Transcript Tsunamis

Homework
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Read pages 56-61
Why are tides considered a dominant
influence on nearshore beach life?
Explain the books statement “the moon
does not exactly rotate around the earth?
How long is a full tidal cycle?
How large does an earthquake have to be
in order for a tsunami watch to be issued?
Define Diurnal, Semidiurnal and Mixed
semidiurnal tides.
“The Great Wave of Kanagawa” by the Japanese artist Hokusai.
Tsunamis
Definition
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Tsunami:
Japanese word: Tsu = Harbour, Nami = Wave
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It is NOT a Tidal Wave, that is the tide and is caused
by the moon, sun etc…
It is a SERIES of fast-moving, LONG WAVELENGTH,
waves generated by large disturbances of the Ocean
below or near the seafloor
Events which cause Tsunami’s
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Tsunamis are waves generated by
displacement of the ocean by impulsive
events
Events known to generate tsunamis:
Submarine earthquakes
Explosive volcanic eruptions
Submarine landslides
Terrestrial landslides that enter water bodies
Impacts of large extraterrestrial objects (e.g.
asteroids or comets) in the ocean
A Tsunami is one large wave
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False
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Tidal waves and Tsunami waves are
the same thing.
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False
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Which would NOT cause a Tsunami
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Landslide
Earthquake
Gravitational Pull
Meteorite
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Submarine earthquakes
Similar to the 2004 Tsunami
Submarine landslides, etc..
Tsunami from Arequipa earthquake,
June 2001
0
0.3m
Usually
takes
about 1516 hours
Image from www.pmel.noaa.gov/~koshi/peru/dcrd/maximum.gif
Facts
A tsunami travels at about 800 km per hour (500
mph) or more in deep water, so it can cross the
Pacific easily in a day, from South America to
Japan.
Wave height (from crest to sine- which is the water
surface) in deep water is usually a meter or less –
it is the long wavelength that is important, so
ships are OK.
The long wavelength means its speed is controlled
by water depth – it acts as a shallow water wave
even in deep water
Waves lose energy at an inverse proportion to their
wavelength.
Longer wavelength = less energy loss.
So – it is fast and doesn’t lose much energy….
When a tsunami enters shallower water the main
control of its speed – water depth – means it slows
down. Phew! BUT energy loss is insignificant and
the height of the wave grows – it is now easily
detectable.
A Tsunami acts as a
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Deep water wave
Shallow water
wave
Interactive wave
Tidal wave
D
1.
A rapidly rising
tide
1 m….
1957 &
1946
When it reaches the shore it may appear as a rapidly
rising and falling tide, or a bore, or one hell of a
towering, breaking wave.
In extreme cases, water levels can rise up to 20
meters or more – and it is not always the first that
is biggest, oh yes – and sometimes the trough
arrives first….. December 2004 Tsunami
Trough
arriving
first
in 1957
This is why
many people
Die
Not
understanding
what will come
next
A bore – April
1st 1946
A biggish wave – 1946,
Hilo
BIGGER? – 1998 Papua New Guinea
About 15 meters
BIGGER?
Hawaii - 1946: 17 m
2004 Tsunami
Destruction……
Damage is highly variable – can extend kms inland
and RUNUP (vertical height above sea level) 30
meters or more, it can also do nothing. Can cause
complete devastation – or nothing.
The natural topography of the coastline is a major
influence on the amount of Damage a Tsunami
creates.
Factors include nearshore beach slope, tide level,
onshore topography, shape of the coast, etc….
Destruction in another part of
Papua New Guinea 4m
Same place – note wooden building in background
The crest of a tsunami always arrives
first
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False
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After all that –
when I say TSUNAMI what do you think?
Do you have a pre-conceived idea of what they
are – even now is it hard to visualize?
The 2004 Tsunami has changed a lot of people’s
ideas about Tsunamis
Some may think they are like hurricanes
The flippant – not so funny anymore
HOTEL SIGN IN NEW ZEALAND
Could this Happen?
And, the “Must get my front row seat” attitude
60
People
Die
…….and then..
Some will realize that reality
is somewhere in the middle of all that.
Let’s have a look
New Zealand & Hawaii’s place in the
World
REMEMBER THE RING OF FIRE?
Warning Buoy
Tsunami Warning
Buoys
Earthquakes in the 20th century
We need some way of knowing what tsunami will do, because
they can obviously be very bad and they are entirely
unpredictable – or are they???
OK – so we cannot predict when and where the generating
process will occur – and we can’t even predict exactly if a
tsunami will be generated from a earthquake.
Examples:
1960 – Large Chilean EQ = Bad tsunami in Hawaii/NZ
2001 – Equally large EQ = 30 cm in NZ, nothing in Hawaii
Hmmmmmmm….
We use geological and historical records, and numerical
modeling to try and understand – and predict.
The geology and history show what happened in the past –
Ultimately the aim is to link models with the real time data
from the tsunami buoys to have some warning of what will
happen.
In the meantime – the models look good, but what do they
tell you?
Ultimately the aim is to link models with the real time data
from the tsunami buoys to have some warning of what will
happen.
In the meantime – the models look good, but what do they
tell you?
They make assumptions about the properties of waves and
therefore – are they based on reality?
A New Zealand case study or two….
New Zealand’s own unique problem
3 possible
Sources:
Earthquakes
Landslides
Volcanoes
ALL CAN BE FOUND IN
NEW ZEALAND !!!!!!!!
Earthquakes- Located on the edge
of the “Ring of Fire
Submarine landslides
3200 km3
150-300 million m3
~0.25 km3
Kaikoura submarine canyon
About
once every
160 years
There is a
submarine
landslide
1833 and
1728
Next???
Water
Level (m)
10
8
6
4
2
0
-2
-4
-6
-8
-10
Oaro
Z
Y
Tsunami generated by a
Kaikoura Canyon landslide
X
R. Walters NIWA
Kaikoura
South
Bay
Goose
Bay
Submarine volcanoes
VOLCANOES
NEW ZEALAND
In more detail
A
-32
150 min
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10
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0
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latitude
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Source
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165
from Walters & Goff,
2003
170
175
longitude
180
Why all this fuss? – Over a “rare” event
• They are
unpredictable
• They can
cause extreme
damage
Okushiri, 1993
• Hawaii
deaths:
• 1877 - 5
• 1946 - 159
• 1960 – 61
•2004 – 250,000
Tsunamis
They are also confounding – with our knowledge of
oceanography we should be able to understand them better.
Right?
They represent almost the “supreme wave” – every
fundamental piece of information on wave dynamics, seafloor
structures, coastal topography, and tectonics all fit together to
make each one unique.
It is that very uniqueness that is the key to really
understanding Oceanography
In Essence if we can understand Tsunamis we will have a
better understanding of Oceanography as a whole
Summary
A tsunami is a SERIES of fast-moving, LONG
WAVELENGTH, waves generated by large disturbances
of the Ocean below or near the seafloor
•(e.g.
submarine earthquakes, submarine landslides,
volcanic eruptions, meteor impact)
Can travel at 800 km per hour (500 mph) or more in
deep water
Damage is highly variable – can extend kms inland and
RUNUP (vertical height above sea level) 30 meters or
more, it can also do nothing
Summary
The natural Shape of a coastline is a major influence on
how the wave comes inland - as is nearshore beach
slope, tide level, onshore topography, shape of the coast,
etc….