Earthquakes and Related Hazards

Question 1

Where do earthquakes happen?

Answer 1

Most occur at plate boundaries:
- divergent & transform: shallow
- convergent: shallow to deep (small to largest)
~1 million detected per year (one every 2 minutes) [~18 per year of magnitude 7 or larger]
Many large cities at risk

Question 2

How do earthquakes happen?

Answer 2

Energy release: Elastic rebound
Stress -> gradual strain build up
Rupture: when elastic limit of material is exceeded
Stored elastic energy released as seismic waves

Question 3

What is an Earthquake?

Answer 3

Vibration of the Earth caused by rapid release of energy in the form of seismic waves

Question 4

what is the Focus/Hypocentre of an earthquake?

Answer 4

exact point of rupture

Question 5

what is the epicentre of an earthquake?

Answer 5

point on Earth directly above focus

Question 6

what type of subduction zone is the cascadia subduction zone?

Answer 6

locked subduction zone

Question 7

How do we measure earthquakes?

Answer 7

Earthquake Magnitude
Seismic Waves (recorded at seismograph)

Question 8

how does earthquake magnitude work?

Answer 8

Various scales (e.g., moment magnitude scale)
Based on amplitude of seismic waves on recording instruments
Reflects energy release (e.g., M6 releases 30 x energy of M5)

Question 9

which type of wave reaches a seismograph first after an eruption?

Answer 9

P (primary) waves arrive 1st

S (secondary) waves arrive 2nd

Question 10

what are P (primary) waves?

Answer 10

• compressional (sound waves)

- compression & dilation of material to transmit wave (particles move back & forth)

Question 11

what are S (secondary) waves?

Answer 11

• shear
• particles move perpendicular to wave direction
• cannot move through liquid/gas

Question 12

how do surface waves travel?

Answer 12

Surface waves travel slowly around the Earth rather than through it
- movement of particles like water waves, ripple the Earth’s surface

Question 13

how do we use P and S waves to calculate distance form the epicentre?

Answer 13

S arrival time minus P arrival time

Question 14

how many seismograph stations do we need to locate an earthquake?

Answer 14

3 or more

Question 15

what are the Primary effects of earthquakes?

Answer 15

Ground motion (shaking)
Amplification
Surface rupture

Question 16

what are the Secondary effects of earthquakes?

Answer 16

Fires
Landslides
Liquefaction
Tsunamis

Question 17

what are the impacts of earthquakes on humans?

Answer 17

10,000 people die per year (average)

Property damage > $30 billion/yr

Question 18

what is amplification?

Answer 18

there is a much greater shaking amplitude in unconsolidated sediment than bedrock

Question 19

what is liquefaction?

Answer 19

Unconsolidated sediment loses its strength, behaves like a liquid
No support of overlying structures

Question 20

what is a tsunami?

Answer 20

A series of waves generated by displacement of a large volume of water (most frequently by an earthquake)

Question 21

How can we minimise earthquake impacts?

Answer 21

Predictions are unreliable.
Understand earthquake (& related) hazards -> long-term “forecast”
- likely earthquake locations, size & frequency
- local conditions (likely amplification, liquefaction..?) -> what areas are most at risk?
Mitigate
- strengthen buildings, prevent collapse
- plan new developments according to hazard maps- earthquake early warning (e.g., Ocean Networks Canada, Uvic)
-community & personal preparedness