tsunami deposits.

What can we learn from studying historical tsunami records?

Historical tsunami data is critical for a basic understanding of tsunami generation, wave characteristics and coastal impacts.

Historical tsunami records support:

• Calibration and validation of tsunami models, which are needed for forecasts as well as inundation and evacuation mapping.
• Tsunami hazard assessments require historical records since it provides insight into tsunami behavior and what might happen during future events. Risk assessments are needed by emergency managers and coastal zone managers for tsunami hazard mitigation and planning.
• Tsunami Warning Centers may use historical records to evaluate the likelihood of tsunami generation following an earthquake and develop the thresholds of tsunami warning products.
• Land use planners and engineers may use the data to develop safer building codes, zoning and other guidance.
• Media use the information when reporting about hazard response and to increase public awareness.
• Historical tsunami data supports key concepts in Earth Science curriculums for grades K-12 and beyond.
• Public education for life-safety actions. For example, historical records have documented that the following “natural warning signs” may precede tsunami waves: (1) strong or long ground shaking; (2) unusual disappearance of water; (3) the roaring sound of the water.

In total, just over a quarter of a million people lost their lives from this tragic event, with another 1.7 million people displaced. In 2017, the cost and damage from the tsunami was estimated to be approximately $13 billion. 

The 9.1 magnitude earthquake in the Indian Ocean was about 19 miles below the ocean floor, and the length of the rupture was roughly 800 miles — basically the size of California’s coastline. The quake displaced a massive volume of water to generate a global tsunami, which reached some areas within 20 minutes and others in 7 hours. The tsunami was observed by more than 100 coastal water-level stations in the Atlantic and Pacific ocean basins. One of the hard-to-grasp statistics from the tsunami were wave heights that reached 167 feet in Indonesia’s Aceh province in northern Sumatra, which resulted in flooding up to 3 miles inland. 

Once the tsunami reached portions of Asian and African coastlines, the waves advanced inland, carried debris, destroyed once-thriving communities and ruined many livelihoods. Imagine: No more homes, basic services, and critical infrastructure such as roads, power lines, bridges, clean water, gasoline/fuel — essentially all the things we rely on in our modern world. The economies of many countries were devastated. The saltwater from the tsunami  damaged soils, crops and vegetation, while completely changing the overall environmental landscape of many Indian Ocean coastal communities. 

An order of magnitude hard to imagine

Since 1900, about 5% of all tsunamis have occurred in Southeast Asia, but the scope and magnitude of the 2004 tsunami set it apart from others.  

Since this catastrophic event, NOAA has made significant improvements to its tsunami detection, forecasting, warning and preparedness capabilities, including:

• U.S. Deep-ocean Assessment and Reporting of Tsunamis (DART) buoys increased from 6 to 39.
• Installed or upgraded 188 coastal water-level stations to support tsunami warning operations.
• Upgraded the Global Seismographic Network to transmit 100% of its seismic data in real time.
• Developed forecasting models to predict the arrival time, duration, height and extent of flooding.
• Expanded the Tsunami Warning Centers’ responsibilities.
• Started the TsunamiReady program to help communities minimize risk through better hazard assessment, planning, education and warning communications.
• In 2007, the U.S. Congress Established a new Act of Law, the Tsunami Warning and Education Act, which was later revised and reauthorized in 2017 as the Tsunami Warning, Education, and Research Act (TWERA), which outlines the responsibilities and goals for NOAA and a successful U.S. Tsunami Warning System and Program. TWERA formally established the National Tsunami Hazard Mitigation Program, a program led by NOAA to advance tsunami preparedness at the local level through partnerships and grant funding for at-risk states and territories. 

NOAA has two tsunami warning centers in the U.S. that are staffed 24 hours a day, 7 days a week, to monitor and alert for tsunamis from earthquakes. Scientists detect the earthquake, observe the tsunami, forecast tsunami impacts, issue tsunami alert messages, conduct public outreach and coordinate with partners to continually improve warning operations with the main mission in protecting life and property from tsunamis.

The National Tsunami Warning Center, located in Palmer, Alaska, serves the continental U.S. (East, West and Gulf Coasts), Alaska and Canada. In Ford Island, Hawaii, the Pacific Tsunami Warning Center (PTWC) serves the Hawaiian Islands and the U.S. Pacific and Caribbean territories, including American Samoa, Guam, Northern Mariana Islands, Puerto Rico, U.S. Virgin Islands and the British Virgin Islands. PTWC also serves as the Pacific Tsunami Warning System’s main tsunami service provider. In this role, PTWC works closely with other international, subregional and national centers to monitor seismic and sea level stations around the Pacific Ocean for large earthquakes and tsunami waves. 

Both tsunami warning centers are part of NOAA’s National Weather Service (NWS). NWS also leads the National Tsunami Hazard Mitigation Program (NTHMP) and the International Tsunami Information Center offsite link (ITIC). The NTHMP is a coordinated U.S. national effort to mitigate the impact of tsunamis through public education, community response planning, hazard assessment and warning coordination. 

How does NOAA monitor and detect tsunamis generated by earthquakes or volcanic eruptions?

NOAA has a suite of coastal water-level stations along the West, East and Gulf Coasts and surrounding Alaska, Hawaii, Puerto Rico and U.S. Territories. These stations collect important data about the height of the ocean at specific coastal locations and monitor tides. The data is then relayed by satellite to NOAA’s warning centers, where it is used to confirm tsunami height and arrival time, and is incorporated into tsunami forecast models.

In addition to vast coastal water-level stations, NOAA has 39 Deep-ocean Assessment and Reporting of Tsunami (DART®) sea-level systems located strategically throughout the Pacific and Atlantic Ocean basins, the Gulf of Mexico and the Caribbean Sea. The DART systems help identify early detection, measurement and real-time reporting of tsunamis in the open ocean waters. 

Data from these coastal and DART stations are then relayed via satellite to the warning centers, where they are used to confirm tsunami height and arrival time and incorporated into tsunami forecast models. 

A constellation of satellite radar altimeters — including NOAA’s partnered missions (Jason-3 and Sentinel-6) — measure sea surface height and roughness. Those measurements provide subtle but detectable signals as tsunamis cross the deep ocean: A small, rapid change in sea level paired with altered surface roughness. The data can support and refine tsunami propagation models, allowing for more accurate predictions of wave travel times and coastal impacts. Additionally, altimeter data can enhance bathymetric models by providing information on ocean bottom topography, which affects tsunami wave speed and direction. This approach is not yet operational at NOAA's Tsunami Warning Centers.

When NTWC and PTWC issue tsunami alerts, what do they mean?

The National Tsunami Warning Center (NTWC) and Pacific Tsunami Warning Center (PTWC) issue four different messages and alerts to notify the public, emergency managers and other local officials and partners about the potential for a tsunami following a possible tsunami-generating event. 

Below are the four types of messages and their definitions:

This action aims to provide consistent tsunami forecasting, alerting, decision-support services and seamless, comprehensive service backup. Substantial progress has been made in recent years, solidifying the requirements for new common analytical software to common operating systems and technical support. The most critical component of the effort to establish a common operating platform is to provide service backup between the two Tsunami Warning Centers if one Tsunami Warning Center becomes inoperable or requests back-up. 

NOAA’s Tribal Relations Team works to strengthen and build relationships with tribes, Alaska Native communities and Native Hawaiian and Pacific Islanders, and works to ensure accountability (by integrating tribal treaties and reserved rights into agency decision-making processes, and upholding Executive Orders), maintain and establish relations and promote awareness of the policies, activities and concerns related to these vital partners. 

Congress has authorized NOAA to lead the National Tsunami Hazard Mitigation Program (NTHMP) through NWS’s Tsunami Program. Tasked with reducing the impact of tsunamis on the nation, the NTHMP is a unique and effective partnership between the NOAA, the Federal Emergency Management Agency (FEMA), the U.S. Geological Survey (USGS) and 28 U.S. coastal states and territories. One of the greatest strengths of the NTHMP is the extraordinary commitment each member brings to the program and the high level of cooperation between federal, state and territory emergency management and scientific partners. This collaboration enhances cost-effectiveness and improves the program’s ability to implement consistent national policies and projects at the local level. The NTHMP has four strategic overarching priorities or themes:

1. Hazard and Risk Assessment 
2. Education and Preparedness 
3. Mitigation and Recovery 
4. Alert, Warning, and Response 

The themes are supported by goals and strategies, which the NTHMP will strive to meet through collaboration between partners and stakeholders. 

New frontiers in tsunami research

The scientists at NOAA’s Pacific Marine Environmental Laboratory (PMEL) in Seattle are working with NWS staff to develop new tsunami detection and modeling methods. The NOAA Center for Tsunami Research at PMEL aims to translate new science into a more accurate, fast and actionable tsunami forecast capability for NOAA’s Tsunami Warning Centers.

PMEL tsunami research mainly focuses on providing accurate inundation forecasts in real-time before flooding from a tsunami reaches threatened coastlines. If timely and accurate, the tsunami inundation estimates are critical for saving coastal residents' lives during a tsunami event. Modeling, real-time tsunami detection and data assimilation are central to PMEL’s tsunami research.

About 80% percent of all known tsunamis are triggered by earthquakes. That is why the current tsunami warning system is focused on earthquake-generated tsunamis. However, landslides, volcanic eruptions and asteroid impacts have caused catastrophic tsunamis. Large storms over the ocean or large water bodies can cause meteotsunamis, which can also be destructive. Specialized models developed by NOAA’s National Center for Tsunami Research can predict coastal inundation from these other tsunamis, too, provided they get accurate information about the size and location of the triggering event. 

Data from atmospheric and weather observations combined with real-time coastal and deep-water tsunami detection, as well as AI-assisted analytics, could provide enough information for models to forecast wave heights before a non-seismic tsunami arrives onshore.

Additionally, NOAA is working with emergency managers to assess long-term tsunami hazards by modeling hypothetical events and to prepare tsunami inundation maps under different scenarios to build tsunami-resilient communities. 

NOAA is collaborating with the American Society of Civil Engineers offsite link to develop the world’s first probability-based tsunami design provisions. Among their goals:

• Establishing building codes requiring structures in inundation zones to be able to withstand wave forces.
• Assessing tsunami debris hazards. One of the most destructive elements of a tsunami results from the debris that is swept up and propelled shoreward, then seaward, by powerful waves. 
• Creating new tsunami inundation (flooding) maps that are based on probabilities of occurrence for different size earthquakes which can be tailored to a given location and take into account hazards amplified by sea level rise and changing coastlines.  

NOAA’s National Weather Service (NWS) has a few programs to help you prepare before, during and after a potential tsunami that could save your life and the lives of your family and friends.

The NWS TsunamiReady program, which started in 2001 and is modeled after the NWS StormReady program, helps communities minimize the risk posed by tsunamis through better risk assessment, planning, education and warning communications. The main goal of the program is to improve public safety by establishing guidelines for a standard level of capability to mitigate, prepare for and respond to tsunamis. TsunamiReady works with communities to help them meet these guidelines, and ultimately become recognized as TsunamiReady by NWS.

Coastal communities should consider conducting a tsunami hazard assessment. This assessment should include computer models and information from past tsunamis to identify and map the areas likely to be flooded, and by how much, during a tsunami. Communities can use the resulting tsunami hazard maps to determine where people and other important community assets (e.g., buildings, facilities, bridges, schools and hospitals) are at risk, so that they can decide where to focus preparedness, response and mitigation efforts.

Communities that understand their tsunami risk are better prepared to protect the public in the event of a tsunami.