January 23 Lecture

Question 1

What happens when waves enter shallow water?

Answer 1

They interact with the ocean floor, the sea floor prevents the water particles from moving in circular orbits and forces them into elongated ellipses.

Question 2

What motion are water molecules near the sea floor restrict to?

Answer 2

Back and forth motions

Question 3

What effect does the the sea floor have on wave speed>

Answer 3

The compression of the wave, as well as friction with the ocean bed slows the forward motion of the wave.

Question 4

What happens to wave speed as water depth decrees?

Answer 4

Wave speed decreases as well

Question 5

When waves enter shallower waters what do they become?

Answer 5

Shallow water or intermediate waves

Question 6

What defines a shallow water wave?

Answer 6

Depth is less than λ/20

Question 7

How do you calculate the speed of shallow water waves?

Answer 7

square root of gH

Question 8

What is does the equation for shallow water waves tell us about how different waves interact at the same depth?

Answer 8

all waves in shallow
water travel at the same
speed in the same depth
water, regardless of period

Question 9

What happens to the velocity, period and wavelength of a wave as they enter shallow water?

Answer 9

the water is slowed but the period of the wave does not change so due to C=λ/T the wavelength must decrease

Question 10

What does a decrease in wavelength of a wave also tell us?

Answer 10

That the wave is getting steeper

Question 11

Due waves approach shore parallel to it ot at an angel?

Answer 11

Waves approach the shore at an angel

Question 12

What effect does the wave coming into the shore at an angel have on the shape of the wave?

Answer 12

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

Question 13

What causes a wave to break?

Answer 13

When wave steepness increases until the wave becomes unstable and breaks 1:7 ratio

Question 14

What is the surf zone?

Answer 14

The area offshore where waves are breaking.

Question 15

What are tsunami’s?

Answer 15

They are long-wavelength, shallow water waves caused by rapid displacmnet of the ocean water

Question 16

What can trigger an earthquake?

Answer 16

Seafloor earthquakes below the subduction zone, they have large vertical movement and generate tsunamis.

Question 17

How can we determine the size of a tsunami?

Answer 17

Their size is partly dependent on the size of the earthquake, An increase of one unit of magnitude.

Question 18

What would a change from a 8.1 to a 9.1 earthquake equate to for a tsunami?

Answer 18

• a 10-fold increase in wave amplitude on a seismogram or

* approximately a 30-fold increase in the energy released.

Question 19

How do tsunamis propagate away from a earthquake zone?

Answer 19

They radiate outward from the zone as a shallow- water ocean wave.

Question 20

How are tsunami’s considered shallow water waves in the open ocean?

Answer 20

Such waves have a wavelength that is greater than the depth of the
water
– Wavelengths often greater 200 km

Question 21

for a tsunami, Is the amplitude small or large in the open ocean

Answer 21

it is small

Question 22

The rate at which a wave loses its energy is inversely related to its what?

Answer 22

Wavelength.

Question 23

What are the characteristic of how a tsunami moves through the open ocean?

Answer 23

A tsunami not only propagates with a high speed, it
also can travel a great, transoceanic distance with only limited
energy loss.

Question 24

What happens when a tsunami approaches he coast line? (two things)

Answer 24

• 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

Question 25

true/false:

The first wave of a tsunami to reach the coast is often the largest and most powerful.

Answer 25

False, the first wave to reach the coast is often not the highest

Question 26

For a tsunami what happens when the crest arrives first?

Answer 26

The wave will arrive first,

it is associated with the upward crustal motion during original seismic event.

Question 27

What happens when the original event to cause the tsunami had a downward crustal motion?

Answer 27

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.