Download Chapter 3: Earthquakes - Unama`ki Training & Education Centre

Chapter 3: Earthquakes
Damaged farm structures in the Edgecumbe, New Zealand
earthquake, 1987.
Videos
Earthquake Destruction
 Earthquakes 101
 Why do Earthquakes Happen?

What is an earthquake?
An earthquake is a trembling or shaking of the
ground caused by the sudden release of energy
stored in the rocks beneath the surface of the
Earth. In other words, the motion of the plates
causes enormous stress on the rock. Like
bending a stick, rock can only bend so far before
it breaks. When the rock breaks, waves of
energy are released through the Earth as an
earthquake.
What are seismic waves?
Seismic waves are waves of energy
produced by an earthquake. There are
several different types of seismic waves.
What is a fault?
A fault is a break between two rock masses.
Faults can be new/active (such as the San
Andreas Fault) or old/inactive (such as the
Aspy Fault in Northern Cape Breton)
What causes an earthquake?
The classic explanation of what causes an earthquake
is called the elastic rebound theory. Over many
years, tectonic forces act on the rock, causing it to
bend. Gradually, the bending becomes more and
more severe. Eventually, the strain exceeds the
breaking strength of the rock, and the rock breaks
suddenly causing an earthquake. The two masses
of rock move past each other along a fault. The
rock may move horizontally, vertically or both.
Recently, however, some scientists believe that faults
are weak, and that only small forces are needed to
cause an earthquake.
Also, extremely deep earthquakes (100-670 km
below the surface) may be caused by minerals
changing from one form to another
Elastic Rebound Theory
Videos
Elastic Rebound Theory (USGS)
 Breaking Stick

In general, where do earthquakes
occur?
Most earthquakes are associated with
movement along obvious fault lines (ex.
San Andreas Fault). But many
earthquakes occur along very old fault
lines (ex. Ontario & Quebec; offshore of
CB) or buried fault lines (ex. Northridge
quake outside of L.A. in 1994).
Earthquakes are also associated with
volcanic eruptions and plate boundaries
(ex. Mid-Atlantic Ridge).
What is the difference between a focus and
an epicentre?
The focus is the point IN the Earth where
seismic waves originate. This is the
centre of the quake and this is where the
first breakage occurs. The point ON the
Earth’s surface directly above the focus is
the epicentre.
Focus vs. Epicentre
What are the two types of seismic waves?
1. Body Waves: These travel through the
BODY of the Earth (or, through the
Earth’s interior), spreading out in all
directions. This happens just like the
exploding Death Star in Star Wars
Episode I.
2.
Surface Waves: Surface waves travel
on the Earth’s surface away from the
epicentre, like water waves when a
pebble is thrown into a pond.
There are two types of body waves:
1.
Primary wave (or P wave)

Compression wave or “pushing” wave in which the rock
vibrates in the same direction.

Just like a sudden push on the end of a stretched
slinky

Travels very fast (4-7 km/sec) INSIDE THE EARTH

Primary waves are the first (or primary) wave after a
quake
Secondary Wave (or S-wave)
 Back & forth (or shearing) kind of wave in which the
rock vibrates perpendicular to the direction of the wave
 Essentially, the rock vibrates back and forth just like
shaking a rope
 Secondary waves are slower (only 2-5 km/sec) and is
the SECOND type of body wave that is formed
2.
Surface Waves


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1.
2.
Surface waves are the slowest waves
that are set off by an earthquake
Surface waves cause the most damage
because they take longer to pass
There are two types of surface waves:
Love Waves
Rayleigh Wave
Love Waves Vs. Rayleigh Waves
1.


Love Waves:
The ground moves side-to-side
horizontally
Tend to knock buildings off their
foundations and destroy highway
overpasses
2.


Rayleigh Waves:
Act just like ocean waves, moving up
and down
Very destructive because they produce
more ground movement and take longer
to pass
How do we determine the location
of an earthquake?
P-waves are faster and always arrive at a
seismograph before the slower S-waves. The
farther the distance the seismograph is located
from the epicentre, the bigger the delay
between the first and second wave. This time
interval can be used to calculate the distance to
a quake. If three different seismograph stations
are used, the exact location of the quake can be
determined. Also, most quakes are shallow
focus quakes, in which the focus of the quake is
0-70 km deep (85% of quakes are shallow).
Measuring the Size of an Earthquake
The size of quakes is measured in two ways. One way is
to measure the intensity of the damage (what kind of
damage and how much damage). The scale that is used
to measure intensity is called the Mercalli Instensity
Scale (pg. 82).
The second method measures the amount of energy
released by the quake. The larger the quake, the more
the ground vibrates and the larger the wiggle on a
seismogram. Scientists can then assign a number called
a magnitude. The scale that is used to measure an
earthquake magnitude is called the Richter Scale.
Richter Scale
Facts About the Richter Scale
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A higher number on the Richter Scale indicates a more
severe earthquake
Largest recorded quake was 8.6 on the Richter Scale
The Richter Scale is a logarithmic scale. This means that
for each increase in the scale, there is actually a 10X
increase in the amount of movement.
Ex. If an Richter Scale 4 (or R4) quake
moves the ground 1cm, an R5 quake would
move the ground 10X more (or 10 cm). And, an R6
quake would move the ground 10X10 cm or 100cm;
R7 would move the ground 10X10X10 or 1000cm;
R8=10000cm; etc.
So, and R8 quake does NOT move the ground twice as
much as an R4 quake. Because the Richter Scale is
logarithmic, an R8 quake actually moves the ground
10,000 more than an R4 quake.
Energy from Earthquakes

In terms of energy, a
10X increase in
ground movement
causes a 32X increase
in the amount of
energy that is
released from a
quake. Look at this
example:
Magnitude
Energy Released
R4
1
R5
1x32=32
R6
32x32=1000
R7
32x32x32=32000
R8
32x32x32x32=1,000,000
Where do Earthquakes Occur on a
Global Scale?
1.
2.
3.
4.
Pacific Rim (80% of the world’s shallow
quakes)
Mediterranean-Himalayan Belt
Mid-Ocean Ridges
Some volcanoes (ex. Hawaii; Mount St.
Helens)
Active Earthquake Zones on a Global Scale
What is a Benoiff Zone?
When one ocean plate slides under the
continental plate forming a trench, many
earthquakes often occur. This zone of
increased seismic activity is called the
Benoiff Zone.
Earthquakes & Plate Boundaries
Earthquakes at Divergent Boundaries:
 Shallow quakes (not very deep)
 Lower magnitude (not as severe)
 Small area affected
 Form rift valleys (like on Mid-Atlantic
Ridge)
 Example: Quakes along the Mid-Atl. Ridge
Earthquakes at Transform Boundaries:
 Often shallow earthquakes
 Can be single or many fault lines
 Example: San Andreas Fault in California
Earthquakes at Convergent Boundaries:
 Wide zones of shallow earthquakes
 Plates may buckle (during mountain building) or
subduct (when trenches form), both of which
create earthquakes
 Example: Marianas Trench near the Philippines;
Himalayas Mountains
Earthquakes Away from Plate Boundaries:
 Very rare; represents only about 5% of quake energy
released in a year
 Also called intraplate earthquakes
 Triggered by a build-up of stress between the crust and
the upper mantle
 Occur in Ottawa region, St. Lawrence River Valley and
around the Grand Banks (off the coast of Louisbourg)
 Earthquake probability in Southern Ontario in the next
50 years:
M557% (occurred in 2010 & 2011)
M66%
M71%
Note: Miriamichi, NB experienced two large quakes (M5.7 &
M5.1) in 1982
Can we predict where earthquakes occur?
There are many ways to detect slight changes in the rock
and in the Earth’s surface:
 Small cracks open up causing small tremors to increase
 Changes in rock magnetism & other properties
 Changing water levels in wells
 Increase in radon gas emissions in wells
 Changes in Old Faithful and other geysers
 Surface of Earth tilts & changes in elevation
 Animal behaviour (dogs howling)
 Analyzing patterns of earthquakes (where they occur &
how frequently)
 Studying places along fault lines where quakes have not
occurred for a long time
Tsunamis
Tsunami is Japanese for “big wave in
harbour”
 A tsunami is a large wave created by an
earthquake, landslide, or meteorite
impact.
 Usually, tsunamis are caused by
earthquakes greater than 6.5 on the
Richter Scale

Videos
Tsunami Formation
 Tsunami Animation
 Alaskan Mega Tsunami
 Japanese Tsunami 2011
 East Coast Tsunami
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