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DOC > NOAA > NWS > PTWC > Frequently Asked Questions (FAQ)
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Frequently Asked Questions (FAQ)
- What causes tsunamis?
- What does the word "tsunami" mean?
- Do "tsunami" and "tidal wave"" mean the same thing?
- How are tsunami waves different from normal ocean waves?
- How many waves are there in a tsunami?
- How fast do tsunamis travel?
- What does a tsunami look like when it reaches the shore?
- Where and how often do tsunamis usually occur?
- How long do tsunamis last when they happen?
- Do all large undersea earthquakes generate tsunamis?
- What is the "wrap-around" effect?
- Can tsunamis be predicted?
- Where should I go for the most accurate earthquake information?
- Why do Tsunami Warning Centers rely upon seismic data?
- What was the biggest earthquake ever recorded?
- Can earthquakes be predicted?
- What is a DART buoy?
- What is a tide station?
- How do Tsunami Warning Centers use sea level data?
- What are the natural warning signs for a tsunami?
- What should I do in the event of a tsunami warning?
- What do I do if I am in a boat at sea or in a harbor during a tsunami?
- How is the Tsunami Warning System organized?
- How does the Tsunami Warning System work?
- How many Tsunami Warning Centers are there?
- What is a Tsunami Warning?
- What is the difference between a "Fixed Regional Warning" and an "Expanding Regional Warning"?
- What is a Tsunami Watch?
- What is a Tsunami Advisory?
- What is a Tsunami Information Bulletin/Statement?
- What are PTWC's responsibilities?
- How does PTWC evaluate earthquakes for tsunami potential?
- How does PTWC disseminate its messages?
- Does PTWC send messages via SMS to mobile phones?
- Does PTWC issue evacuation orders?
- Can PTWC predict earthquakes and tsunamis?
- What are some other tsunami FAQ websites?
- 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.
Top of Page - What does the word "tsunami" mean?
"Tsunami" comes from the Japanese words for harbor ("tsu") and wave ("nami").
Top of Page - 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.
Top of Page - 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.
Top of Page - 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.
Top of Page - 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.
Top of Page - 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.
Top of Page - 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.
You can search the NGDC tsunami database for more information on historical tsunamis.
Top of Page - 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.
Top of Page - 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.
Top of Page - 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.
Top of Page - 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. You can learn more on the NCTR website: http://nctr.pmel.noaa.gov/tsunami-forecast.html
Top of Page - Where should I go for the most accurate earthquake information?
USGS National Earthquake Information Center
Top of Page - 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.
Top of Page - 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.
Top of Page - 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
Top of Page - What is a DART buoy?
DART systems consist of an anchored seafloor bottom pressure recorder and a companion moored surface buoy for real-time communications (Gonzalez et.al, 1998). An acoustic link transmits data from the recorder on the seafloor to the surface buoy. The data are then relayed via a GOES satellite link to ground stations (Milburn, et al., 1996), which send the data streams to NOAA's Tsunami Warning Centers, the National Data Buoy Center, and the Pacific Marine and Environmental Laboratory. Additional information on the DART systems can be found at http://nctr.pmel.noaa.gov/Dart/
Top of Page - What is a tide station?
Tide gauges (aka: sea level gauges) are generally located on piers in harbors along coastlines throughout the world. These are primarily used for monitoring the tides for navigation purposes, but the tsunami warning centers also rely upon this data to monitor sea level and determine whether tsunamis were generated for a given earthquake.
Top of Page - 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.
Top of Page - What are the natural warning signs for a tsunami?
- Severe ground shaking from local earthquakes may cause tsunamis.
- As a tsunami approaches shorelines, water may recede from the coast, exposing the ocean floor, reefs and fish.
- Abnormal ocean activity, a wall of water, and an approaching tsunami create a loud "roaring" sound similar to that of a train or jet aircraft.
If you experience any of these phenomena, don't wait for official evacuation orders. Immediately leave low-lying coastal areas and move to higher ground.
Top of Page - What should I do in the event of a tsunami warning?
- If you are in school and you hear there is a tsunami warning, you should follow the advice of teachers and other school personnel.
- If you are at home and hear there is a tsunami warning, you should make sure your entire family is aware of the warning. Your family should evacuate your house if you live in a tsunami evacuation zone. Move in an orderly, calm and safe manner to the evacuation site or to any safe place outside your evacuation zone. Follow the advice of local emergency and law enforcement authorities.
- If you are at the beach or near the ocean and you feel the earth shake, move immediately to higher ground. DO NOT wait for a tsunami warning to be announced. 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. A tsunami from a local earthquake could strike some areas before a tsunami warning could be announced.
- Tsunamis generated in distant locations will generally give people enough time to move to higher ground. For locally-generated tsunamis, where you might feel the ground shake, you may only have a few minutes to move to higher ground.
- High, multi-story, reinforced concrete hotels are located in many low-lying coastal areas. The upper floors of these hotels can provide a safe place to find refuge should there be a tsunami warning and you cannot move quickly inland to higher ground. Local Civil Defense procedures may, however, not allow this type of evacuation in your area. Homes and small buildings located in low lying coastal areas are not designed to withstand tsunami impacts. Do not stay in these structures should there be a tsunami warning.
- Offshore reefs and shallow areas may help break the force of tsunami waves, but large and dangerous waves can still be threat to coastal residents in these areas. Staying away fro all low-lying coastal areas is the safest advice when there is a tsunami warning.
Top of Page - 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.
Top of Page - How is the Tsunami Warning System organized?
The world's tsunami warning systems are coordinated by the United Nations Intergovernmental Oceanographic Commission (IOC) through the International Tsunami Information Centre (ITIC). More information on each tsunami warning system is available on their respective websites:
- Pacific Tsunami Warning System (established 1968)
- Indian Ocean Tsunami Warning System (established 2005)
- North Eastern Atlantic, Mediterranean, and Connected Seas Tsunami Warning System (established 2005)
- Caribbean Sea and Adjacent Regions Tsunami Warning System (established 2006)
The eventual goal is to form a global tsunami warning system as a fully participating component of the Global Earth Observation System of Systems (GEOSS).
Top of Page - How does the Tsunami Warning System work?
coming soon
Top of Page - How many Tsunami Warning Centers are there?
The U.S. has two tsunami warning centers: the Pacific Tsunami Warning Center in Hawaiʻi and the West Coast and Alaska Tsunami Warning Center in Alaska. The relative areas of responsibility for these two centers are explained on PTWC's Responsibilities page. The Japan Meteorological Agency also has a well-developed tsunami warning system with multiple warning centers. Following the 2004 Indian Ocean tsunami, other warning centers have been established in Indonesia, Australia, and New Zealand. Efforts are currently underway to establish tsunami warning centers in the Caribbean and Mediterranean Seas.
Top of Page - What is a Tsunami Warning?
The highest level of tsunami alert. Warnings are issued due to the imminent threat of a tsunami from a large undersea earthquake or following confirmation that a potentially destructive tsunami is underway. They may initially be based only on seismic information as a means of providing the earliest possible alert. Warnings advise that appropriate actions be taken in response to the tsunami threat. Such actions could include the evacuation of low-lying coastal areas and the movement of boats and ships out of harbors to deep water. Warnings are updated at least hourly or as conditions warrant to continue, expand, restrict, or end the warning.
Top of Page - What is the difference between a "Fixed Regional Warning" and an "Expanding Regional Warning"?
These types of warnings are issued to the Pacific Ocean only. If PTWC issues a Fixed Regional Warning, only the countries within 1000 km (621 miles) of the earthquake epicenter are placed in a warning. All other countries are advised of the earthquake but are not placed in a warning or watch. If PTWC issues an Expanding Regional Warning, the countries within 3 hours of the predicted tsunami arrival time are placed in a warning, and countries within 3-6 hours of the predicted tsunami arrival time are placed in a watch. Every hour, PTWC will issue new bulletins to update the lists of countries within a warning or watch.
Top of Page - What is a Tsunami Watch?
The second highest level of tsunami alert. Watches are issued by the TWCs based on seismic information without confirmation that a destructive tsunami is underway. It is issued as a means of providing an advance alert to areas that could be impacted by destructive tsunami waves. Watches are updated at least hourly to continue them, expand their coverage, upgrade them to a Warning, or end the alert. A Watch for a particular area may be included in the text of the message that disseminates a Warning for another area.
Top of Page - What is a Tsunami Advisory?
The third highest level of tsunami alert. Advisories are issued to coastal populations within areas not currently in either warning or watch status when a tsunami warning has been issued for another region of the same ocean. An Advisory indicates that an area is either outside the current warning and watch regions or that the tsunami poses no danger to that area. The Center(s) issuing the Advisory will continue to monitor the event, issuing updates at least hourly. As conditions warrant, the Advisory will either be continued, upgraded to a watch or warning, or ended.
Top of Page - What is a Tsunami Information Bulletin/Statement?
A text product issued to inform that an earthquake has occurred and to advise regarding its potential to generate a tsunami. In most cases, a Tsunami Information Bulletin indicates there is no threat of a destructive tsunami, and are used to prevent unnecessary evacuations as the earthquake may have been felt in coastal areas. A Tsunami Information Bulletin may, in appropriate situations, caution about the possibility of a destructive local tsunami. A supplemental Tsunami Information Bulletin may be issued if important additional information is received such as a sea level reading showing a tsunami signal. A Tsunami Information Bulletin may also be upgraded to a watch or warning if appropriate.
Top of Page - What are PTWC's responsibilities?
See Responsibilities page.
Top of Page - How does PTWC evaluate earthquakes for tsunami potential?
First, we use seismic data to determine the earthquake hypocenter (location, depth) and magnitude. If the earthquake is underwater, less than 100 km in depth, and above a certain magnitude threshold, PTWC will issue the appropriate tsunami message (see About PTWC Messages). As more seismic data becomes available, PTWC can use more sophistocated methods to measure the earthquake's slowness, which correlates with its tsunamigenic potential.
Top of Page - How does PTWC disseminate its messages?
Official message dissemination paths include the GTS (Global Telecommunication System), AFTN (Aeronautical Fixed Telecommunication Network), AWIPS (Advanced Weather Interactive Processing System), EMWIN (Emergency Managers Weather Information Network), NWW (NOAA Weather Wire), NOAA Weather Radio, RANET, Email, Fax, and Telex. PTWC also maintains RSS feeds of its messages and is planning for other XML feeds in the future.
Top of Page - Does PTWC send messages via SMS to mobile phones?
No. At this time, PTWC does not directly send its messages to mobile phones. Some emergency managers in Pacific, Asian, and African countries can receive PTWC messages via SMS on thier mobile phones through the RANET Project. Since PTWC messages are available to the general public via RSS feeds, one could get PTWC messages on a mobile phone using one of the many RSS-to-SMS services on the Internet (for example: Yahoo! Alerts, ZapTXT, Rasasa, FeedBeep). Another option is the USGS Earthquake Notification Service (ENS), which allows you to receive custom automated earthquake alerts via email or SMS.
Top of Page - Does PTWC issue evacuation orders?
In the case of a tsunami warning issued to Hawaiʻi, PTWC makes the decision whether evacuations are necessary, and the evacuations are coordinated by civil defense authorities. Outside Hawaiʻi, PTWC has no responsibility or authority to directly warn the public or issue evacuation orders. Local civil defense authorities are the ones who receive PTWC's messages and take appropriate action, which could include sounding sirens, evacuating beaches, and activating the Emergency Alert System.
Top of Page - Can PTWC predict earthquakes and tsunamis?
No. Earthquakes and tsunamis cannot be predicted. However, once a large earthquake does occur, PTWC can forecast the tsunami behavior if one had been generated.
Top of Page - What are some other tsunami FAQ websites?
International Tsunami Information Centre (ITIC)
NOAA West Coast & Alaska Tsunami Warning Center FAQ
NOAA Center for Tsunami Research FAQ
Pacific Tsunami Museum FAQ
American Red Cross Tsunami FAQ
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