Most tsunamis are caused by large earthquakes below or near the ocean floor, but can also be caused by landslides, volcanic activity, certain types of weather, and near-earth objects. In the deep ocean, tsunami waves are often barely noticeable, but can move as fast as a jet plane, over 500 mph. As they enter shallow water near land, they slow down and grow in height, and currents intensify.

Most tsunami waves are less than 10 feet high, but can exceed 100 feet in extreme events. When a tsunami comes ashore, 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 like a very low, low tide. A large tsunami can flood low-lying coastal areas more than a mile inland. The first wave may not be the largest or most damaging. Waves may repeatedly flood and recede from the land for many hours.

A tsunami can be very dangerous to life and property on the coast. It can produce unusually strong currents, rapidly flood the land, and cause great destruction. The flow and force of the water and debris it carries can destroy boats, vehicles, and buildings; cause injuries; and take lives as the tsunami moves across land and returns to the sea. Dangerous currents and flooding may last for days. Even small tsunamis can be dangerous. Strong currents can injure and drown swimmers and damage or destroy boats in harbors.

Tsunamis can strike any U.S. coast, but risk is greatest for states and territories with Pacific and Caribbean coastlines. 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. Tsunamis can happen any time, any season. They can be generated far away (across the ocean) or locally. Local tsunamis can arrive just minutes after generation.

The NOAA Tsunami Program runs the U.S. Tsunami Warning System. This includes monitoring for tsunamis and the earthquakes that cause them to provide timely and accurate tsunami messages. NOAA’s success in fulfilling this important mission relies on the ability to quickly detect a tsunami, which is accomplished through networks of advanced observation systems.

NOAA operates two tsunami warning centers, which are staffed 24 hours a day, 7 days a week. The National Tsunami Warning Center serves the continental United States, Alaska, and Canada. The Pacific Tsunami Warning Center 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 Pacific and Caribbean.

When an earthquake occurs, seismic networks provide information about its location, depth, and magnitude to help the warning centers determine if it may have generated a tsunami and if they should issue tsunami messages. The U.S. Geological Survey and its partners operate the primary U.S. seismic networks.

If the earthquake meets certain criteria, the warning centers use sea-level data to determine the existence of a tsunami and refine or cancel messages. NOAA has established and maintains two essential sea-level observation networks.

NOAA’s National Data Buoy Center operates and maintains the U.S. network of Deep-Ocean Assessment and Reporting of Tsunami (DART) systems, which were developed by NOAA’s Pacific Marine Environmental Laboratory for the early detection, measurement, and real-time reporting of tsunamis in the open ocean. The U.S. network is part of a larger international network.

Closer to shore, networks of coastal water-level stations are used to confirm tsunami arrival time and height. In the United States, most of these stations are operated and maintained by NOAA’s Center for Operational Oceanographic Products and Services. Several others are operated by the tsunami warning centers.

If an earthquake meets certain criteria, the warning centers issue U.S. messages (alerts) through multiple channels to emergency managers, other officials, news media, and the public. They issue threat messages to international partners in the Pacific and Caribbean to help them understand the threat to their coasts so they can determine the level of alerts to issue.

There are four levels of tsunami alerts in the United States: Information Statement, Watch, Advisory, and Warning. When the centers issue Tsunami Warnings, they are broadcast through local radio and television, wireless emergency alerts, NOAA Weather Radio and NOAA websites (like Tsunami.gov). They may also come through outdoor sirens, local officials, text message alerts, and telephone notifications.

If a tsunami is detected, the warning centers run tsunami forecast models developed by NOAA’s Pacific Marine Environmental Laboratory and the warning centers. These models use real-time information from the observation systems 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 data and additional seismic analysis, help the warning centers decide whether to cancel messages or adjust them. In the United States, these forecasts also provide local officials with actionable information that can guide decisions about population evacuation, including pedestrian and traffic routes and beach and road closures.

To support forecast and warning capabilities, NOAA’s Center for Coasts, Oceans, and Geophysics develops high-resolution coastal digital elevation models, which depict Earth’s solid surface. The center also serves as the long-term archive for national and international tsunami data, a natural hazards image database, and the global historical tsunami database. It is used to identify regions at risk, validate tsunami forecast models, help position DART systems and coastal water-level stations, and prepare for future events.

Although tsunamis cannot be prevented, there are things that communities and the public can do before, during, and after a tsunami that can save lives. A comprehensive and effective warning system requires that communities and individuals are prepared to respond to tsunamis and have taken steps to reduce potential impacts.

In the United States, much of this work is conducted through the National Tsunami Hazard Mitigation Program (NTHMP), a federal/state partnership led by NOAA that also includes the Federal Emergency Management Agency, the U.S. Geological Survey, and 28 U.S. states and territories. The NTHMP works to reduce the impact of tsunamis through preparedness and mitigation activities that include public outreach and education, community planning (e.g., evacuation and land-use), hazard assessment, and warning guidance.

Established in coordination with the NTHMP, NOAA’s TsunamiReady program helps communities prepare for tsunamis and minimize tsunami-related losses through better planning, education, and awareness. The program is voluntary, and communities must meet certain guidelines to be recognized as TsunamiReady.

National Weather Service Forecast Offices work with communities to support their tsunami preparedness efforts and help them become TsunamiReady. They also support the NOAA Tsunami Program by educating the public, local officials, and the media about tsunamis and tsunami safety.

NOAA also provides education and preparedness services to international partners in the Pacific and the Caribbean through the International Tsunami Information Center offsite link and the Caribbean Tsunami Warning Program, respectively.

NOAA is a global leader in the development of tsunami detection technologies and modeling tools. While significant efforts were underway to detect and model tsunamis prior to the 2004 Indian Ocean tsunami, the event spurred the development of a national tsunami research to operations plan.

The U.S. tsunami warning system has come a long way in recent years. Continued investments to advance the technology and strengthen partnerships have resulted in a robust and effective system, but more remains to be done to ensure the delivery of the best messages possible. NOAA is constantly developing new and improved technologies and tools to better safeguard coastal communities. Current research efforts are addressing the science-based needs of the tsunami warning centers and aim to identify, develop, and rapidly transition technologies and modeling tools into tsunami warning center operations.

These efforts include refining tsunami detection technologies to improve capability and reduce production and operating costs. A fourth generation DART system (4G) has been undergoing testing since 2013. Like the earlier systems, the DART 4G system will detect and measure both distant and local tsunamis but will be able to detect and measure a tsunami very close to the earthquake source, which will provide valuable information to warning centers faster than ever before.

Other examples include exploring the use of undersea fiber-optic cables and GPS-based tsunami monitoring to augment the DART network and looking at how to better detect, observe, and forecast tsunamis caused by landslides and weather-related events. In addition, NOAA depends on social science research to refine the content of its messages and outreach to encourage appropriate public response and improve the effectiveness of the warning system.