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.
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.
Earthquakes which produce horizontal sea floor
motion will generate little to no tsunami.
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.
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 will not be 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 West Coast
and Alaska Tsunami Warning Center’s (WC/ATWC) responsibilities?
The main mission of the WC/ATWC is to help
protect life and property from tsunami hazard by providing tsunami
information and warning messages to its area-of-responsibility (AOR).
The WC/ATWC AOR is the U.S. West, Alaskan,
Atlantic, and Gulf of Mexico coasts as well as the coast of British
Columbia in Canada.
Develop new processes and techniques to improve
response times, accuracy, and bulletin content to residents in the AOR.
Increase community preparedness and public
tsunami education through the TsunamiReady program and outreach.
Staff consists of six full-time personnel who
provide on-site coverage during regular work hours and provide “on-call”
service after hours.Two personnel
are always within 5 minutes travel to the center.
For more information on the center, see
http://wcatwc.arh.noaa.gov/subpage3.htm.
When are warnings issued?
Warnings are issued when a potentially
tsunami-producing earthquake over the threshold magnitude (7.0 in the
Pacific AOR, 6.75 in the Atlantic AOR) occurs in the AOR.
Warnings are also 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 is based on
the size of the earthquake and tsunami travel times throughout the AOR.
Warnings are issued within 10 minutes after earthquake
occurrence.
Do all large earthquakes,
greater than magnitude 7, generate tsunamis?
No, only those which induce large vertical sea
floor displacements, or those that trigger landslides which displace
significant amounts of sea water, trigger tsunamis.
How does the WC/ATWC
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 incrementally,
or expanded to cover the entire coast in the event of a major tsunami.
Tsunami warnings include estimated wave arrival
times for key coastal locations in the warned area.
What is a tsunami watch?
A tsunami watch is an alert issued to areas
outside the warned area.
The geographical extent of the watch area is
based on the size of the earthquake and tsunami travel times throughout
the AOR.
The watch will either be upgraded to a warning
in subsequent bulletins or cancelled depending on the severity of the
tsunami.
Tsunami watches include estimated wave arrival
times for key coastal locations in the watch area.
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 the tsunami hazard zone has been delineated
for your area, stay out of that area.
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 WC/ATWC issues tsunami warnings, but the
warnings and subsequent evacuations are implemented by state and local
emergency management.
Mariners in deep water 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 does the
WC/ATWC evaluate in order to issue tsunami bulletins?
WC/ATWC acquires seismic data from various
seismic networks throughout its AOR.This data is processed, automatically and interactively, to quickly
determine the tsunami-potential of an earthquake.
Bulletins are issued based initially on this
first analysis of seismic data.
If a tsunami could have been generated, sea
level data and historical tsunami information are analyzed to estimate
impact level.
Based on impact estimations, supplemental
bulletins are issued.
How are warnings 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 FEMA’s 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 WC/ATWC and NWS
web sites (http://wcatwc.arh.noaa.gov/message.shtml) and are issued
through a public e-mail list server.
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 the WC/ATWC predict
earthquakes and tsunamis?
No, earthquakes can not be
predicted.
Once an earthquake has
occurred, the arrival time of a tsunami, if generated, can be determined
accurately.
Tsunami wave heights can not be accurately
predicted near the earthquake 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).
Three of the seven largest earthquakes ever
recorded occurred in Alaska.
Has WC/ATWC’s AOR
experienced any 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
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,
and 1975.
US
Atlantic Coast: Tsunamis were
recorded in 1755, 1884, 1886, and 1929.
How are maximum expected
wave heights 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://www.pmel.noaa.gov/tsunami-hazard/)
has funded hazard assessment investigations for many west coast, Alaska,
and Hawaii communities.The
process will be extended to the Atlantic coast.The WC/ATWC is presently performing preliminary studies on
U.S. Atlantic and Gulf coast hazard.
National Weather Service Mission: "The
National Weather Service (NWS) provides weather,
hydrologic, and climate forecasts and warnings for
the United States, its territories, adjacent waters
and ocean areas, for the protection of life and
property and the enhancement of the national economy.
NWS data and products form a national information
database and infrastructure which can be used by
other governmental agencies, the private sector,
the public, and the global community."
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