Transcript Hazards Chapter3b

CHAPTER 3B
TSUNAMI WAVES
The term tidal wave is deeply entrenched in the
expressions of both the general public and the
popular press
Further, it is frequently and incorrectly applied to
two large wave types that have nothing to do with
the tides
Tsunami: (seismic sea wave) Japanese for
“tsu”: harbor “nami”: wave; name given to a
seismic sea wave generated as an indirect
effect of an undersea earthquake, landslide or
volcanic eruption felt over great distances
Fitting to use “harbor wave” for this
phenomena since greater heights possible in
the physically confined locations
Interesting (and a bit confusing) that tsunami is also
the Japanese word for tidal wave
(1)
If you are interested in experiencing a tsunami, the
Hawaiian Islands are a good choice
… the Islands rarely if ever experience an
earthquake (did just have a 6.6 on 10/15/06),
since 1819 have experienced about 40
tsunami episodes
… location makes it a target
These occurrences can generate a concentric ring
of waves similar to those resulting from objects
thrown into the water
These seismographically-induced waves may be
unnoticeable in the open sea, but upon reaching
shallow waters they begin to grow to massive
proportions – may reach 30 m (100’) of more
They have awesome kinetic energy from the mass
of the volumes of water that constitute them
Aside
A tsunami is not to be confused with a tidal wave
… which if viewed as in common perception as a
mountainous thundering wall of water – is a
misnomer as nothing like it exists in nature, and if
we are to take the term literally – the term itself
means nothing
Tidal wave is not to be confused with storm surge
either
(2)
Storm surge: (the result of atmospheric low
pressure) – a “dome” of water rising beneath
low pressure cells (hurricanes / cyclones /
typhoons) and which cause extensive coastal
flooding when they make landfall
… ex: Bangladesh 1970, a cyclone and
accompanying storm surge killed and
estimated 500,000 persons
… we do storm surge when we do hurricanes
Formation of Tsunamis
Most tsunamis originate along the Pacific Ring of
Fire region as a consequence of tectonic activity
… so, at least theoretically, any seismic activity
can result in a tsunami anywhere along its
24,000 mi length
It is fitting to use tsunami for this class of seismic
wave, because the Japanese Islands combine the
two essentials for their production and for
destruction
(1) tectonically active
(2) steeply sloping continental shelf
Tsunamis originate in ocean regions where
submarine volcanic, seismic or tectonic activities
are frequent or where sudden ocean floor
subsidence or collapse or mass movement of
sedimentary material occur
--- type of displacement determines whether the leading
edge of the wave is a wave trough (floor collapse) or a
wave crest (floor doming)
Quakes which disturb the sea commonly generate
sufficient displacement to create powerful waves
Concentric rings of waves race across the open
ocean at speeds up to 500 mph
These waves are frequently unseen / unnoticed
--- VERY long waves --- may surpass 100 mi [wave
length] and 20 min crests [period]
--- less than 3’ in height
[open ocean, a wave of that size with a wavelength
of 100 mi or more would not register … following
pictures are 2’- 4’ waves]
Because the sea is fluid [no pun intended] any
disturbance displaces the entire water column
and wave energy of a tsunami is distributed from
sea surface to sea floor
Once it begins to move into shallower water the
energy it is carrying [a 2’ wave of < L/20 depth 100 mi
long] is compacted and the wave has no where to
go but up
Aside
… try this: in the open ocean with a depth of [say]
12,000’ the wave moves at 656’/sec
… near shore the ocean depth decreases to 330’
and friction reduces wave speed to about
99’/sec
… at 165’ of depth wave speed is reduced to
about 72’/sec all the water piling behind has
to go somewhere – up
[or if you like, Tad is driving at 70 mph [just believe it] on I-40
when mutant orange barrels force him to slow to 40 mph…
then 20 mph… what must happen to traffic following him?
What beautiful strains of vocalization must follow him?]
--- upon reaching shallow water they begin to
grow rapidly (maybe to 200’)
… ultimate height will be variable according to bottom
structure and slope, configuration of shoreline, etc.
Sometimes the only warning of an approaching
tsunami is a sudden increase or decrease in
ocean level
Some Notable Tsunami Disasters
Worldwide tsunamis occur on average at least once
a year
--- the Pacific alone had 270 damaging tsunamis
between 279 A.D. and 1946
--- worldwide the average is one tsunami per year
Possibly the most dramatic event occurred about
3,400 years ago – destruction of volcanic island –
Stronghyle (also known as Thera)
[present day island of Santorini, Agean Sea]
leaving a caldera and 3 arc-shaped smaller
islands
? It has been put forward to be Atlantis
? Direct factor in decline of Minoan (Crete) culture
? May be part of biblical history (the Flood)
July 21, 365 A.D. an earthquake in the eastern
Mediterranean likely caused a seafloor collapse
--- record of receding waters around Sicily,
Greece, and Egypt
--- returning water inundated low-lying areas
--- City of Alexandria was reported to be the most
heavily damaged
… 50,000 people died, fishing boats relocated
miles inland
… the famous 600’ lighthouse was destroyed
June 1692, - Port Royal, Jamaica
--- three tremors create tsunamis registered to 24’
--- 100’s drown; total causalities reach 1,600+
[would love to dive here; won’t let you]
November 1, 1755 – Lisbon, Portugal
--- quake; fire; tsunamis
--- 50,000+ was killed
August 27, 1883 – most famous – eruption and
ultimate destruction of the volcanic island –
Krakatoa (Krakatau)
--- over a 3 mo period the island’s 3 volcanoes
began to erupt
--- collapse of the largest volcano (Rakata) after
19 straight days of eruption allowed seawater
into its partially collapsed magma chamber
--- massive explosives resulted in particles and
steam columns to 83.000’ (25,300m)
--- resulting tsunamis waves 20’-130’ overtopped
areas of Sumatra and Java
--- 36,000+ were killed; 200 towns and villages
were destroyed
--- water level recorders as far away as Cape Horn
(7800 mi) and Panama (11, 400 mi) were
triggered
--- estimated that seismic waves moved in excess
of 400 mph
April 1, 1946 – Hilo, HI
--- 160 persons killed; $25 mill damage
36’ waves on Oahu; 56’ waves on Hawaii
--- waves came in 15 intervals
--- a disturbance in the Aleutian Trench of Alaska
produced a series of tsunamis
[ waves destroyed the Scotch Gap Lighthouse (33’)
and its radio mast (108’) in the Aleutians]
--- waves not only struck the windward shores,
but were refracted and diffracted into high
waves on the leeward shores
Irony
Stranded in Hawaii during the April 1946 Hilo tsunami was Dr.
Francis Shepherd, a marine geologist, who was part of a
group of scientist on their way to Bikini for an atomic bomb
test
? A social comment by God?
Citizens of Islands were warned to evacuate 6 hrs before the
waves arrived – few listened
The Pacific Tsunami Warning System came about because of
Hilo, but as early as 1923 seismologists at the Hawaiian
Volcano Observatory were accurately predicting the arrival
of these seismic waves
• Not a “disaster” but noteworthy
July 9, 1958 – Lituya Bay, Alaska
--- a giant rockfall created a seiche
[an oscillating wave in a lake or bay whose period can
last minutes or several hours]
--- sent a wall of water up alternating slopes of the
bay to heights of 1,700’ (520m)
… believed to be the highest known waves in
history
Storm Surge
Origin of Storm Surge
Storm surge (or storm tide) period of excessive high
water along a coast associated with changes on
atmospheric pressure and wind action on the
surface of the ocean
storm surge, especially in association with
normal high tide conditions, can be disastrous to
low-lying coastal areas
Storms at sea (esp. cyclones and hurricanes) are
centered around areas of intense low pressure
Under such storms, lowest pressure at its center
allows the sea surface to rise in comparison to
the storm’s periphery
--- this difference can result in a sea level
difference of as much as 39”
[13x denser than water a 3” decrease in mercury
pressure will raise water levels 13x greater]
… additionally, inwardly spiraling winds serve
to “drag” the sea surface upward
As the storm moves shore-ward, drag on the
surface of the sea pulls this “dome” of water with
the storm and piles the leading edge of the water
against the increasingly shallow shore
… this increases the height of the water and
produces the surge
Water continues to pile against the shore
steepening the slope of the wave until the
tendency of the water to flow downward and back
to sea is equal to the force of the wind driving the
water ashore
Interesting
If water along the shore is deep (a steep shore line), the surge
will not reach the ocean bottom and some of its waters will
escape downward and seaward --- decreasing the height of
the surge wave
If the water along the shore is shallow (shallow shore line), the
shore-ward surge will reach to the ocean’s bottom, no water
will escape, and the surge wave will grow higher
* Surge waves may last for several hours until the storm at sea
diminishes
Storm Surge Disasters
Bay of Bengal – bowl-shaped northern region of the
Indian Ocean [India and Bangladesh]
Storm waters in the Indian Ocean move to confined
Bay of Bengal where they are forced to rise
[esp. is they occur during monsoon conditions]
Area is alluvial delta only 12’ in elevation in places
1876: 100,00 to 250,000 dead
1970: 500,000 dead
1985: 10,000 dead
1991: 139,000 dead
Atlantic-Gulf Coastal Plain
Shallow coastal waters surrounding the United States have
caused major problems
1900: Galveston, TX, 13’ over the island, destroying the city
and killing 5000
1969: Hurricane Camille, surge est.s as high are 25”+,
reputed to have reversed the flow of the Mississippi River
1989: Hurricane Hugo, hit Charleston, SC with 16.5’ surge
1992: Hurricane Andrew, until Katrina the worst in terms of
damage and with 135 mph winds had almost no surge – it
apparently was moving too fast from Bahamas to FL and
kept running over its surge
North Sea (1953)
A slow moving mid-latitude cyclone over the North
Atlantic pushed water into the shallow North Sea
In combination with high tide conditions and
narrowing of the sea to the south, sea levels rose
6’ (eastern British Isles) to 11’ (Netherlands),
brought widespread destruction and killed more
than 1800 persons