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1906 San Francisco earthquake
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What are Earthquakes?
• The shaking or trembling caused by the sudden
release of energy
• Lithospheric plates move suddenly along faults
in the crust
• Usually associated with faulting or breaking of
rocks
• Continuing adjustment of position results in
aftershocks
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What is the Elastic Rebound Theory?
• Explains how energy
is stored in rocks
– Rocks bend until
the strength of the
rock is exceeded
– Rupture occurs and
the rocks quickly
rebound to an
undeformed shape
– Energy is released
in waves that
radiate outward
from the fault
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The Focus and Epicenter of an Earthquake
• The point on a fault
with the greatest
movement (where
the ‘quake originates)
is called the focus
• The point directly
above the focus on
the surface is the
epicenter
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Earthquake Impacts
• Ground shaking depends on:
– Distance to epicenter
– Area’s type of geology
– Size of the earthquake
• Liquefaction
– Vibrations cause ground water to rise and
turn solid ground into liquid
– Structures may sink into the ground
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• Ground Shaking
– amplitude, duration, and damage increases
in poorly consolidated rocks
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Ocean Earthquakes
-Tsunamis• Earthquakes can occur on faults in the ocean
• If the ‘quake is large enough, the energy can
cause a tsunami to form
• Tsunamis are fast moving waves that are no
higher than regular waves in deep water
• However, when tsunamis approach the
shore, they are forced to slow down, causing
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the wave to build into a huge wave!
• The Alaska earthquake of 1964 created
a tsunami that affected the coast of BC,
Washington, California, and Hawaii
• Port Alberni, on Vancouver Island
received major damage
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It took the tsunami 4.1 hours to get to Port
Alberni from Alaska
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The Economics and Societal Impacts of EQs
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Damage in Oakland, CA, 1989
Building collapse
Fire
Tsunami
Ground failure
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Where Do Earthquakes Occur and How Often?
• ~80% of all earthquakes occur in the
Pacific Rim
• Most of these result from convergent
boundaries (subduction)
• ~15% occur in the Mediterranean-Asiatic
belt
• Remaining 5% occur in the interiors of
plates and on spreading ridges
• Over 150,000 quakes strong enough to
be felt are recorded each year
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What are Seismic Waves?
• Seismology is the study of earthquakes and
their effects
• The energy released from a ‘quake travels in
the form of waves
• Two types:
– Body waves
• P and S
– Surface waves
•L
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Body waves
• Body waves travel through ground
• Faster than surface waves
• Primary (P) waves
– 6 km/s
– Move through any material (solids and liquids)
– Cause ground to stretch and compress in direction of
travel
• Secondary (S) waves
– 3.5 km/s
– Cause ground to squeeze/stretch at right angles to
direction of motion
– Travel through solids but NOT liquids!
– Cause more damage than P waves
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Surface waves
• Surface waves aka Love (L) waves
• Travel less quickly than body waves
• Travel along surface in rolling action
– Similar to ripples in a pond
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• P waves refract
(bend) as they
pass through
different earth
layers
• S waves
disappear at
bottom of
mantle (can’t
travel in liquid
outer core)
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Earthquake waves
• P- waves (primary)
– Fastest moving waves
– Compression wave
– Can travel through both
solids and liquids
• S-waves (secondary)
– Travel slower than p waves
– Shear wave
– Can only travel through
solids (not liquids)
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Body Waves: P and S waves
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Surface Waves: R and L waves
•
Surface Waves
– Travel just below or along the ground’s surface
– Slower than body waves; rolling and side-to-side
movement
– Especially damaging to buildings
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Seismographs record
earthquake events
•As the ground shakes during a ‘quake, the pen
remains motionless
•Produces a seismogram
•Gives time, duration, magnitude (amount) of
shaking
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How is an Earthquake’s Epicenter Located?
Seismic wave behavior
– P waves arrive first, then S waves, then L and R
– Average speeds for all these waves is known
– After an earthquake, the difference in arrival times at a
seismograph station can be used to calculate the distance
from the seismograph to the epicenter.
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How is an Earthquake’s Epicenter Located?
Time-distance graph
showing the average
travel times for P- and
S-waves.
The farther away a
seismograph is from
the focus of an
earthquake, the longer
the interval between
the arrivals of the Pand S- waves
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How is an Earthquake’s
Epicenter Located?
• Three seismograph
stations are needed to
locate the epicenter of an
earthquake
• A circle where the radius
equals the distance to the
epicenter is drawn
• The intersection of the
circles locates the
epicenter
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How are the Size and Strength of an Earthquake Measured?
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•
Intensity
– subjective measure
of the kind of
damage done and
people’s reactions
to it
– lines identify areas
of equal intensity
– Eg: Mercalli Scale
Modified Mercalli Intensity Map
– 1994 Northridge, CA earthquake,
magnitude 6.7
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How are the Size and Strength of an Earthquake Measured?
• Magnitude
– Richter scale
measures total
amount of energy
released by an
earthquake;
independent of
intensity
– Amplitude of the
largest wave
produced is
corrected for
distance
– Each step up on
scale = 10x larger
vibrations
– No upper limit
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Can Earthquakes be Predicted?
Earthquake Precursors
– changes in elevation or tilting of land surface,
fluctuations in groundwater levels, magnetic field,
electrical resistance of the ground
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Can Earthquakes be Predicted?
Earthquake Prediction Programs
– include laboratory and field studies of rocks before, during,
and after earthquakes
– monitor activity along major faults
– produce risk assessments
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Earthquake Risk in BC
•Pacific Plate trying to slide past North American
Plate along Queen Charlotte Fault
•Stress is building
•Juan de Fuca Plate subducting under North
American Plate
•Stress is building
•Major earthquakes in this area occur about every
150-300 yrs
•The last one was in 1700
•The “Big One” is coming and there is nothing we
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can do about it!