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Earthquakes and
Plate Tectonics
Some Definitions
• An earthquake is the
vibration of Earth
produced from the
rapid release of
energy
• The focus is the
location of this
energy release
• The epicenter is the
location on Earth’s
surface above the
focus
• The energy released at a focus
radiates out in all directions
• Nearly 1 million earthquakes occur
worldwide each year
• Most of which occur along plate
boundaries
Plate Boundaries
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Where Earthquakes Occur
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• Sliding plates cause deformation in the
crustal rocks on both sides of a fault.
• Friction impedes movement of the
plates.
• This causes energy to be stored (future
earthquake). The amount of energy
stored equals the magnitude of the
quake.
• Eventually the stored energy is released
(earthquake) at the weakest point
(focus).
• Deformed rock snaps back (elastic
rebound).
Elastic Rebound
Theory
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• Aftershocks
– Are additional movements and
adjustments in the rocks
following the main quake
• Foreshocks
– Are several small quakes that
can precede a major earthquake
• Not all movement along faults is
horizontal
• Vertical displacement is also
possible
• Some earthquakes occur at so
great a depth that no surface
displacement occurs
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Measuring Earthquakes
• Use a seismograph
machine
• Measure P, S, (bodywaves) and L (surfacewaves)
Types of Vibrations
• Two Types
– Surface Waves (L): travel along the
outer layer of
Earth
– Body Waves (P, S): travel through
Earth’s interior
• Two Types of Body Waves
– Primary (P): Travel by pushing and
pulling rocks in the
direction the waves are
traveling.
– Secondary (S): Travel by shaking the
particles at right
angles to their
direction of travel.
P Waves
S Waves
• P waves can travel through all
types of matter.
• P waves arrive at the recording
station before S waves.
• Fluids will not transmit S waves
(so they cannot travel through the
outer core).
Surface Waves
• Have an up and down motion as
well as a side to side motion.
• The side to side motion causes
most of the structural damage to
buildings and their foundations.
• Because they are confined to a
narrow region near the surface
they retain their maximum
amplitude longer (so they are more
dangerous).
• They travel more slowly than body
waves.
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Finding the Location of an
Earthquake
• Requires three or more seismic
stations reporting.
• The epicenter is found where their
circles intersect.
Travel-Time Graph
uses the difference in
arrival times of the first
P and S waves to
calculate how far away
the epicenter is.
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Earthquake Depth
• Earthquakes originate at depths
ranging from 5 to nearly 700 km
• They are classified as:
– shallow (< 60 km)
– Intermediate (between 60 to 300 km)
– Deep (> 300 km)
• About 90% of all quakes occur at
depths < 100 km
• All very strong quakes are shallow
• Quakes generated at ocean ridge
systems are shallow and weak
Magnitude of Earthquakes
• Richter Scale
– Logarithmic for amplitude (not
energy!)
– So an increase of 1 unit corresponds
to a 32 times increase in energy
released.
Magnitude, Effects, and
Number per year
Magnitude
Effect
< 2.5
Generally not
felt but
recorded
Often felt but
minor damage
2.5 – 5.4
5.5 – 6.0
Slight
structural
damage
Number per
year
900 000
30 000
500
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Magnitude
Effect
6.1 – 6.9
Can be
destructive to
populated
areas
Major quake
Serious
damage
Total
destruction
7.0 – 7.9
> 7.9
Number per
year
100
20
Every 5 to 10
years
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Earthquake Destruction
• Amount depends on:
– The intensity and duration of the
vibrations
– The nature of the material upon
which the structure rests
– The design of the structure
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Bay Area California, 1989
• Soft sediments amplify the
vibration more than solid bedrock
Other Risks
•
•
•
•
Structural Collapse
Fire
Landslide
Tsunami
Earthquake Hazards: Ground Shaking
Mexico City, 1985
• 8.1 Richter scale (7.5
aftershocks), but epicentre was
located off coast of Mexico –
effects only lasted 2-3 minutes
and should have been mild.
• Mexico City built on water
saturated lake sediment
• Standard of buildings were poor
• Population highly concentrated
• Shaking caused water-sediment
to behave and flow like a liquid
• Buildings move in rhythm with
ground – 800 buildings
collapsed, many ‘pancaked’
• 20,000 deaths 40,000 injured
• 100,000 homes destroyed
• Cost $4 billion
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Earthquake Hazards: Fire Storm
San Francisco, 1906
• IX on the Mercalli scale,
caused by movement on San
Andreas fault
• Areas of city built on
sediments damaged more
than bedrock areas
• Shaking burst gas pipelines
that then caught fire
• Mostly wooden buildings
• So many fires started that
people could not cope
• Fires burned uncontrollably
for 4 days
• ~3000 deaths mostly trapped
in fires
• Cost $500 billion (today’s
money)
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Earthquake Hazards: Liquefaction
Niigata, Japan, 1964
• Magnitude 7.5 on Richter
scale
• Buildings built on water
logged soil
• Shaking of soil breaks down
cohesive structure of soil –
it can behave and flow like
liquid
• Soil de-waters and water
can bust through surface
• Building foundations sink
into ground
• Strong concrete tower
blocks remain internally
undamaged – most were
jacked back up!
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Tsunamis
• Most result from the vertical
displacement of the ocean floor
during an earthquake.
• They travel at speeds of 500 to 800
km per hour
• While far away from shore, wave
height can be as small as 1 metre.
• Upon entering shallow water, wave
height grows to 30+ metres.
• The first warning is a rapid
withdrawal of water from beaches.
• This provides 5 to 30 minutes of
warning.
• The first wave is not always the
largest.
• Volcanic eruptions can also
generate tsunamis.
Earthquake Hazards: Tsunami
Anchorage, Alaska, 1964
• 8.6 on Richter scale,
occurred off coast of
Alaska, biggest ever in N
America
• Tsunami killed 107 people
along coast in Alaska.
• 6 m wave crashes across
California as a result.
• Wave spread out across
Pacific to S. America and
Japan.
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Tsunami damage in Hawaii: From 1960 Chile
earthquake, 15 hours later
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Tsunami spreads across Pacific
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Indonesia Tsunami
December 26, 2004
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The first five hours …
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Hours after the quake.
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Countries Affected
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Phuket – December 2004
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• An earthquake, 9.0 on Richter scale,
occurred off the west coast of Northern
Sumatra. It was later upgraded to 9.3.
• Tsunamis killed thousands.
• Thousands went missing.
• More than 1 million people were displaced
from their homes.
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• The earthquake set off a series of
aftershocks as the plate boundary
continued to shift.
• At least 15 aftershocks between
magnitudes 5.8 and 7.3 happened in the
next 12 hours.
• Aftershocks continued for months as the
plates settled into their new positions.
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• From this info, scientists have estimated
that the rupture zone was over 1200 km
long and up to 100 km wide, with most
of the movement in the southernmost
400 km of that zone.
• At the focus, it is estimated that the
plates moved about 20 m.
• The ocean floor moved about 10 m
WSW and was uplifted several metres.
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• Within hours tsunamis had hit
coastal areas around the Indian
Ocean as far away as 4500 km
from the epicenter
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Epicentre
Epicentre
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Previous Earthquakes
Earthquakes in area
since 1900
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Previous Tsunamis
Previous Tsunamis
in Area
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1 Month of
Aftershocks
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Earthquake Waves Inside
Earth
• Use information generated from P
and S waves to determine:
– thicknesses,
– states, and
– densities of various layers
• Crystalline rock will transmit
seismic waves more rapidly than a
layer of unconsolidated material.
• Seismic waves move faster with
depth because rock has been
squeezed into a more compact,
elastic material.
• Recall that P waves are “push-pull”
waves in that they oscillate in the
same direction as their direction of
motion.
• Recall that S waves oscillate at
right angles to their direction of
motion
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P Waves
S Waves
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• Recall that S waves cannot travel
through liquids.
• Recall that P waves travel faster
than S waves.
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• When seismic waves pass from one
material to another the wave is
bent (refracted) at the boundary.
• Also, some of the energy bounces
back (reflected) from the
boundary.
• Increased pressure will also refract
the waves.
• As a result of seismic waves, earth
scientists discovered:
– Crust (5 – 60 km thick)
– Mantle (2900 km thick)
– Outer core (2250 km thick)
– Inner core (2 400 km thick) or
(1 200 km radius)
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• The Mohorovicic discontinuity
(MOHO) separates crustal rock
from mantle rocks.
• It ranges in depth from 8 km
(under the oceans) to 50 km
(under the mountains).
• A shadow zone is a region that
does not receive P or S waves.
• A shadow zone is found on the
opposite side of Earth from a focus.
• A shadow zone is the result of
refraction occurring at the outer
core.
S-waves cannot
move through
liquids.
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P-waves move
through liquids but
are refracted.
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