Week 4

Question 1

Megathrust Earthquakes

Answer 1

Special type of Convergent Boundary Earthquake, where the fault plane is almost horizontal
Mw 7+

Question 2

Megathrust Earthquakes (of the Past)

Answer 2

Tsunami Evidence
Submerged Coastlines
Large Underwater Landslides

Question 3

Tsunami Evidence

Answer 3

Finding geology that has been affected by a tsunami is a strong indicator of a nearby earthquake source

Question 4

Submerged Coastlines

Answer 4

Drowned forests are a common feature of plates that show a loss of stored stress, where the plate is allowed to un-deform and flatten to its original position, sometimes lowering the elevation of land

Question 5

Large Underwater Landslides

Answer 5

Underwater landslides are often strong indications of these hugely powerful events and the amount of shaking they produce

Question 6

The Cascadia Subduction Zone

Answer 6

• Subduction occurs where the Juan de Fuca plate moves under the North America Plate
• Strike-Slip occurs along the San Andreas and Queen Charlotte faults
• Since the spreading centre is so close to the subducting edge of the Juan de Fuca plate, the plate remains hot and buoyant, causing it to subduct at a very shallow angle and setting the stage for a megathrust environment

Question 7

The 1700 Cascadia Megathrust Earthquake

Answer 7

• Full rupture, breaking the entire 1100km-long Cascadia Subduction Zone and causing vertical displacement of ~20m, leading to drowned forests and the production of a very large tsunami event
• Drowned Forests, Carbon dating, Indigenous records, and Tsunami records prove date. 9 Mw

Question 8

The Sunda Megathrust Zone

Answer 8

• Subduction of the Australian and Indian Plates under the Burma and Sunda Microplates has created a very shallow megathrust margin
• 2004 Sunda Megathrust Earthquake, which ruptured with a MW 9.2 and resulted in ~230,000 casualties
• Damaged ocean floor to cause tsunami and earths wobble, slowing down Earth rotation

Question 9

Producing Tsunami

Answer 9

• Waves caused by a large, abrupt displacement of water; from Japanese: tsu-nami (‘harbour-wave’)
• Waves can travel up to 700km/h in the open ocean

Question 10

Imamura-Iida Scale

Answer 10

The tsunami magnitude scale used in this course; based on the max wave height in the open ocean ranging from -1 up to 4

Question 11

Shoaling Effect

Answer 11

Reduction in the water depth (like up a beach) increases friction under the tsunami wave, causing the waves to grow in height

Question 12

Draw-Down

Answer 12

The dramatic sea level drop along a coast prior to tsunami striking

Question 13

Run-Up

Answer 13

The maximum vertical height of the tsunami wave above shore level

Question 14

PTWC

Answer 14

‘Pacific Tsunami Warning Center’; an international response centre that issues tsunami warnings and watches for at-risk areas after M6.5+ earthquakes occur in or around the Pacific basin

Question 15

1964 Lituya Bay Tsunami

Answer 15

• A rockfall at the head of Lituya Bay produced a 150m-tall wave, reaching speeds of 150-210 km/h
• When the wave reached the end of the bay, it surged up to 524m tall
• Evidence of two previous, larger tsunami from Lituya Bay (1874, 1936)

Question 16

1946 Aleutian Island Earthquake and Tsunami

Answer 16

• Two subduction zone ruptures under the Aleutian Islands, the larger being a M8.1 earthquake, resulted in a tsunami more than 30m high striking the island chain
• After killing a lighthouse crew on Unimak Island, the tsunami rushed across the ocean at 780km/h and struck Hawaii less than 5 hours later, producing up to 17m high waves
• The tsunami resulted in 159 deaths in Hawaii, and prompted serious study of earthquakes and associated deadly tsunami