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EARTHQUAKES
Did you know…
 Globally, there are
over 50o ooo
earthquakes each
year
 100 000 can be felt
 100 cause damage
to Earth’s surface
 Introduction video
What is an earthquake?
 Trembling or shaking of
Earth’s crust created by a
release of energy stored in
the rocks beneath the
surface
 Rocks between two plates
bend until the strain is too
great, and then the rock
breaks, releasing seismic
waves that produce
shaking.
Focus: actual location
in the earth where
Where?
movement originates
Epicentre: point on the surface directly
above the earthquake's focus VIDEO
• The seismic waves produced by an
earthquake radiate out from its focus.
• These waves cause the earth to vibrate in
response to the pushing and pulling forces
that are applied to them.
• Two category of waves (each with
subsets):
– Body waves: Primary(P), Secondary(S)
– Surface waves: Love(L), Rayleigh (R)
SEISMIC WAVES: Body Waves
• Body
waves arrive first since they are the fastest. Body
waves are broken down into two types:
•Primary (P) waves, which cause the rock particles through
which they pass to shake back and forth (compression
effect) and can result in a noise similar to a sonic boom.
•Secondary (S) waves make the particles vibrate both
vertically and horizontally (shearing effect).
• Note that Body waves travel mostly downwards into the
interior of the earth and are rarely felt by humans. They
disappear as soon as they hit a gas (atmosphere).You can hear
Body waves, but it is the next set of waves that actually move
outwards along the ground – causing damage.
SEISMIC WAVES: Surface Waves
Surface waves arrive last and as their name implies they
travel along the surface of the earth. It is these waves that
cause damage. They begin with:
•Love/Long (L) waves that move the ground from side to
side, again, in a shearing effect. These are followed by:
• Rayleigh (R) waves which are similar to ocean waves.
These cause surface materials to move in a vertical circle
just as a floating object would move as a sea wave passes
under it. These waves are responsible for most of the
damage to buildings.
• The greatest shaking occurs near the centre of large
earthquakes. It is here that all the waves originate.
Measuring and Predicting
• A seismograph is used to detect and record
seismic waves
• There are more than 10 000 seismograph
stations worldwide
• Video:
How seismographs
work
MEASURING THE DAMAGE
The violence of an earthquake can be measured using
two different scales:
The Richter scale - devised in 1932 by Charles
Richter
The Richter scale actually uses a numerical value. It is
important to note that this scale is logarithmic in nature
meaning that the numbers measure in factors of 10. In
other words, an earthquake measuring a 4 on the
richter scale is 10x as damaging as an earthquake that
measured 3 on the richter scale.
The Mercalli scale - devised in 1905 by a gentlemen
named Mercalli. The Mercalli scale is a visual scale
based on the destruction that occurs.
Earthquake Hazards
-Ground shaking causes buildings to collapse
Earthquake Hazards
-Liquefaction occurs when loose soil temporarily
turns from a solid to a liquid during an earthquakeresembles quicksand and will not support buildings
Earthquake Hazards
- Tsunami- a series of large destructive sea waves
caused by an earthquake under the ocean floor
Earthquake Hazards
- Aftershocks- a series of smaller earthquakes that
occur after the main tremor. Usually smaller than
initial quake but can cause considerable damage
-Fire- due to broken gas and water mains and fallen
electrical wires
Earthquakes in Canada?
 VIDEO
 There are 3-4 earthquakes in Canada each DAY
 Most can only be detected by seismographs
 Only one person has died in Canada as a result of an
earthquake in Montreal in 1732
 Read the case study about Vancouver, answer
questions # 1,2,5