Coast and Alaska
Frequently Asked Questions
Last updated: 6/2011
What is a tsunami?
tsunami is a series of waves with a long wavelength and period (time
between crests) generated by a large, impulsive displacement of sea water.
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.
are often incorrectly called tidal waves; they have no relation to the
daily ocean tides.
are tsunamis generated?
are generated by any large, impulsive displacement of the sea level.
most common cause of a tsunami is sea floor uplift associated with an
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?
the average, two tsunamis occur per year throughout the world which
inflict damage near the source.
every 15 years a destructive, ocean-wide tsunami occurs.
Can strike-slip (horizontal motion)
earthquakes trigger tsunamis?
approximately 10-15% of damaging tsunamis are triggered by strike-slip
type of earthquake is less likely to trigger a tsunami than one with
waves are likely generated by associated landslides or motion of a sloping
generated by strike-slip earthquakes normally affect regions near the
What does the word "tsunami"
(soo-NAH-mee) is a Japanese word meaning harbor wave.
How fast do tsunamis travel?
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).
travel approximately 475 mph in 15,000 feet of water. In 100 feet of water
the velocity drops to about 40 mph.
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?
range in size from inches to over a hundred feet.
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
tsunamis propagate into shallow water, the wave height can increase by
over 10 times.
heights vary greatly along a coast.The waves can be amplified by shoreline and bathymetric (sea floor)
large tsunami can flood low-lying coastal land over a mile from the coast.
What does a tsunami look like when it
a tsunami appears as a rapidly advancing or receding tide.
some cases a bore (wall of water) or series of breaking waves may form.
How is a tsunami different from a
waves usually have periods (time between crests) between 5 and 15
seconds. Tsunami periods normally
range from 5 to 60 minutes.
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
Center's (WC/ATWC) responsibilities?
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).
WC/ATWC AOR is Puerto Rico, the Virgin Islands, Canada, and the ocean coasts of all
U.S. states except Hawaii.
new processes and techniques to improve response times, forecast accuracy,
and message content to residents in the AOR.
community preparedness and public tsunami education through the
TsunamiReady program and outreach.
more information on the center, see http://wcatwc.arh.noaa.gov/.
What is the tsunami threat level for the U.S. East and Gulf of Mexico
What is the tsunami threat level for the Alaska Arctic and west (Bering Sea)
tsunami threat level for the Alaska Bering Sea and Arctic coasts is very low when compared
to the southern coast of Alaska.
Arctic and Bering Sea Talking Points
for more information.
When are warnings or advisories issued?
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.
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.
are normally issued approximately 5 minutes after earthquake occurrence.
Do all large earthquakes, greater than
magnitude 7, generate tsunamis?
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,
How does the WC/ATWC respond to
landslides which generate tsunamis are triggered by large
earthquakes. In this case, local
warnings will be issued based on the earthquake size.
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?
tsunami warning indicates that a tsunami may be imminent and that coastal
locations in the warned area should prepare for flooding.
initial warning is typically based solely on seismic information.
the tsunami is recorded on sea level gages, the warning will be cancelled,
restricted, expanded, or downgraded to an advisory.
indicate that flooding up to the maximum expected limit is possible and
residents should follow their local emergency management instructions.
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?
tsunami advisory indicates a tsunami which may produce strong currents and
is dangerous to those in or very near the water is expected.
inundations are not expected in areas under advisory status.
will be cancelled, extended, or upgraded to a warning depending on the
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?
tsunami watch is an early alert issued to areas which may later be
impacted by a tsunami.
impact is normally at least three hours away for regions within a tsunami
watch will either be upgraded to a warning or advisory in subsequent messages
or cancelled depending on the severity of the tsunami.
within a watch area should stay alert for further information regarding
What are nature's signs that a tsunami
may be imminent?
ground shaking for 20+ seconds near the coast.
sudden sea level withdrawal.
may be accompanied by loud, booming noises.
Where should I go in the event of a
tsunami warning or large, local earthquake?
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.
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.
WC/ATWC 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?
in deep water (600 feet or greater) should stay at sea.
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?
acquires seismic data from various seismic networks throughout its
AOR. These data are processed to
quickly determine the tsunami-potential of an earthquake.
are issued based initially on this first analysis of seismic data.
a tsunami may have been generated, sea level data, tsunami models, and
historical tsunami information are analyzed to estimate impact level.
on impact estimations, supplemental messages are issued.
How are tsunami messages issued?
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
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.
US Coast Guard relays the message via radio.
warnings are posted on the WC/ATWC and NWS web sites and are issued
through a public e-mail list server (http://wcatwc.arh.noaa.gov/).
coastal communities supplement the basic notification systems with sirens,
automatic phone messaging systems, etc. Contact your local emergency management officials to determine
Can the WC/ATWC predict earthquakes and
- No, earthquakes cannot be predicted.
- Once an earthquake has occurred, the arrival time of
a tsunami, if generated, can be determined accurately.
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
largest recorded earthquake occurred in Chile
(9.5) in 1960.
second largest earthquake recorded was the 1964 Alaskan earthquake (9.2).
more information on earthquakes
Has WC/ATWC's AOR experienced any
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.
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.
Tsunamis were recorded in 1755, 1884, 1886, and 1929.
Rico and the Virgin Islands experienced damaging tsunamis in 1867 and 1918.
for the NOAA/National Geophysical Data Center tsunami catalog.
How is the maximum expected tsunami
inundation determined for a specific location?
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.
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
National Tsunami Hazard Mitigation Program (http://nthmp.tsunami.gov/)
has funded hazard assessment investigations for many U.S.