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Tri-fold Brochure of City Specific Info
Tsunamis, also known as
seismic sea waves (mistakenly called "tidal waves"), are a series of enormous
waves created by an underwater disturbance such as an earthquake. A tsunami can
move hundreds of miles per hour in the open ocean and smash into land with waves
as high as 100 feet or more, although most waves are less than 18 feet high.
From the area where the
tsunami originates, waves travel outward in all directions much like the ripples
caused by throwing a rock into a pond. In deep water the tsunami wave is not
noticeable. Once the wave approaches the shore it builds in height. All tsunamis
are potentially dangerous, even though they may not damage every coastline they
strike. A tsunami can strike anywhere along most of the U.S. coastline. The most
destructive tsunamis have occurred along the coasts of California, Oregon,
Washington, Alaska and Hawaii.
Tsunamis are most often
generated by earthquake-induced movement of the ocean floor. Landslides,
volcanic eruptions, and even meteorites can also generate tsunamis. If a major
earthquake or landslide occurs close to shore, the first wave in a series could
reach the beach in a few minutes, even before a warning is issued. Areas are at
greater risk if less than 25 feet above sea level and within a mile of the
shoreline. Drowning is the most common cause of death associated with a tsunami.
Tsunami waves and the receding water are very destructive to structures in the
run-up zone. Other hazards include flooding, contamination of drinking water and
fires from gas lines or ruptured tanks.
Local Tsunamis
Tsunamis can be categorized as local and Pacific-wide. Typically a Pacific-wide
tsunami is generated by major vertical, ocean bottom movement in offshore deep
trenches. A local tsunami can be a component of the Pacific-wide tsunami in the
area of the earthquake or a wave that is confined to the area of generation
within a bay or harbor and caused by movement of the bay itself or landslides.
The local tsunami may be the most serious threat as it strikes suddenly,
sometimes before the earthquake shaking stops. Alaska has had six serious local
tsunamis in the last 80 years and Japan even more. The West Coast of Canada and
the United States have not had one in that time; however most scientists agree a
tsunami is possible.
Therefore, it is imperative that those people near shore who feel a strong
earthquake should be moved to higher ground immediately. A strong quake is one
lasting 30 seconds or longer and causes difficulty in standing. The Alaska
Tsunami Warning Center will begin issuing its WARNING to the West Coast in
approximately 25 minutes, but that is not quick enough for a local tsunami; so,
when in doubt, areas should be evacuated.
A
tsunami threat to the City of Newport Beach is considered low to moderate.
A United States Government study reports that:
· Dangerous tsunamis would most likely originate in the
Aleutian and Chilean trenches. Newport Beach has southwestern facing beaches and
is vulnerable to tsunamis or tidal surges from the south and west.
·
Predicted wave heights, exclusive
of tide and storm generated wave heights are:
For a 100 year occurrence
For a 500 year occurrence
4.0 feet
minimum 6.8 feet minimum
6.6 feet
average 11.4 feet
average
9.2 feet
maximum 16.0 feet maximum
·
A tsunami is normally not one
wave, but a series of waves, which may occur over a period of ten hours. The
first wave may not be the largest.
·
In 1964, following the Alaska 8.2 earthquake, tidal surges of
approximately 4 feet to 5 feet hit the Newport Harbor area causing moderate
damage.
What to do before a tsunami
- Know the terms used by
the West Coast/Alaska Tsunami Warning Center (WC/ATWC - responsible for
tsunami warnings for California, Oregon, Washington, British Columbia, and
Alaska) and the Pacific Tsunami Warning Center (PTWC - responsible for tsunami
warnings to international authorities, Hawaii, and the U.S. territories within
the Pacific basin).
- Advisory
- An earthquake has occurred in the Pacific basin, which might generate a
tsunami. WC/ATWC and PTWC will issue hourly bulletins advising of the
situation.
- Watch
- A tsunami was or may have been generated, but is at least two hours travel
time to the area in Watch status.
-
Warning
- A tsunami was or may have been generated, which could cause damage;
therefore, people in the warned area are strongly advised to evacuate.
-
Listen to radio and/ or television for more
information and follow the instructions of your local authorities.
-
Immediate warning of tsunamis sometimes comes
in the form of a noticeable recession in water away from the shoreline. This
is nature’s tsunami warning and it should be heeded by moving inland to higher
ground immediately
-
If you feel an earthquake in a coastal area,
leave the beach or low-lying areas. Then turn on your radio to learn if there
is a tsunami warning.
-
Know that a small tsunami at one beach can be a
larger wave a few miles away. The topography of the coastline and the ocean
floor will influence the size of the wave.
-
A tsunami may generate more than one wave. Do
not let the modest size of one wave allow you to forget how dangerous a
tsunami is. The next wave may be bigger.
- Prepare for possible
evacuation. Be familiar with
evacuation routes. Determine where you would go and how you
would get there if you needed to evacuate. See the “Evacuation” and “Emergency
Planning and Disaster Supplies” chapters for information.
What to do during a tsunami
-
If you are advised to evacuate, do so
immediately.
-
Stay away from the area until local authorities
say it is safe. Do not be fooled into thinking that the danger is over after a
single wave - a tsunami is not a single wave but a series of waves that can
vary in size.
- Do not go to the
shoreline to watch for a tsunami. When you can see the
What to do after a tsunami
-
Stay away from flooded and damaged areas until
officials say it is safe to return.
-
Stay away from debris in the water, it may pose
a safety hazard to boats and people.
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