Earthquakes and Related Hazards Flashcards
Where do earthquakes happen?
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
How do earthquakes happen?
Energy release: Elastic rebound
Stress -> gradual strain build up
Rupture: when elastic limit of material is exceeded
Stored elastic energy released as seismic waves
What is an Earthquake?
Vibration of the Earth caused by rapid release of energy in the form of seismic waves
what is the Focus/Hypocentre of an earthquake?
exact point of rupture
what is the epicentre of an earthquake?
point on Earth directly above focus
what type of subduction zone is the cascadia subduction zone?
locked subduction zone
How do we measure earthquakes?
Earthquake Magnitude Seismic Waves (recorded at seismograph)
how does earthquake magnitude work?
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)
which type of wave reaches a seismograph first after an eruption?
P (primary) waves arrive 1st
S (secondary) waves arrive 2nd
what are P (primary) waves?
- compressional (sound waves)
- compression & dilation of material to transmit wave (particles move back & forth)
what are S (secondary) waves?
- shear
- particles move perpendicular to wave direction
- cannot move through liquid/gas
how do surface waves travel?
Surface waves travel slowly around the Earth rather than through it
- movement of particles like water waves, ripple the Earth’s surface
how do we use P and S waves to calculate distance form the epicentre?
S arrival time minus P arrival time
how many seismograph stations do we need to locate an earthquake?
3 or more
what are the Primary effects of earthquakes?
Ground motion (shaking)
Amplification
Surface rupture
what are the Secondary effects of earthquakes?
Fires
Landslides
Liquefaction
Tsunamis
what are the impacts of earthquakes on humans?
10,000 people die per year (average)
Property damage > $30 billion/yr
what is amplification?
there is a much greater shaking amplitude in unconsolidated sediment than bedrock
what is liquefaction?
Unconsolidated sediment loses its strength, behaves like a liquid
No support of overlying structures
what is a tsunami?
A series of waves generated by displacement of a large volume of water (most frequently by an earthquake)
How can we minimise earthquake impacts?
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
