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ATOC 185 Final Exam Notes
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TSUNAMISTsunamis
- Damage after march 2011 earthquake in japan
- Wave generate when there is an offset in the ocea floor
- Ocean floor offset
- Water is also offset
- Wave displaces
- Tsunami wave generated
- Mechanism to cause the offset
- Japenese word, meaning harbour wave
- Wave in open water is moving, harmlessly
- Only when wave is close to shoreline that is becomes destructive
- Tidal waves, completely wrong
- Nothing to do with tidal wave
- Nothing to do with tides
- If u have a high tide, impact will be greater
- But tides are completely separate from tsunami wave
Hazard and risk
- Quite common and dangerous
- Kill thousands of people over a decade
- 1990s alone, few thousand people killed
- Almost all occur in pacific ocean basin
- Around pacific ring
- Pacific ring of fire
- Subdction zones
- Tsunamis and subduction zone very important
- Driven by earthwauke activity but also volcano
- Boxing day tsunami, powerful greater than magnitude 9 earthquake
- Sumatra indonesia
- Radiated across indian ocean basin
- Entire indian ocean basin
- Quarter of million people killed by wave
- In indonesia, india and sri lanka
- More recently, march 2011 mag 9 earthquake in japa, large tsunami coast of japan
- Swept across pacific ocean basin
- South america, hawaii…
Important subduction association
- Pacific ring of fire subdction zone association
- Generating waves
- Most of action is here
- Can occur in atlantic and indian
, - But focus on subduction big earthquakes and sometimes volcanoes
- Cascadia pacific north west, subduction zon
- Epotential for generating large earthquakes mag 9
- Tsuniami is also great
- In the region, a lot of hazard impct
- Doesn’t take much time for wave to be generated then hit coast
- Lot of people on pacific north west coast
Principles of tsunamis
- Structure of wave
- Any wave
- Bathtub or shoreline
- Crest of peak of wave
- Trough
- Amplitude from still water line to crest or peak
- Wave height troph to peak
- Wavelength of wave
- Distance from one peak to the next
- One trough to next
- Take home point
- Wavelength of tsunamis in open water especially
- Can be very very large
- Tens ot hundred of km
- Hard to conceive of
- Distance is ton large to see the wave length compares to normal ocean wave 50-100m
Speed of wave
- Look at distance wave for wave to move from point a to be
- Crest travel to point b
- Travel time, wave length, is called the wave period
- To caltuclate speed, wavelength divided bby period
- Near shoreline vx near open water
- Speed of wave shallow water
- Open water: speed, square root of gravitational constant multiply by wave length, divided
by 2 pie
- Calculations, see that tsunamis waves mvoe at hundred of km/hour, 700-1000km/h, hard
to conceive of
- Jet plane 700-1000km/hr, smae speed as tsunami almost
Tsunamis in open ocea
- Wave lengths are large
- Sense the bottom of seas floor
- May be very deep, few thousand m deep, but bc of large scale of wave, it senses bottom
of sea floor
, - Topography
- Mid ocean ridge may influence direction of tsunami
- But can be steers by nature of topography of sea floor
- But if it falt then no
Distant v local tsunami
- Distant: generated far away, earthqauke in japan, waves moves to central america, time
to warn people, tsunami is on the way
- Local: generated close to where people live, subduction zone, earthquake generated by
subduction process, tsunami goes to beach, happens quickly, only 30 mins tog et out of
the way, hard to warn people properly
Why do tsunami waves travel faster in deep water compared to shallow water?
- The speed of the wave in deep water is based on the wavelength of the tsunami which
can be very large (greater than 100km), whereas in shallow water the speed is limited by
the depth of the water.
Principle 2
Shallow water
- Deep to shallow
- Things are quished
- Waves compressed
- Speed of wave slows down
- Wavelength decrease, amplitude increase
Amplitutde
- Maybe a meter in wavelength
- Dominated by long wavelength
- As wave approaches shoreline, amplitude grows
- 1m in open ocean is 20-40m
- Grows enormously
- As waves arrives on shore
- Displacement of sea floor, displace volume of water
- Generation of wave with long wavelength
- Wave moes to shorem wavelength less and less
- Amplitude increases enormously
- Wave floss the shoreline
Arrival of a tsunami ona coast
- Some waves break
- But many don’y break
- Only break if hight of wave greater than 1/7 of wavelength
- But height doesnt approach wavelength in tsuami
- Flood of water
- Less as waves, more as floods of water comingin
, Drawdown
- Panel of arrival of 2004 indian ocean tsunami
- When wave arrives, crest of wave arrive first or trough
- If crest arrives then water flood comes inland
- But trough first, height ocean decreased, water recedes, low tide in fast forward motion
- Down and down nand down, sometimes see seabed
- Drawdown effect
- Whent rough arrives frist draw down effect attracts people
- Level of water lower than low tide
- Then the crest arrives
- Flood of water comes with the tsunami and people can be killed
- People have to know what to do when they see draw down, need to go in opporosite of
wave
Long wavelengths and the coast
- Bc wavelength is so lon, could take long for wav to crest
- Keeps flooding inland
- Wave pumping water inland
- Finally the wave crests
- Finally waster withdraw
- But can take long
- Minutes or tens of minutes
- Typically not one single wave incoming, series of waves
- Wave comes, recedes, again and again
- Series of waves can go on for hours
- Ifa a tsunami arrive, not the end of the event, there will likely be other waves
- Stay where u are in a safe zone
- Wait for the all celar to sound
- Whole sequence quill take hours
Wave runup
- How deep the water is
- What is the profile of the sea floor
- Gradual?
- Coastline?
- Lines we see, contours, of the tsunami generated during eruption
- Wide space of countrous, 10 minute contour
- Wider the space, the faster the wave is moving
- Fast in open water, slowing down as it gets closer to coastline
- Irregular coastline plays important role in how wave arrives
Inland penetraion of tsunamic along flat-line coastline
- X is inland penetration as it flood inland
- A is the hegiht of the wave, amplitude, near the coastline
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