MIDTERM CH. 5

Question 1

1. Tsunami

Answer 1

A large wave caused by a seismic event.

Question 2

2. Where is a tsunami not a hazard?

Answer 2

In the open ocean, where this low, long fast wave is generated, it is not a hazard.

Question 3

3. Where is a tsunami most hazardous?

Answer 3

These waves are a significant hazard when they approach a coast.

At a coastline, as water depth increases, the energy of this wave and the water it is displacing is forced upward.

Wave length decreases, but wave height increases. (You end up with a very tall wave (several meters tall))

This much larger wave then breaks against the shore.

Question 4

Tsunami characteristics

Answer 4

Tsunamis have:

• very small amplitudes (1-3 meters)
• very long wave lengths (hundreds of kilometres)
• very fast speeds (hundreds of kilometres per hour)

Question 5

4. How are tsunamis generated?

Answer 5

Tsunamis are generated when energy is transferred from the crust to the water - a seismic event.

This transfer of energy may be the result of:

1. Earthquake
2. Volcanoes
3. Mass wasting

Question 6

5. How does an earthquake generate a tsunami?

Answer 6

During an earthquake, the crust beneath the ocean must move vertically.

As the surface of the ocean returns to normal - the hole fills in - a series of waves are generated that travel outward from this point of origin.

Question 7

6. How does a volcano generate a tsunami?

Answer 7

1. An explosive eruption next to or beneath the ocean can generate a tsunami.
2. The collapse of an underwater volcano as the magma chamber empties during an eruption.
3. Underwater landslides from a volcano between eruptions. (Movement of a large mass of rock down the flank of a volcano can create a wave)

Question 8

7. What particular features of a shield volcano contributes to the creation of a tsunami? (part 1) (see diagrams 1-3)

Answer 8

As a shield volcano forms, it does so on the sea floor.

Because lava cools very quickly on contact with water, the rock that forms has little to no crystalline structure.

Also, pillow structures may form.

As a result, the rock that forms the underwater part of the volcano is structurally weak.

Question 9

8. How does a landslide or rockfall generate a tsunami

Answer 9

The movement of a large mass of rock beneath or into the water can cause a tsunami.

Question 10

9. What two factors control the formation of a tsunami by a rockfall?

Answer 10

The size of a wave generated by mass wasting will depend on two things:

1. The volume of rock that moves
2. The heigh that it falls from.

Question 11

Pillow structures (see diagram)

Answer 11

These are spherical or bulbous shapes in the rock.

Question 12

7. What particular features of a shield volcano contributes to the creation of a tsunami? (part 2) (see diagram)

Answer 12

Once the volcano reaches seal level and above, the rate of cooling is slower and the crystalline structure of the rock is better and the rock is stronger.

As a result, there is a solid cap of rock on top of weaker rock beneath it. In places like Hawaii, this exposed (above sea level) rock is prone to failure.

If enough rock suddenly falls into the ocean, a tsunami can be generated.

Question 13

10. What is run-up height? (see diagram)

Answer 13

Elevation on land that the water reaches

Question 14

11. What is run-out distance? (see diagram)

Answer 14

Distance a wave reaches horizontally on land.

Question 15

12. Trimline

Answer 15

The maximum height and distance reached on land by the wave as recorded by the damaged done to vegetation, usually trees, by the water.

Question 16

13. Describe the events that took place in Lituya Bay, Alaska, July 9, 1958.

Answer 16

In this fjord, an earthquake generated a large landslide from a mountain next to the ocean.

Upon impact with the water, a 150 meter wave was generated.

There were 6 people in the bay and 4 survived.

The run up height was 524m.

Question 17

14. Burin Peninsula, Newfoundland, Nov 18 1929.

Answer 17

On this date, a large earthquake happened in the Gulf of St. Lawrence.

As a result, a large underwater land took place off the coast of Newfoundland near the Burin Peninsula.

This landslide create a tsunami that caused significant damage and the loss of 29 lives.

Question 18

15. What are tsunami laid sediments? (part 1)

Answer 18

Waves break against a shoreline because they run out of room. And as the wave breaks, water travels onto the land.

Water slows down until it reaches zero velocity at the maximum run-up height, run-out distance.

At this point, some water may stay on the land - pooled on the surface - or infiltrate into the ground.

As a result, less water flows back into the ocean.

This return flow may also be slower because it is driven only by the force of gravity.

Question 19

15. What are tsunami laid sediments? (part 2)

Answer 19

Sediment originally on the coast (e.g. beach) may be eroded and transported on land.

It may remain there because return flow is too weak to move it back to the ocean.

(see diagram)

The end result is a layer of sediment - a tsunami deposit - that is thicker and coarser (larger grains) near the coast and thinner and thinner (smaller grains) further inland.

While a lot of coarser sediment can be moved where wave energy is high near the coast, further inland where wave energy is less only smaller amounts of finer sediment is moved.

The structure of a tsunami deposit will reflect the different processes as water moves on land and returns to the ocean.

Question 20

Seiche (see diagram)

Answer 20

A wave generated in an enclosed body of water that behaves like a tsunami.

However, a seiche is wind generated, not seismic.

At A, water is pushed up against the shoreline by the wind.

Eventually, this extra water flows back into the lake.

As the moving water bounces back and forth across the lake basin it creates waves that impact the shoreline.

Question 21

Bc Tsunami deposits

Answer 21

Massive, normal graded and horizontal bedding structures that reflect changing flow conditions.