EPSC 185 Lecture Notes - Lecture 9: Pressure Sensor, Plate Tectonics, Waikiki

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Tsunamis:
Tsunamis - wave that is generated when there is some sort of off set in the ocean floor
and the water in the ocean is also off set. When the water in the ocean is displaced, the
tsunami wave is generated. We have to have some mechanism to have that off set. There
are a number of mechanisms that displace water.
-The term tsunami is a Japanese word meaning harbour wave
-It was named so because the wave is harmless until it enters a harbour
-It is frequently called a “tidal wave”, but it has nothing to do with tides (it is true that if
you have high tide situation and a tsunami comes in, the impact will be greater but the
movement of the tides is separate from the tsunami wave).
-Tsunamis can hit with little or no warning (very dangerous and common)
-4,000 people were killed between 1990 and 2000
-More than 25,000 were killed in 2004 alone
-The most prone areas are those associated with earthquakes and volcanoes, mainly in
subduction zones
-During the 1990’s alone a few thousand people were killed because of tsunamis. They occur
around the pacific ocean base (around the pacific rim or ring of fire) where there are
subduction zones. The association between tsunamis and subduction zones is important
and mainly driven by earthquake activity but also volcanoes.
-26 December 2004 boxing day tsunami (1/4 million fatalities): very powerful earthquake off the
coast of Sumatra in Indonesia. It radiated across the Indian ocean but a lot of people lost their
lives in India and other countries.
-11 March 2011 (21,000 fatalities): magnitude 9 earthquake off the coast of Japan created a
large Tsunami. Off the pacific ocean but also hit South America, Hawaii and North America.
-The all important subduction association (ring of fire): where most of the action is.
-The subduction zone of Cascade has potential for very large offshore quakes
-There is a great danger of locally generated tsunamis here, since they travel so fast
-Many large cities are found on this coast
Principles behind tsunamis:
-The wavelength of a tsunami can exceed 200 km
-Normal ocean waves have wavelength of about 100 m
-Key terms: trough; peak or crest; wave height; amplitude
-The amplitude is from the still water line to the crest or peak of the wave
-The wave height is from the trough to the peak
-The wave length is the distance from one peak to the next or from one trough to the next
-Take home point: wave length of tsunamis especially in open water (where the water is deep),
can be very large (not something you can see in comparison to a normal ocean wave as the
distance is too large)
-Speed - we can look at the distance it takes for the wave to move from point A to point B
—> that travel time is called the wave period.
-To calculate the speed of a wave = wavelength/period
-Speed x period = wavelength
-Wavelength - distance from one peak to another, often really large
-T = wave period, i.e., time it takes for a pair of wave crests to pass by a certain point
-Tsunami speed in shallow water (at shore): speed is related to the square root of the
gravitational constant multiplied by the water depth.
-Tsunami speed in deep (ocean) water: square root of the gravitational constant
multiplied by the wave length divided by two pie
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-Tsunamis travel very quickly relative to normal waves
-Speed can reach 1,000km/hr in the ocean ocean (normal ocean wave: ~90km/hr)
-Tsunamis in the open ocean have very large wavelengths (they can sense the bottom of the
sea floor), typically ranging from tens to hundreds of km
-Hence they can “feel” the ocean bottom at any water depth
-Their crests can undergo bending around higher seafloor topography, which can focus or
defocus the wave
-The topography of the sea floor for example the mid ocean ridge which can influence in part the
direction of the Tsunami. If the topography that is flat, it does not affect the tsunami.
-Distant tsunami (a tsunami that is generated far away): as a wave propagates across the
ocean, there is time to warn people
-Local tsunami (very close to where people live). In a subduction zone where a tsunami is
generated, there is very little warning time (15-30 minutes typically)
—> Tsunami moves so quickly that it is vital in the case of a local situation to warn people
properly.
-A tsunami wave that moves from deep to shallow water. The waves are compressed and
pushed together and the speed of the wave slows down and the wave length decreases but the
amplitude increases.
-In shallow water, tsunami waves pile up
-As a result, velocities and wavelengths decrease
-But at the same time, amplitudes can increase enormously
-In deep water, wave amplitudes are generally less than 1 meter
-But in shallow water, amplitudes can reach 40 meters or more above normal sea level
-The wave will break when its heigh exceeds roughly one seventh of its wavelengths
-But many tsunamis do not break (comes in as a flood of water)
-Initially, there may be a rise or fall (drawdown) in sea level (which may attract people, to their
great misfortune)
—> When the wave arrives, the crest or trough of the wave may arrive first. If the crest
arrives first then the water comes inland. If the trough comes first then instead of the
height of the water increasing, it decreases (the water recedes and it attracts people but
then the crest arrives with the water and then it causes more damage)
-Due to its long wavelength, it may take a long time for a tsunami wave to crest (The water
comes in and hits the shore line and keeps flooding inland, especially if the shoreline is flat. The
wave keeps pumping water inland and then the wave finally crests and the water withdraws. It
can take a long time)
-Tsunami is usually a series of waves not just one wave (can go on for hours). If you
survive a tsunami, its probably not the end of the event. There will probably be more that
arrive.
-The wave then may remain high for several minutes
-And a tsunami is actually made up of a series of waves
-So it may take a while (hours) for the crests of successive waves to reach the shore,, so don’t
go surfing
-Wave runup (is quite complicated) and depends on several factors:
-Water depth
-Sea floor profile (gradual or steep)
-Shape of coastline (focussing of energy, tsunamis travelling up rivers) - continuous or irregular
x max: limit of landward incursion (m) - inland penetration as it floods inland from the coastline
A: wave amplitude close to shore (m) - height/amplitude near the close line
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Document Summary

Tsunamis - wave that is generated when there is some sort of off set in the ocean oor and the water in the ocean is also off set. When the water in the ocean is displaced, the tsunami wave is generated. We have to have some mechanism to have that off set. There are a number of mechanisms that displace water. The term tsunami is a japanese word meaning harbour wave. It was named so because the wave is harmless until it enters a harbour. Tsunamis can hit with little or no warning (very dangerous and common) 4,000 people were killed between 1990 and 2000. More than 25,000 were killed in 2004 alone. The most prone areas are those associated with earthquakes and volcanoes, mainly in subduction zones. During the 1990"s alone a few thousand people were killed because of tsunamis.

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