Week 5 Lecture: Earthquakes

Question 1

List the components of an earthquake at its center

Answer 1

• Eppicenter
• Focus
• Fault Plane
• Seismic Waves
• Fault Scarp
• Fault Trace

Question 2

Where are the Epicenter and Focus located in relation to the surface of the Earth?

Answer 2

Focus: underground centerpoint on the fault plane where the earthquake originates

Epicenter: projected surface point directly above the focus

Question 3

What kind of fault is the San Andreas Fault?

Answer 3

Transform Fault

Question 4

Explain the Elastic Rebound Theory

Answer 4

Extreme shear stress builds on either side of a fault. If external rigidity is exceeded, then a rupture along the fault line releases energy in a seismic wave before the rocks snap back to original position.

Question 5

List some internal Seismic Energy sources

Answer 5

• Earthquake faulting
• Buried Explosions
• Magma movements

Question 6

List some external Seismic Energy sources

Answer 6

• wind
• waves
• meteorite impacts

Question 7

What are the three stages of an earthquake

Answer 7

• Foreshock
• Mainshock
• Aftershock

Question 8

What are the two subcategories of Seismic Waves?

Answer 8

• Body Waves
• Surface Waves

Question 9

What are the two subcategories of Body Waves?

Answer 9

• Primary (P-waves)
• Secondary (S-waves)

Question 10

What are the two subcategories of Surface Waves?

Answer 10

• Love Waves
• Rayleigh Waves

Question 11

Differentiate Body and Surface Waves

Answer 11

Body waves travel through the Earth’s interior and spread outward from focus point in all directions

Surface waves travel in the crust and on the surface and spread outward from the epicenter

Question 12

List the order in which each wave occurs during an earthquake

Answer 12

• P-Waves
• S-Waves
• Surface Waves

Question 13

Describe and compare P and S-waves

Answer 13

P-waves:
- are compression waves in which rocks vibrate back and forth
- travel at 4-7 km/sec
- can also occur in liquids and gases

S-waves
- travel in shearing motion where rocks vibrate perpendicular to direction of wave
- travel at 2 -5 km/sec
- cannot occur in liquids and gases

Question 14

Do Body or Surface waves cause more damage?

Answer 14

Surface waves cause more damage (Love and Raleigh waves travel together)

Question 15

Describe and compare Love and Rayleigh waves

Answer 15

Love waves:
- side to side motion in horizontal plane
- perpendicular to direction of wave

Rayleigh waves:
- up and down motion
- opposite to direction of wave propagation

Question 16

How does Lithology affect the speed of seismic waves?

Answer 16

the velocities of seismic waves are slower through loose and uncompact soil/rock and faster through hard/compact rock

Question 17

How is an earthquake epicenter located?

Answer 17

• The time interval between arrival of P-wave and S-waves is measured
• Three seismometers measuring this interval are used to triangulate the position

Question 18

How does the Richter scale work?

Answer 18

• A vertical scale of interval between S and P-waves (in seconds) is placed on one side
• A vertical scale of amplitude (in mm) is placed on the other side
• In between the two scales, a vertical Richter magnitude scale is placed
• A line is drawn b/w the two outer scales, and the resulting line intercept on the Richter scale is read

Question 19

The Richter scale is logarithmic

Answer 19

Just remember that

Question 20

Describe the action of the plates in the 2011 Japan Earthquake

Answer 20

• Earthquake was result of thrust faulting near convergent plate boundary of three plates
• Pacific plate subducted under both Eurasian Plate and Phillipine Plate
• Phillipine also was subducting under Eurasian Plate

Question 21

What was the slip length and resulting water uplift that caused a tsunami during the Japan Earthquake?

Answer 21

• 18m slip length
• 4m uplift

Question 22

What unit of earthquake measurement is more common now than Richter Scale?

Answer 22

Moment magnitude (Mw)

Question 23

What is the equation for the Moment magnitude scale?

Answer 23

Moment = rigidity x area x slip

Question 24

What earthquake scale uses 12 stages of perception

Answer 24

The Modified Mercalli Intensity scale

Question 25

List and describe the two categories of earthquake effects

Answer 25

Primary effects: associated with ground movement during earthquake and seismic energy (Building collapse or damage)

Secondary effects: hazards related to response to primary effects (landslides, liquifaction etc.)

Question 26

Explain Liquefaction effects

Answer 26

• saturated soil turns into quicksand during earthquake
• sediments flow like fluids
• buildings etc sink and tip over

Question 27

How does liquefaction occur?

Answer 27

water in sediment completely surrounds all grains and eliminates grain to grain contact

Question 28

Describe the steps of a Tsunami

Answer 28

• underwater earthquake causes a break in sea floor that pushes water up
• wave generated of very long wavelength
• as water gets shallower, wave slows down and amplitude increases sharply

Question 29

How fast do tsunami waves travel in open ocean?

Answer 29

600 to 700 km/hr

Question 30

Describe two scientific methods of Earthquake Prediction?

Answer 30

• construction of seismic risk maps based on history, intensity, and likelihood of past earthquakes
• consideration of tectonic plate boundaries and major fault zones

Question 31

List some ways an Earthquake can be predicted

Answer 31

• probability map
• seismic gap (areas where earthquake hasn’t happened in a while)
• land elevation and tilting
• well water fluctuations
• bizarre animal behaviour
• foreshocks

Question 32

List and describe 3 factors of Earthquake Effects

Answer 32

Intrinsic factors: magnitude/type of earthquake, potential for tsunami etc

Geologic factors: underlying soil or bedrock etc

Societal factors: quality of construction, emergency response, population preparedness