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Frequently
Asked Questions And Articles
The last major Pacific wide tsunami
occurred in 1964. The rare occurrence
of a Pacific wide tsunami in recent
times makes them increasingly important
to understand as more and more people
live and play in coastal areas.
Currently, many people are not aware
of the actual threat tsunamis pose to
many coastal areas throughout the Pacific.
This section has been designed to answer
many of the common questions concerning
the nature of tsunamis; their occurrence
here in Hawaii and the Pacific region;
and what scientists and civil authorities
have done to improve our understanding
and prevent loss of life from this destructive
natural phenomenon.
Where
is Hilo, Hawaii?
Hilo,
pronounced (hee-low), is located on
the windward (eastern) coast of the
island of Hawaii, 200 miles southeast
by air from Honolulu, the state capitol.
Nestled between the flanks of the volcanic
peaks Mauna Kea and Mauna Loa, Hilo,
and its 45,000 residents surround Hilo
Bay, the second largest deep water port
in the island chain. To island residents,
Hilo is most famous for its rainfall
(about 120 inches per year), hence agriculture
is the dominant industry in the surrounding
area.
The
geographic isolation of the Hawaiian
Islands has resulted in unique assemblages
of plants and animals found nowhere
else on Earth. Surrounded by Pacific
Ocean, the history of the Islands has
been shaped by the interactions between
land and sea. Hilo occupies a unique
spot in this history, having been frequently
subjected to devastating tsunami waves.
In terms of property damage and loss
of human life from tsunamis, Hilo surpasses
all other areas in Hawaii, and consequently
has the reputation as the tsunami capital
of the United States.
Hilo
is largely affected by tsunamis for
many reasons, one of which is the local
topography and bathymetry. The
orientation of the Hawaiian Ridge and
coastline, with respect to the direction
and approach of a tsunami, plays an
important role. Also, small funnel-shaped
bays, like Hilo Bay, harness the tsunami
wave energy and amplify the heights
of the waves. That leads to greater
destruction.
What
does the word tsunami mean?
A
tsunami is a Japanese word which translates
as "harbor wave", now used internationally
to refer to a series of waves traveling
across the ocean with extremely long
wavelengths ( up to hundreds of miles
between wave crests in the deep ocean).
When these waves approach shore, the
speed of the wave decreases as they
begin to "feel" the bottom. It
is at this time that the height of the
wave drastically increases. As
the waves strike shore they may inundate
low-lying coastal areas resulting in
mass destruction and in many instances
loss of life. Often a tsunami is incorrectly
referred to as a tidal wave. Tidal waves
are simply the periodic movement of
water associated with the rise and fall
of the tides produced by the gravitational
attraction of the sun and moon. Tsunamis
have no connection with the weather
nor with tides.
There
are two words used to describe tsunamis.
"Kai e'e" is a general word for tsunami
waves and "Kai mimiki" used to describe
the withdraw of the water before the
Kai e'e arrives. Please
note though that the withdraw of the
water is actually the trough of the
tsunami reaching shore.
What
causes a tsunami?
Oceanographers
often refer to tsunamis as seismic sea
waves as they are usually the result
of a sudden rise or fall of a section
of the earth's crust under or near the
ocean. A seismic disturbance can displace
the water column, creating a rise or
fall in the level of the ocean above.
This rise or fall in sea level is the
initial formation of a tsunami wave.
Tsunami
waves can also be created by volcanic
activity and landslides occurring above
or below the sea surface. These types
of activity produce tsunamis with much
less energy than those produced by submarine
faulting. The size and energy of these
tsunamis dissipates rapidly with increasing
distance from the source, thus resulting
in more local devastation.
This
is a difficult topic to research,
because much of the information surrounding
nuclear testing is classified.
During the Cold War there was fear
of tsunamis produced by the detonation
of nuclear bombs on the continental
shelf off the East Coast of the US.
A nuclear bomb was never detonated
on the shelf, however a huge explosion
did generate a tsunami during World
War I causing vast destruction
Any large disturbance that displaces
a large volume of water can be a potential
cause of a tsunami.
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How
is a tsunami wave different from a normal
wave?
The
waves you see at the beach are generated
by wind blowing over the sea surface.
The size of these waves depends on the
strength of the wind creating them and
the distance over which it blows. Generally
the distance between these waves, known
as the wavelength, ranges from a couple
of feet to perhaps a thousand feet.
The speed of these waves as they travel
across the ocean ranges from a few miles
an hour up to sixty miles an hour in
some instances.
Tsunami
waves resulting from physical mechanisms
( see above question) behave much differently
than wind generated waves. The magnitude
of the disturbance causing the tsunami
is the primary factor influencing the
size and strength of the waves. The
height of the wave when it is generated
is very small, usually less than a few
feet. The distance between successive
wave crests or the wavelength however,
is much larger than that of a normal
wave and may be hundreds of miles apart.
Depending on the depth of the water
in which the tsunami is traveling, it
may attain speeds of up to 500 miles
an hour.
When
the waves of a tsunami approach land,
their appearance and behavior become
dependent on several local factors.
Two of the most important factors are
the topography of the seafloor and the
actual shape of the shoreline. As a
tsunami encounters shallow waters surrounding
the shoreline, its height can increase
from a meter or less to over 20 meters.
Wave heights can also be increased when
concentrated on headlands or when traveling
into bays having wide entrances that
become progressively more narrow. The
presence of an offshore coral reef can
dissipate the energy of a tsunami, decreasing
the impact on the shoreline. Normal
wind swell may ride atop of a tsunami
wave thereby increasing wave height.
The
image most people have of a tsunami
is a large, steep wave breaking on the
shore. This image is hardly if ever
the case. Most tsunamis appear as an
advancing tide without having a developed
wave face, resulting in rapid flooding
of low-lying coastal areas. Sometimes,
a bore can form during which an abrupt
front of whitewater will rapidly advance
inland much similar to the tidal bore
formed at the mouth of large rivers.
Another
rare event that may result from a tsunami
is a standing wave or seiche. A seiche
occurs in bodies of water that are partially
or completely enclosed, such as Hilo
Bay, creating a standing wave that continually
sloshes back and forth. The appearance
of a seiche would be very similar to
what happens when you place a glass
of water on the table; the water rocks
back and forth before settling. When
a seiche is generated by a tsunami,
subsequent tsunami waves may arrive
in unison with a seiche resulting in
an increase in the drawdown in sea level
and a dramatic increase in wave height.
Seiche waves may continue several days
after a tsunami.
Once
generated, a tsunami wave in the open
ocean can travel with speeds greater
than 500 miles an hour. These waves
can travel across the Pacific Ocean
in less than one day. Locally generated
tsunamis can reach coastlines in just
minutes.
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.
When
a tsunami approaches a coastline, the
wave begins to slow down and increase
in height, depending on the topography
of the sea floor. Often the first
signs of a tsunami are a receding water
level caused by the trough of the wave.
In some instances though, a small rise
in the water level just before the recession,
has been observed. Regardless,
the incoming wave approaches much like
the incoming tide though on a much faster
scale. The maximum vertical height
to which the water is observed with
reference to sea level is referred to
as run-up. The maximum horizontal
distance that is reached by a tsunami
is referred to as inundation.
The
wave height of a tsunami can be highly
variable in a local area depending on
the underwater topography, orientation
to the oncoming wave, the tidal level,
and the magnitude of the tsunami. Because
direct physical measurement of a tsunami
wave would be a life threatening event,
the most common method for determining
tsunami wave height is by measuring
the runup, the highest vertical point
reached by the wave. Runup heights
are measured by looking at the distance
and extent of salt-killed vegetation,
and the debris left once the wave has
receded. This distance is referenced
to a datum level, usually being the
mean sea level or mean lower low water
level. The reference to mean lower low
water is more significant in areas with
greater tidal ranges such as in Alaska
where a smaller tsunami wave can be
more devastating during a high tide
than a larger wave at low tide.
Whether
a tsunami is generated in the North
or South Pacific, it has the potential
to effect all shores of the Hawaiian
Islands. As large tsunami waves approach
the islands, they may refract or bend
around the islands 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 or bending around,
thereby increasing wave height of the
approaching wave. Therefore, when a
tsunami warning is issued from an earthquake
in Chile, Alaska, or Japan, inhabitants
along all shores of the Islands should
take the necessary precautions.
Yes.
Tsunamis have been recorded to occur
in all the major oceans of the world.
However, this phenomenon is mainly restricted
to the Pacific basin, an area surrounded
by volcanic island arcs, mountain chains
and subduction zones earning the nickname
the "ring of fire", as it is the most
geologically active area on the planet.
The amount of activity in this region
makes it much more susceptible to submarine
faulting and subsequent tsunami events,
whereas the Indian and Atlantic oceans
are far less geologically active, with
some exceptions, and therefore the occurrence
of tsunamis is rare.
The
lack of a warning during the 1946 tsunami
that devastated many coastal areas in
Hawaii, led scientists and governmental
agencies to establish the Pacific Tsunami
Warning System (PTWS), for the Hawaiian
Islands and United States territories
in the Pacific by 1948. The main
objectives of this system are: To detect
and locate the existence all possible
tsunami causing earthquakes by the use
of properly monitored seismographs;
to ensure that a tsunami actually exists
by measuring water level changes at
tide-gauging stations located throughout
the Pacific; and finally, to determine
the time of arrival of the tsunami and
to provide an adequate warning for evacuation
procedures.
What
is the difference between a Tsunami Watch and a Tsunami Warning?
A
Tsunami Watch is automatically declared
by the warning center for any earthquake
having a magnitude of 7.5 or larger
on the Richter scale (7.0 or larger
in the Aleutian Islands) and located
in an area where a tsunami can be generated.
Notification of and Civil Defense agencies
begins, followed by limited public announcements
by the local media. Data from tidal
gauge stations is awaited for confirmation
of the actual existence of a tsunami.
Reports
on wave activity from the tide-gauging
stations nearest to the earthquake epicenter
is requested by the warning center.
If the stations report that there is
no observed tsunami activity, the Tsunami
Watch is canceled. If these stations
report that a tsunami has been generated,
a Tsunami Warning is issued for areas
which may be impacted in the next hour.
At this time the public is informed
of the ensuing danger by the emergency
broadcast system. Evacuation procedures
are implemented, and sea going vessels
are advised to head out to sea, where
in deep waters they will not be affected
by the tsunami.
How
many warnings
have been issued by the Pacific Tsunami
Warning Center since it was established?
The
Pacific Tsunami Warning Center has issued
a total of 20 warnings since it was
first established in 1948. Of these
20, 5 resulted in significant Pacific-wide
tsunamis. Even though all significant
Pacific-wide tsunami events have been
detected since 1948, 61 people perished
when they failed to heed the warning
for the 1960 tsunami that struck Hilo.
Since 1964, there have been no significant
Pacific-wide tsunami events.
Because
a tsunami can strike at any time, being
adequately prepared and knowing what
to do beforehand could save your life.
Hawaii State and County Civil Defense
agencies provide maps of evacuation
zones and information on how to be prepared
for this type of natural disaster in
the front pages of the telephone book.
If you are at the beach and you feel
an earthquake or observe a rapid withdrawal
of the sea and think a tsunami may be
coming, head for higher ground immediately.
When a tsunami warning has been issued
do not attempt to use the telephone
or head to low-lying areas to view the
oncoming waves. Remember, tsunamis
travel at very fast speeds across the
ocean; therefore once a warning has
been issued you should evacuate immediately.
Your
tsunami survival kit is generally the
same for all natural disasters. Here
is a list of suggested supplies:
an extra supply of prescription medicines,
non-perishable dietary foods, ice chest,
a minimum of 2 quarts of water per person
per day, pet food, candles/flashlight,
matches, blankets/sleeping bags, extra
cash, clothing, eyeglasses, personal
hygiene items, special items for infants,
elderly and disabled family members,
quiet games or books/toys for children,
important papers- driver’s license,
special medical information, insurance
policies, and property inventories,
First aid kit and water purification
kit
Can
the arrival time
of a tsunami be accurately predicted?
When
a tsunami is generated offshore the
wave will behave as a shallow water
wave. A shallow water wave is one that
travels through water having a depth
less than 1/20 of its wavelength. Knowing
that the average ocean depth is roughly
three miles, oceanographers can determine
the speed of the tsunami, and calculate
the time it will take to travel between
any two points. This information has
led to the development of travel-time
charts that make it possible to predict
the arrival time of a tsunami wherever
it is generated. Due to the high
speeds of these waves, a tsunami can
travel across the Pacific Ocean is less
than one day! Areas near the epicenter
of earthquakes, landslides or volcanic
activity are most vulnerable to the
effects of a tsunami as they cannot
be properly warned by the Tsunami Warning
Center of the coming danger.
What
has been the most destructive tsunami to strike the Hawaiian Islands
in recent history?
Early
in the morning on April 1, 1946, an
earthquake with a reported magnitude
of 7.1 occurred in the Aleutian Islands
off of Alaska. Almost five hours
later the largest and most destructive
tsunami waves in reported history struck
the Hawaiian Islands. Maximum runups
were reported to be 54 feet in Molokai,
and 55 feet in Pololu Valley on the
Big Island. Waves in some areas
penetrated more than half a mile inland.
Between wave crests, the drawdown is
reported to have exposed some areas
of the seafloor 500 feet in the seaward
direction. A total of 159 tsunami-related
fatalities resulted from this destructive
event. Many were curious school children
who ventured into the exposed reef area,
not knowing the receding water to be
a sign of an approaching tsunami.
No warning was possible nor given for
this tsunami.
How
many Pacific-wide tsunamis have struck the Hawaiian Islands
in recent history?
This
century, there have been 13 significant
tsunamis impacting Hawaii. These
tsunamis were generated by earthquakes
occurring along the geologically active
margins of the Pacific basin. Maximum
recorded runups were 55 feet on the
Big Island and 54 feet on Molokai (see
above) during the 1946 tsunami, and
53 feet in Kauai during the 1957 tsunami.
The last Pacific-wide tsunami occurred
in 1964.
How
many locally generated tsunamis have occurred in
the Hawaiian Islands in recent history?
On
the Big Island there have been several
significant tsunamis resulting from
local earthquakes or submarine landslides.
The most recent and devastating of these
tsunamis occurred in the early morning
hours on November 29, 1975. Within
a little over an hour, two earthquakes
jolted the island. The first, located
three miles inland of Kamoamoa village
in Volcanoes National Park, had a Richter
magnitude of 5.7. The second,
centered two miles offshore of the Wahaula
heiau (also in the park area) was much
more violent having a Richter magnitude
later to be determined as 7.2.
The result of this earthquake was a
10 foot subsidence of the shoreline
and the second most destructive local
tsunami ever to be recorded in Hawaii.
Campers
in the remote Volcanoes National Park
coast at Halape were awakened by the
violent shaking of the first quake unknowing
that a second and more severe quake
would follow in just over an hour later.
Some of them had barely gotten back
to sleep when the second quake shook
so violently that standing was nearly
impossible. Within 30 seconds,
the first of five tsunami waves struck
Halape. Two campers, one an adult
with a group of Boy Scouts, the other
a fisherman, did not survive. Nineteen
others were injured. The maximum
runup height was 47 feet at Keauhou
Landing and 26 feet at Halape, 1.9 miles
to the southwest.
In
Hawaii, methodology was developed at
the University for determining the maximum
expectable inundation of our shores
for worst-case tsunamis, drawing on
the records compiled by the Joint Institute
for Marine and Atmospheric Research
for many years. These historical
data are used in mathematical analyses
to predict maximum wave heights along
the coast; these heights are then used
in numerical models involving the topography
(land contours) to map the inundation
in each location.
In
coordination with the Civil Defense
officers on each island, a final map
is prepared showing the actual evacuation
zones. The zones extend inland from
the inundation limit to the nearest
landmark such as a road, which can be
used by public and police to identify
the areas which must be evacuated to
ensure safety. When the sirens sound,
people are routed to safety until officials
determine that hazardous wave action
has ceased.
The
zones are published in the front of
the telephone directories for each Hawaiian
island. UH-scientists have recently
done a similar analysis for the Humboldt
Bay area in California which may be
applied by their emergency management
officials. A few location in
Japan have well marked zones, based
on historical inundation but for most
of the Pacific, authorities simply call
for evacuation of "low lying areas".
It
is interesting that for Hilo itself,
there are such complete (block-by-block)
records of inundation (1946-1964) that
they have been used to determine evacuation
zones with only minor analysis.
In fact, these records have been extensively
used to test the computer models developed
to predict tsunami wave heights and
inundation. If the model can adequately
re-create a previous event, there is
more assurance it can be used to predict
future events elsewhere.
Since
I don't live in
an inundation area why should I be concerned?
The
shoreline areas of the Hawaiian Islands
are no doubt the main attraction for
visitors and residents alike.
Much of the state's commerce and recreation
involves the surrounding ocean and therefore
it is very important for all of us to
acknowledge the threat a tsunami would
impose on our lives. Even though
you may live in an area that is not
threatened by a tsunami directly, you
would most definitely be impacted by
its effects.
The
last Pacific-wide tsunami to impact
Hawaii occurred over 30 years ago.
During this period of tsunami quiescence,
beach usage especially among children
and teenagers has increased. It remains
essential that this age group, having
never experienced the destructive and
deadly forced of tsunamis, be properly
informed and aware of what to do and
where to go in such an event.
I
own a boat: what should
I do when there is a tsunami warning?
Boats
are safer from tsunami damage while
in the deep ocean rather than moored
in a harbor. U.S. Coast Guard
guidelines suggest deployment to water
depths of at least 1,200 feet (200 fathoms).
However, do NOT risk your life and attempt
to get underway if it is too close to
the first wave arrival time.
Anticipate slowdowns caused by traffic
gridlock and hundreds of other boaters
heading out to sea.
Tsunami
References
Libraries:
Hilo, Kailua-Kona, Lihue, Kahului, Kahuku,
Hawaii Kai, Waianae, Hawaii State Library,
Salt Lake, and Molokai.
For more information e-mail at info@tsunami.org or call the Hawaii State
Public
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