January 23 Lecture Flashcards
What happens when waves enter shallow water?
They interact with the ocean floor, the sea floor prevents the water particles from moving in circular orbits and forces them into elongated ellipses.
What motion are water molecules near the sea floor restrict to?
Back and forth motions
What effect does the the sea floor have on wave speed>
The compression of the wave, as well as friction with the ocean bed slows the forward motion of the wave.
What happens to wave speed as water depth decrees?
Wave speed decreases as well
When waves enter shallower waters what do they become?
Shallow water or intermediate waves
What defines a shallow water wave?
Depth is less than λ/20
How do you calculate the speed of shallow water waves?
square root of gH
What is does the equation for shallow water waves tell us about how different waves interact at the same depth?
all waves in shallow
water travel at the same
speed in the same depth
water, regardless of period
What happens to the velocity, period and wavelength of a wave as they enter shallow water?
the water is slowed but the period of the wave does not change so due to C=λ/T the wavelength must decrease
What does a decrease in wavelength of a wave also tell us?
That the wave is getting steeper
Due waves approach shore parallel to it ot at an angel?
Waves approach the shore at an angel
What effect does the wave coming into the shore at an angel have on the shape of the wave?
This usually means one part of the wave is in shallow water, while the rest is in deeper water
The wave is thus refracted or bent
resulting in waves normally ending up reaching the shore almost parallel to it
What causes a wave to break?
When wave steepness increases until the wave becomes unstable and breaks 1:7 ratio
What is the surf zone?
The area offshore where waves are breaking.
What are tsunami’s?
They are long-wavelength, shallow water waves caused by rapid displacmnet of the ocean water
What can trigger an earthquake?
Seafloor earthquakes below the subduction zone, they have large vertical movement and generate tsunamis.
How can we determine the size of a tsunami?
Their size is partly dependent on the size of the earthquake, An increase of one unit of magnitude.
What would a change from a 8.1 to a 9.1 earthquake equate to for a tsunami?
- a 10-fold increase in wave amplitude on a seismogram or
* approximately a 30-fold increase in the energy released.
How do tsunamis propagate away from a earthquake zone?
They radiate outward from the zone as a shallow- water ocean wave.
How are tsunami’s considered shallow water waves in the open ocean?
Such waves have a wavelength that is greater than the depth of the
water
– Wavelengths often greater 200 km
for a tsunami, Is the amplitude small or large in the open ocean
it is small
The rate at which a wave loses its energy is inversely related to its what?
Wavelength.
What are the characteristic of how a tsunami moves through the open ocean?
A tsunami not only propagates with a high speed, it
also can travel a great, transoceanic distance with only limited
energy loss.
What happens when a tsunami approaches he coast line? (two things)
- Its speed decreases as water depth decreases
- Its amplitude increases as the energy is converted from kinetic into potential
- waves can easily reach 10+m in amplitude
true/false:
The first wave of a tsunami to reach the coast is often the largest and most powerful.
False, the first wave to reach the coast is often not the highest
For a tsunami what happens when the crest arrives first?
The wave will arrive first,
it is associated with the upward crustal motion during original seismic event.
What happens when the original event to cause the tsunami had a downward crustal motion?
The trough will proceed the crest (AKA it will come before the crest )
This causes the water along the shore to receed by up to 3-4 m over 2-3 minutes.
