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 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?
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.
"Tsunami"
comes from the Japanese words for harbor ("tsu") and wave
("nami").
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
The
largest recorded earthquake in the world was a magnitude 9.5 (Mw) in Chile on May
22, 1960.
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
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/
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.
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.
1.
Severe
ground shaking from local earthquakes may cause tsunamis.
2.
As
a tsunami approaches shorelines, water may recede from the coast, exposing the
ocean floor, reefs and fish.
3.
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.
·
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.
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.
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).
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.
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.
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.
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.
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.
Tsunami
information, issued in a Tsunami Information Bulletin, is to inform that an
earthquake has occurred and to advise regarding its potential to generate a
tsunami. In most cases there is no threat of a destructive tsunami, and the
information is used to prevent unnecessary evacuations as the earthquake may
have been strongly felt in coastal areas. The information may, in appropriate
situations, caution about the possibility of a destructive local tsunami for
coasts located near an earthquake epicenter (usually within 100 km). Because it
takes 10-20 minutes for PTWC initial bulletins to be issued, they are typically
not effective for a local tsunami that can be onshore in just minutes. In such
situations, however, the information can be useful to local authorities so they
can at least investigate if a tsunami has occurred and if so quickly initiate
recovery procedures. Supplemental tsunami information may be issued if, for
example, a sea level reading showing a tsunami signal is received.
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.
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.
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.
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.
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.
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
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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