Tsunami Frequently Asked Questions
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What is a tsunami ?
* A tsunami is a series of waves with a long wavelength and period (time between crests) generated by a large, impulsive displacement of sea water.
* Time between crests of the wave can vary from a few minutes to over an hour, but generally are in the range of 15 to 30 minutes.
* Tsunamis are often incorrectly called tidal waves; they have no relation to the daily ocean tides.
How are tsunamis generated ?
* Tsunamis are generated by any large, impulsive displacement of the sea level.
* The most common cause of a tsunami is sea floor uplift associated with an earthquake.
* Tsunamis are also triggered by landslides into or under the water surface, and can be generated by volcanic activity and meteorite impacts.
How often do tsunamis occur ?
On the average, two tsunamis occur per year throughout the world which inflict damage near the source.
Approximately every 15 years a destructive, ocean-wide tsunami occurs.
Can strike-slip (horizontal motion) earthquakes trigger tsunamis ?
* Yes, approximately 10-15% of damaging tsunamis are triggered by strike-slip earthquakes.
* This type of earthquake is less likely to trigger a tsunami than one with vertical motion.
* The waves are likely generated by associated landslides or motion of a sloping bathymetric feature.
* Tsunamis generated by strike-slip earthquakes normally affect regions near the source only.
What does the word "tsunami" mean ?
* Tsunami (soo-NAH-mee) is a Japanese word meaning harbor wave.
How fast do tsunamis travel ?
* Tsunami velocity depends on the depth of water through which it travels (velocity equals the square root of the product of the water depth times the acceleration of gravity).
* Tsunamis travel approximately 475 mph in 15,000 feet of water. In 100 feet of water the velocity drops to about 40 mph.
* A tsunami travels from the central Aleutian Is. to Hawaii in about 5 hours and to California in about 6 hours, or from the Portugal coast to North Carolina in about 8.5 hours.
How big is a tsunami ?
* Tsunamis range in size from inches to over a hundred feet.
* In deep water (greater than 600 feet), tsunamis are rarely over 3 feet and are not normally noticed by ships due to their long period (time between crests).
* As tsunamis propagate into shallow water, the wave height can increase by over 10 times.
* Tsunami heights vary greatly along a coast.The waves can be amplified by shoreline and bathymetric (sea floor) features.
* A large tsunami can flood low-lying coastal land over a mile from the coast.
What does a tsunami look like when it reaches shore?
* Normally, a tsunami appears as a rapidly advancing or receding tide.
* It some cases a bore (wall of water) or series of breaking waves may form.
How is a tsunami different from a wind-generated wave ?
* Wind-generated waves usually have periods (time between crests) between 5 and 15 seconds. Tsunami periods normally range from 5 to 60 minutes.
* Wind-generated waves break as they shoal and lose energy offshore. Tsunamis act more like a flooding wave. A twenty foot tsunami is a twenty foot rise in sea level.
What are the National Tsunami Warning Center's (NTWC) responsibilities ?
* The main mission of the NTWC is to help protect life and property from tsunami hazard by providing tsunami information and warning messages to its area-of-responsibility (AOR).
* The NTWC AOR is Puerto Rico, the Virgin Islands, Canada, and the ocean coasts of all U.S. states except Hawaii.
* Develop new processes and techniques to improve response times, forecast accuracy, and message content to residents in the AOR.
* Increase community preparedness and public tsunami education through the TsunamiReady program and outreach.

What is the tsunami threat level for the U.S. East and Gulf of Mexico coasts ?
* The tsunami threat level for the U.S. East and Gulf of Mexico coasts is low when compared to U.S. Pacific and Caribbean coasts.
* See U.S. East and Gulf of Mexico Talking Points for more information.
What is the tsunami threat level for the Alaska Arctic and west (Bering Sea) coasts ?
* The tsunami threat level for the Alaska Bering Sea and Arctic coasts is very low when compared to the southern coast of Alaska.
* See Arctic and Bering Sea Talking Points for more information.
When are warnings or advisories issued ?
* Warnings are issued when a potentially tsunami-producing earthquake over the threshold magnitude (7.0 in the Pacific AOR, 6.5 in the Atlantic AOR) occurs in the AOR.
* Warnings or advisories also may be issued when potentially tsunami-producing earthquakes over magnitude 7.5 occur outside the AOR and are likely to impact the AOR.
* The geographic extent of the warning or advisory is based on the size of the earthquake, tsunami travel times throughout the AOR, and expected impact zones.
* Warnings are normally issued approximately 5 minutes after earthquake occurrence.
Do all large earthquakes, greater than magnitude 7, generate tsunamis ?
* No, only those which induce large sea floor displacements capable of disturbing the sea level over a wide area, or those that trigger landslides which displace significant amounts of sea water, trigger tsunamis.
How do the tsunami centers respond to landslide-generated tsunamis ?
* Many landslides which generate tsunamis are triggered by large earthquakes. In this case, local warnings will be issued based on the earthquake size.
* In some cases, sub-sea landslides will occur with little to no seismic energy release (e.g., Skagway, AK 1994). Historically, these events have been locally destructive with impacts occurring within minutes. The Tsunami Warning System is not set up to respond to this type of event.
What is a tsunami warning ?
* A tsunami warning indicates that a tsunami may be imminent and that coastal locations in the warned area should prepare for flooding.
* The initial warning is typically based solely on seismic information.
* After the tsunami is recorded on sea level gages, the warning will be cancelled, restricted, expanded, or downgraded to an advisory.
* Warnings indicate that flooding up to the maximum expected limit is possible and residents should follow their local emergency management instructions.
* Warnings are issued when the earthquake information or tsunami forecasts indicate that a wave over 1 meter in amplitude is expected, possible, or ongoing.
What is a tsunami advisory ?
* A tsunami advisory indicates a tsunami which may produce strong currents and is dangerous to those in or very near the water is expected.
* Large inundations are not expected in areas under advisory status.
* Advisories will be cancelled, extended, or upgraded to a warning depending on the event severity.
* Advisories are issued when the tsunami forecast is in the range of 0.3 to 1 meter, or an observed tsunami is in the range of 0.5 to 1.0 meters.
What is a tsunami watch ?
* A tsunami watch is an early alert issued to areas which may later be impacted by a tsunami.
* Tsunami impact is normally at least three hours away for regions within a tsunami watch.
* The watch will either be upgraded to a warning or advisory in subsequent messages or cancelled depending on the severity of the tsunami.
* People within a watch area should stay alert for further information regarding tsunami threat.
What are nature's signs that a tsunami may be imminent ?
* Hard ground shaking for 20+ seconds near the coast.
* A sudden sea level withdrawal.
* Tsunamis may be accompanied by loud, booming noises.
Where should I go in the event of a tsunami warning or large, local earthquake ?
* Know evacuation routes and potential hazard zones for your area. Most coastal communities have an evacuation plan and designated safe areas. This information can be obtained from your local emergency official.
* If no tsunami hazard zone has been established or you don't know what it is, as a rule of thumb move to 100 feet above sea level or 1 mile inland.
* The NTWC issues tsunami warnings, but the warnings and subsequent evacuations are implemented by state and local emergency management.
What do I do if I'm in a boat at sea or in a harbor during a tsunami event ?
* Mariners in deep water (600 feet or greater) should stay at sea.
* Those in shallow water or harbors should move to deep water if there is enough time and weather conditions are suitable.
What information do the tsunami centers evaluate in order to issue tsunami bulletins ?
* Seismic data is acquired from various seismic network. These data are processed to quickly determine the tsunami-potential of an earthquake.
* Messages are issued based initially on this first analysis of seismic data.
* If a tsunami may have been generated, sea level data, tsunami models, and historical tsunami information are analyzed to estimate impact level.
* Based on impact estimations, supplemental messages are issued.
How are tsunami messages issued ?
* Warnings are broadcast through standard National Weather Service (NWS) dissemination methods such as the NOAA Weather Radio-All Hazards, the Emergency Alert System, and the Emergency Managers Weather Information Network.
* State Emergency Service Agencies receive the message through the FEMA National Warning System and the NOAA Weather Wire. The states immediately pass warnings to local jurisdictions.
* The US Coast Guard relays the message via radio.
* The warnings are posted on the web and are issued via e-mail.
* Many coastal communities supplement the basic notification systems with sirens, automatic phone messaging systems, etc. Contact your local emergency management officials to determine local procedures.

Can earthquakes and tsunamis be predicted ?
* No, earthquakes cannot be predicted.
* Once an earthquake has occurred, the arrival time of a tsunami, if generated, can be determined accurately.
* There is not normally enough time to accurately predict tsunami heights near the source. Away from the source, tsunami wave heights can be estimated based on mathematical tsunami models and observed wave heights.
What was the biggest earthquake ever recorded ?
* The largest recorded earthquake occurred in Chile (9.5) in 1960.
* The second largest earthquake recorded was the 1964 Alaskan earthquake (9.2).
* See http://www.earthquake.usgs.gov/ for more information on earthquakes
What are some of the most damaging tsunamis ?
* Alaska: Major tsunamis were generated along the Alaskan coast in 1946, 1957, 1958, 1964, and 1965. Many other locally generated tsunamis occurred prior to those dates.
* US/Canada West Coast: Tsunamis were generated locally in 1812, 1873, 1878, 1927, 1930, 1946 and 1949. The west coast was also impacted by tsunamis generated in other regions in 1877, 1946, 1952, 1957, 1960, 1964, 1975, 2006, and 2010.
* US/Canada Atlantic Coast: Tsunamis were recorded in 1755, 1884, 1886, and 1929.
* Puerto Rico and the Virgin Islands experienced damaging tsunamis in 1867 and 1918.
* See http://www.ngdc.noaa.gov/hazard/tsu.shtml for the NOAA/National Geophysical Data Center tsunami catalog.
How is the maximum expected tsunami inundation determined for a specific location ?
* In areas where the maximum potential source is known (e.g., areas with an active subduction zone offshore), tsunami generation, propagation, and runup can be mathematically modeled and maximum wave heights estimated. Or, if the area's largest expected earthquake has occurred in recorded times, the historical records can be used to constrain expectations.
* In areas where the maximum potential source is unknown and no historic events have occurred (e.g., parts of the U.S. Atlantic coast), maximum expected wave height is difficult to determine. Many potential sources must be considered to determine the maximum expectations.
* The National Tsunami Hazard Mitigation Program (http://nws.weather.gov/nthmp/) has funded hazard assessment investigations for many U.S. communities.

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