Lab 6 Lecture

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Earthquakes
Lab 6
Concepts
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Diastrophism
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Types & causes of stress
5 types of folds
5 types of faults
Earthquakes
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Focus, epicenter
Types of energy waves
Methods of measuring earthquakes
Determining the magnitude of an earthquake
Determining the epicenter of an earthquake
Real-world example: New Madrid Fault
Diastrophism

Definition: deformation of earth’s crust

Deformation without movement
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Jointing: Fracture of rock without displacement
Affects resistance of rock to erosion (weakens)
Deformation with movement
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Folding: bending rock without breakage
Faulting: fracture of rock with displacement (either
vertical or horizontal movement)
Diastrophism
Types & Cause of Stress

3 Types of Stress
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Compressional: Rocks move together (convergent motion)
Tensional: Opposite movement (divergent motion)
Shearing: Tearing (transform motion)
Causes of stress
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Confining pressure
Temperature

Extreme heat  folds the rock without breakage

Extreme cold  fractures the rock
Strength/Composition of rock
Time
Diastrophism
5 types of folds
1) Monocline: one-sided slope. Slight bend
in otherwise parallel layers of rock.
2) Anticline: simple symmetrical upfold,
resembles an arch. Due to compression.
3) Syncline: rock is warped downward –
due to compression.
4) Overturned: upfold that has been
pushed so vigorously from one side that
it becomes over-steepened.
5) Overthrust: pressure was great enough
to break the over-steepened area and
cause a shear (a break).
Diastrophism
5 types of faults

Normal: One block is
displaced up, the other
down. Due to tension.

Reverse: A block is pushed
up and over the other. Due
to compression.

Strike-slip: Adjacent blocks
are displaced laterally.
Movement is entirely
horizontal. Due to shearing.
Diastrophism
5 types of faults (con’t)

Graben: Subsidence of
one middle block (it drops
down). Due to tension.

Horst: 2 reverse faults
push a middle block up.
Due to compression.
Earthquakes
Atlanta
New Madrid Fault
Earthquakes

Sudden vibration within lithosphere from a quick
release of energy
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Result of rock moving due to folding or faulting
From point of origin (focus), energy is transmitted to
surrounding rock by waves
Focus: Origin of stress and energy
release.
Epicenter: Surface location of focus
(directly above the origin).
Earthquakes
Types of Energy Waves

Body Waves
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Occur first. These are the initial waves emitted from the
earthquake. These occur in a specific order.


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1st wave: Primary “P” wave.
2nd wave: Secondary “S” wave.
Surface Waves

Occur after the body waves. These affect the surface of the
earth (we typically feel these).


Type 1: Love wave.
Type 2: Rayleigh wave.
Earthquakes
Types of Energy Waves: Body Waves

Primary Wave (P wave)
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Expansion & contraction of rock
as wave moves through it
Fastest body wave
Moves through solid rock and fluids
(e.g., ocean/water)
Secondary Wave (S wave)
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
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Wave moves through rock up and
down and side-to-side
Slower than P wave
Can only move through solid rock
Earthquakes
Types of Energy Waves: Surface Waves
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Love Wave
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Rolling/swaying effect
on surface
Moves the ground from
side-to-side
Fastest surface wave
Rayleigh Wave
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Rolls along ground like
an ocean wave
Type most often felt
during quakes
Earthquakes
Methods of measuring earthquakes
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Modified Mercalli Intensity Scale
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Measures “intensity” of earthquake (e.g., the amount of
shaking felt and the damage done).
Very subjective: depends on the viewer’s description of the
earthquake event! Based on observations.
Richter Scale


Measures the “magnitude” of earthquake (the energy
waves released).
Based on readings from a seismograph, and examining the
actual energy waves.
Modified Mercalli
Intensity Scale
I
Not felt
II
Felt only by persons at rest
III/IV
Felt by persons indoors only
V/VI
Felt by all: some damage to
plaster/chimneys
VII
People run outdoors, damage to poorly
built structures
VIII
Well-built structures slightly damaged,
poorly-built structures suffer major
damage
IX
Buildings shifted off foundation
X
Some well-built structures
destroyed
XI
Few masonry structures remain
standing, bridges destroyed
XII
Damage is total: waves seen on ground,
objects thrown into air
Richter Scale
Logarithmic Scale: Each increase in
magnitude is 10x more energy
released
• 5.0 is 10x greater than 4.0
• 5.0 is 100x greater than 3.0
• 5.0 is 1000x greater than 2.0
Earthquakes
Determining the magnitude of an earthquake

A seismologist reviews data taken by
a seismograph.
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Two important pieces of data to
record:
 Lag Time: difference in time
between the P wave and the S
wave (when each is picked up by
the seismograph).
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Designated as “S – P”
Given in seconds.
Amplitude: the size of the largest S
wave (the height of the wave).

Given in millimeters.
Earthquakes
Determining the magnitude of an earthquake
Result from a seismograph machine:
Seismogram
Earthquakes
Determining the magnitude of an earthquake
Take the
information
from a
seismogram
(lag time and
amplitude)
and
apply it on a
nomograph.
Earthquakes
Determining the location of an earthquake: Triangulation
Receive data from at least three seismographs…
Station 1: Eq occurred 10km from station
Station 2: Eq occurred 5 km from station
Station 3: Eq occurred 8 km from station
Plot them all together, and your
intersecting point is the epicenter!
Earthquakes
Example: New Madrid Fault
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Earthquakes & aftershocks of 1811 and
1812
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Tremors caused the Mississippi River
to flow backwards
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Caused Reelfoot Lake to be formed
Felt far away - steeple bells rang in
Boston
Sparse population in that region
accounted for a lack of serious damage
Richter scale had not been established,
but would have registered 8.5 to 9.0
A projected 60% chance for a(nother)
damaging earthquake by 2020 and
90% by 2040
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