Chapter 13 Earthquakes and Earth’s Interior
The crust of the Earth is made up of floating tectonic plates- huge
continent-sized chunks of solid rock floating on molten rock. Rock
masses along the boundaries of these plates are constantly exerting
huge forces as they butt up against each other.
These pressures can be in any combination of horizontal and vertical
directions. Although the pressure is constantly being exerted, once
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the pressure gets too great; a breaking point is reached and the
plates actually move. This happens along fault lines. The potential
energy (stored) that had been building up for some time finally
‘erupts’ into kinetic energy (motion)…the quake.
Cause- The Elastic Rebound Theory
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Definitions
seismologist – a scientist who studies earthquakes.
seismograph – a device that measures quakes.
seismogram – the paper tracing produced by the device.
epicenter – the place on the surface above the focus of the
earthquake.
magnitude- the strength of an earthquake.
focus – the location on the fault where there is the greatest amount
of movement. (Normally located many kilometers beneath the
surface.)
Effects
• Outward ripple effect
• Groundwaves up to 2 m high
• Buildings and bridges collapse
• Landslides
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• Fissures
• Tsunamis up to 70 m high
• Aftershocks sometimes greater than the initial quake
• Ruptured gas lines and fire
• Ruptured water mains
• Contaminated water supply – disease
Measuring earthquakes
Mercalli Scale – a classification based on observable phenomena
and damage caused by the quake. (I to XII) Very inaccurate because
it depends upon the observer and quality of building construction.
Richter Scale – a classification based on seismograph readings
where every whole number increase reflects a ten-fold increase in
ground motion and a thirty-fold increase in vibration energy.
Richter
Ground
Vibration
Number
Vibration
Energy
0
background
0
1
10
30
2
100
900
3
1000
27000
If the earth’s crust was of uniform density, the ripple effect would
decrease uniformly as you moved away from the epicenter. However,
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solid bedrock shifts very little compared to loose soil. Because of this,
quake intensities can vary widely from region to region.
P-waves – these waves move quite quickly and so are the first felt
ones felt. This is considered the quake. (Push-pull pressure waves)
S-waves – these waves move slowly, and although they originated at
the focus at the same time as the P-waves, they arrive later. These
are what cause aftershock. (Secondary, Side-to-side or shear waves)
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The above diagram is meant to show the effects
of Primary and Secondary waves on the Earth’s
crust.
Using P & S Waves to Find Epicenters
Because P and S-waves travel at different speeds, the difference in
their arrival time can be used to estimate the distance the epicenter of
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a quake is from the recording device (seismograph). If three
seismographs at remote locations record the same quake, the three
estimated distances can be used to pinpoint the epicenter
B
A
C
Where all the radii of the
estimated distances
cross, determines the
epicenter
• Activity 13D (II) and Discussion
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The Earth’s Interior / Plate Tectonics and Earthquakes
How do the tectonic plates move?
As magma is heated near the core of the Earth, it expands and rises
toward the surface, where it cools and sinks back toward the center
of the earth. This produces an up and down cycling of magma called
convection current. As the magma reaches the top. It cycles past
the crust where it ‘grabs’ the tectonic plate and pushes it along. Since
this cycling is continuous, so is the pressure applied by the magma.
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13.4 Earthquakes in BC
High risk earthquake zones have three characteristics:
The west coast of BC showing the Juan de Fuca
subduction plate (shaded) and other rifting faults.
•
Located near a plate boundary
• There is a history of earthquakes in the vicinity
• There are numerous active faults
The majour fault-lines are in the Pacific Ocean off the coast of BC
(See above map). These faults are part of the San Andreas fault
system which runs through California and is responsible for their
lively earthquake history. Movement in off-shore fault could produce
upheavals or drops in the ocean floor. This would most likely
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generate tsunami waves which would devastate the coastline,
causing huge geographic and environmental changes.
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13.5 Predicting Quakes
There are a number of indicators that a quake may be near. These
include changes in rock
a) density
b) electrical conductivity
c) gas content
It appears that just before the fault slips, very tiny cracks appear in
the rock surrounding the focus, these fill with water and are
responsible for the above ‘hints’.
Unfortunately it is not quite as simple as that and so far no real
success.
Using Earthquakes to Map the Earth’s Interior
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Similar to how whales, bats and other animals use sound to ‘echolocate’ prey, seismologists can use waves generated by earthquakes
to tell them about the shape and composition of the Earth’s interior.
Similar to sound waves, P and S-waves will change speed and
direction as they go from one area of density to another.
Seismologists have used these differences to map Earth’s interior
and to conclude that the Earth has a solid core of nickel and iron
surrounded by an outer core of liquid nickel and iron with a thick
mantle and a very thin crust (sorta’ sounds like a pizza!)
Activity 13E (Part II)
How many Sphere’s Does the Earth Have?
Three. (Okay, I’m lying…there are many subdivisions for each of the
following, but for the time being lets leave it at:
• atmosphere – air. 100 km thick
• hydrosphere – water. up to 11 km thick
• lithosphere – ground. about 6,400 km thick
The lithosphere is (again, for the time being) divided into four
subdivisions
a) crust – thin rock layer. 2-8 km under the oceans and 20 –
50 km thick on land.
b) mantle – mixed solid/liquid in constant motion, cycling
through convection currents. ~2,900 thick.
c) outer core – liquid iron and nickel at about 3,000oc. ~
2,200 km thick
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d) inner core – solid iron and nickel at about 6,000oc. the
tremendous overlying pressure keeps the inner core solid.
~1,300 km in radius.
Activity 13E (II) and Discussion
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