Frequently Asked Questions (FAQ)

What causes tsunamis?

Anything that rapidly displaces a large volume of water can cause a tsunami. Typically, tsunamis are caused by underwater earthquakes, but landslides, volcanic eruptions, calving icebergs, and (very rarely) meteorite impacts can also generate tsunamis. These types of events can cause large disturbances in the surface of the ocean, and when gravity pulls the water back down, the tsunami is born.

What does the word "tsunami" mean?

"Tsunami" comes from the Japanese words for harbor ("tsu") and wave ("nami").

Do "tsunami" and "tidal wave"" mean the same thing?

No, but unfortunately some people commonly refer to tsunamis as "tidal waves", which is incorrect since tsunamis have nothing to do with the tides. Tides result from the gravitational attraction of the Moon and Sun.

How are tsunami waves different from normal ocean waves?

Both normal ocean waves and tsunami waves can be described by their period (time between two waves), wavelength (horizontal distance between waves), amplitude (wave height), and speed. Normal ocean waves are caused by the wind, weather, tides, and currents. They have periods of 5-20 seconds, wavelengths of 100-200 meters (300-600 feet), and travel at speeds of 8-100 km per hour (5-60 miles per hour). Tsunami waves have much longer periods of 10 minutes to 2 hours, wavelengths of 100-500 km (60-300 miles), and travel at speeds of 800-1000 km per hour (500-600 miles per hour). The amplitude of normal waves and tsunami waves are similar in deep ocean water, but near shore, tsunami waves can be much larger with heights of 10 meters (32 feet) or more. Furthermore, normal ocean waves only involve motion of the uppermost layer of the water, but tsunami waves involve movement of the entire water column from surface to seafloor. This means a normal wave is like a small ripple on top of the ocean, but tsunamis are like the entire ocean getting deeper all at once.

How many waves are there in a tsunami?

A tsunami generally consists of a series of waves, often referred to as the tsunami wave train. The amount of time between successive waves, known as the wave period, is only a few minutes; in some instances, waves are over an hour apart. Many people have lost their lives after returning home in between the waves of a tsunami, thinking that the waves had stopped coming.

How fast do tsunamis travel?

Tsunami wave speed is controlled by water depth. Where the ocean is over 6,000 meters (3.7 miles) deep, unnoticed tsunami waves can travel at the speed of a commercial jet plane, over 800 km per hour (500 miles per hour). Tsunamis travel much slower in shallower coastal waters where their wave heights begin to increase dramatically.

What does a tsunami look like when it reaches the shore?

As the leading edge of a tsunami wave approaches shore, it slows dramatically due to the shallower water. However, the trailing p art of the wave can still be moving rapidly in the deeper water. This results in a "piling up" of the tsunami energy, and the tsunami wave height grows. The wave looks and acts like giant river of water on top of the ocean that floods the shore.

Where and how often do tsunamis usually occur?

Major tsunamis occur about once per decade. Based on historical data, about 59% of the world's tsunamis have occurred in the Pacific Ocean, 25% in the Mediterranean Sea, 12% in the Atlantic Ocean, and 4% in the Indian Ocean.

Tsunamis have been reported since ancient times. They have been documented extensively, especially in Japan and the Mediterranean areas. The first recorded tsunami occurred off the coast of Syria in 2000 B.C. Since 1900 (the beginning of instrumentally located earthquakes), most tsunamis have been generated in Japan, Peru, Chile, New Guinea and the Solomon Islands. However, the only regions that have generated remote-source tsunamis affecting the entire Pacific Basin are the Kamchatka Peninsula, the Aleutian Islands, the Gulf of Alaska, and the coast of South America. Hawaiʻi, because of its location in the center of the Pacific Basin, has experienced tsunamis generated in all parts of the Pacific.

The Mediterranean and Caribbean Seas both have small subduction zones, and have histories of locally destructive tsunamis. Only a few tsunamis have been generated in the Atlantic and Indian Oceans. In the Atlantic Ocean, there are no subduction zones at the edges of plate boundaries to spawn such waves except small subduction zones under the Caribbean and Scotia arcs. In the Indian Ocean, however, the Indo-Australian plate is being subducted beneath the Eurasian plate at its east margin. Therefore, most tsunamis generated in this area are propagated toward the southwest shores of Java and Sumatra, rather than into the Indian Ocean.

How long do tsunamis last when they happen?

Since a tsunami consists of a series of waves, the danger can last for many hours. Large tsunamis are generally recorded for a few days following the event.

Do all large undersea earthquakes generate tsunamis?

No. The earthquake must cause significant vertical deformation of the seafloor in order for a tsunami to occur. Tsunamis are more likely to happen from shallower and larger earthquakes than deeper or smaller ones.

What is the "wrap-around" effect?

As large tsunami waves approach islands, they may refract or bend around them and diffract through the channels between the islands as well. The ability of a tsunami wave to bend around and through the islands is called the wrap-around effect. During the wrap-around effect, the energy of the tsunami often decreases resulting in smaller wave heights. Sometimes tsunami waves will reflect off of a land mass instead of bending around, thereby increasing wave height of the approaching wave.

Can tsunamis be predicted?

Since earthquakes cannot be predicted, we do not know when a tsunami will happen before the earthquake occurs. Once a large potentially tsunamigenic earthquake does occur, we can forecast tsunami arrival times and wave heights through the use of computer modeling.

Where should I go for the most accurate earthquake information?

USGS National Earthquake Information Center

Why do Tsunami Warning Centers rely upon seismic data?

Seismic waves travel about 100 times faster than tsunami waves. This means, we can detect and study the earthquake much more quickly than the tsunami itself.

What was the biggest earthquake ever recorded?

The largest recorded earthquake in the world was a magnitude 9.5 (Mw) in Chile on May 22, 1960.v

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Can earthquakes be predicted?

No. Neither the USGS nor any other scientists have ever predicted a major earthquake. They do not know how, and they do not expect to know how any time in the foreseeable future. However, based on scientific data, probabilities can be calculated for potential future earthquakes. For example, scientists estimate that over the next 30 years the probability of a major EQ occurring in the San Francisco Bay area is 67% and 60% in Southern California

How do Tsunami Warning Centers use sea level data?

Sea level data are used to confirm the generation of a tsunami and to predict the tsunami hazard for locations where the waves have yet to strike. They are critical to the Warning Centers' ability to provide timely and accurate tsunami watches, warnings and cancellations.

What are the natural warning signs for a tsunami?

What do I do if I am in a boat at sea or in a harbor during a tsunami?

Since tsunamis are imperceptible in the open ocean, do not return to port if you are at sea and a tsunami warning has been issued for your area. Tsunamis can cause rapid changes in water level and unpredictable dangerous currents in harbors and ports. If there is time to move your boat or ship from port to a location where the water is more than 400 meters (1300 feet) deep, you should consider:

• Most large harbors and ports are under the control of a harbor authority and/or a vessel traffic system. Keep in contact with the authorities should a forced movement of vessels be directed.
• Smaller ports may not be under the control of a harbor authority. In this case, be sure you have enough time to move your vessel safely to deep water. Small boat owners may find it safest to leave their boat at the pier and physically move to higher ground, particularly in the event of a locally-generated tsunami. Severe weather conditions (rough seas outside of the harbor) could present a greater hazard to small boats, so physically moving yourself to higher ground may be the only option.
• Damaging wave activity and unpredictable currents can affect harbors long after the initial tsunami impact on the coast. Contact the harbor authority before returning to port to verify that conditions in the harbor are safe for navigation and berthing.

What is a Tsunami Warning?

A tsunami warning is issued by PTWC when a potential tsunami with significant widespread inundation is imminent or expected. Warnings alert the public that widespread, dangerous coastal flooding accompanied by powerful currents is possible and may continue for several hours after arrival of the initial wave. Warnings also 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. To provide the earliest possible alert, initial warnings are normally based only on seismic information.

What is a Tsunami Watch?

A tsunami watch is issued to alert emergency management officials and the public of an event which may later impact the watch area. The watch area may be upgraded to a warning or canceled based on updated information and analysis. Therefore, emergency management officials and the public should prepare to take action. Watches are normally issued based on seismic information without confirmation that a destructive tsunami is underway.

What is a Tsunami Advisory?

A tsunami advisory is issued due to the threat of a potential tsunami which may produce strong currents or waves dangerous to those in or near the water. Coastal regions historically prone to damage due to strong currents induced by tsunamis are at the greatest risk. The threat may continue for several hours after the arrival of the initial wave, but significant widespread 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 are normally updated to continue the advisory, expand/contract affected areas, upgrade to a warning, or cancel the advisory.