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Lecture 10

GG231 Lecture Notes - Lecture 10: Pacific Tsunami Warning Center, Tsunami Warning System, 2004 Indian Ocean Earthquake And Tsunami


Department
Geography
Course Code
GG231
Professor
Rob Milne
Lecture
10

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LESSON 4: TSUNAMIS – CHAPTER 4, 12.1-12.5
4.1 Description and Processes
Tsunami
oSeries of waves caused by displacement of large volume of water, typically in an ocean or a large
lake
Tsunami waves
oDO NOT resemble normal sea waves because wavelength is longer
oWavelength in oceans are about 200 km compared to 10’s meters in normal waves
Amplitudes are less than 1 meter
oHeights can reach 10s of meters in large events
Tsunami misconceptions
oMisconceptions revolve around the origin of tsunamis
oTsunami consists of single wave that curls over and crashes on shore
Waves are actually turbulent, and oncoming surges of debris-laden water
It is a series of waves separated by minutes to more than an hour
Events that can trigger a tsunami:
oEarthquakes and landslides trigger most disastrous tsunamis
oExplosive volcanic eruption
oImpact in ocean of asteroid or comet
Earthquake- triggered Tsunamis
oCan cause tsunami by displacing the seafloor or the floor of a large lake, or even by triggering a
landslide
oDisplacing of seafloor happens when earth’s crust moves up or down rapidly during earthquake
along a fault
oTakes earthquake magnitude of 7.5 or larger to generate tsunami
4-step formation and development of a tsunami:
1. Earthquake rupture in seafloor pushes water upwards, starting the tsunami
This rupture doesn’t have to be only by earthquake but can happen with landslide, volcano
etc.
2. Tsunami moves rapidly in deep ocean reaching large speeds (950 km/hr and height<1m)
Height stays less than 1m but spaces between crests are often large (waves spaced far
apart)
3. Comes near land and slows speed (45km/hr) and gets squeezed upwards, increasing height
Water depth, velocity, and spaces between wave crests decrease
Increase in the wave height
4. Heads in land destroying all in its path
Sometimes trough of wave arrives first, causing the sea to recede and expose the seafloor
Bore: when one wave overtakes another, steep wall of water/bore is created
oHigh landward-flowing wave caused by collision of tidal currents
Run-up: maximum horizontal and vertical distances that the largest wave reaches as it
travels inland
oOnce run-up is reached the wave returns back to open ocean in strong flow
Edge waves:
Other types of waves a tsunami can generate
oTravel along shore
oInteraction between tsunami waves and complex can cause wave amplification
This causes the second or third wave to be larger than first
Distant tsunami/tele-tsunami:
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oTravels 1000 of km’s across open ocean before reaching land affects shorelines from; few km-
1000km from its source
oStrikes remote shorelines with little loss of energy MORE HAZARDOUS
Local tsunami:
oAffects shorelines near the source of the earthquake
oDistance of affected shoreline can be few km- 100 km or more
Subduction Earthquakes
oTrigger most distant tsunamis
oBut can produce both distant and local effects
Nearby Earthquakes
oCan be especially deadly because they arrive at shoreline soon after the quake, with little or no
warning
Ex: 7.8 M earthquake in Japan caused tsunami only 2-5 mins after the quake with no
warning
Calculating velocity of tsunami waves:
oEqual to square root of the product of the acceleration of gravity and the water depth
oAcceleration of gravity is approx.10m/sec^2
December 2004- Tsunami Indonesia
oPrior to this event not a lot of people knew what tsunami mean
o230 000 people were killed, many injured, and millions displaced in dozen countries surrounding
Indian Ocean (Sri Lanka, India, Thailand)
oNo warning system in place
o9.1 M subduction quake – struck off of the Indonesian island of Sumatra
Third largest in historic time Chile in1960 and Alaska in 1964
Occurred along fault that separates the Indo-Australia and Burma plates
Strain was accumulating for more than 150 years along the plates and resulted in a
convergence
The movement produces tsunami moving to Indonesian islands within minutes and other
countries hours later
Countries along Indian ocean didn’t have tsunami warning system (unlike Pacific)
Had an effective warning system been in place- many lives could’ve been saved
Landslide-triggered Tsunami:
oTsunamis triggered by the movement of a large mass of material into a lake or ocean which in
return generates the wave
oMany of these landslides are initiated by earthquakes
oJuly 1998- North Shore of Papua New Guinea:
Earthquake of 7.1 M happened and triggered a submarine landslide which cause the
tsunami
Tsunami happened at Sissano Lagoon (50 km away)
12000 people homeless and 2100 dead
This event highlighted the effects of a tsunami produced by the combination of a earthquake
and landslide
Earthquake alone would have not been able to generate the large tsunami
oNovember 1929 – Newfoundland, Grand Banks
Canada set off a huge submarine slump that travelled across Atlantic ocean and triggered
tsunami in south Carolina and Portugal damaged 40 coastal communities
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oJuly 1958- Lituya Bay
Most spectacular landslide-generated tsunami of 20th century
Earthquake rockslide rockslide plunged into the bay tsunami
The waves were elevated to 525 m
Destroyed forests on its path
Swept 3 fishing boats 1/3 was swept and spit into ocean
Volcanic-triggered Tsunamis
oLeast common of the triggers
oEruptions have potential to generate large tsunamis
oSmaller tsunamis can be triggered by large volcanic mudflows that enter sea after eruption
These are less hazardous
August 1883 – Krakatua
oSecond most deadly tsunami in history was caused by eruption of Krakatoa
Krakatoa- volcano between Indonesian island of Java and Sumatra
Triggered tsunami along shores of Sunda Strait that killed over 36000 people
Human Impact-triggered Tsunami
oNot a lot of human-generated hazards
oHuman-made explosions are the best example
EX: Halifax explosion in 1917
Damage is confined to the immediate blast area
Ship collided with a relief ship in Halifax Harbour
Fire from ignited cargo caused explosion and created tsunami-like wave
Natural Function Services-triggered Tsunami
oOther hazards that act to rejuvenate the vegetation cover, creating new habitats and possibly over
time increasing biodiversity
oIn areas with high development and population release of chemicals and material into water and
land systems can be detrimental and add onto tsunami
4.2 Geography of Risk
Ocean shores and lakeshores can experience a tsunami
oSome coasts re more at risk
Due to location with respect to earthquakes, landslides, and volcanoes
Coasts in proximity to major subduction zone or directly across ocean basic capable of
generating 9 M quake are at greatest risk
Pacific Ocean- subduction generated tsunamis
o85% of recorded tsunamis have been from here
oThere are a lot of subduction zones that surround much of the Pacific
oAreas @ pacific basin at greatest risk:
Japan
Kamchatka
Hawaii
Islands in southern and west Pacific
Chile
Peru
Mexico
North east Pacific coast form Alaska to northern California
oOther areas at great risk:
Mediterranean and eastern Indian Ocean
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