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Chapter 12 Teacher Notes – Deformation and Earthquakes
Lesson #1- Law of Superposition
Using a few basic principles, scientists have developed laws to determine the order and age in which rock
layers formed.
The Law of Superposition states that a sedimentary rock layer is older than the layers above it and younger
than the layers below it if the layer are not disturbed.
Law of Crosscutting Relationships
When rock layers have been disturbed by faults or intrusions, determining relative age may be difficult.
In such cases scientists apply the Law of Cross-Cutting Relationships which states a fault or igneous
intrusion is always younger than the rock layers it cuts though.
Folds
- When a rock is compressed or squeezed inward it will fold.
- Although a fold commonly results from compression, it can also from as a result of shear stress.
3 Types of Folds - .
(1) Syncline
They occurs at a convergent boundary.
Compressed Rock fold the surrounding rock upward into a bowl shape.
The youngest layer is in the center of the this fold and if the softer rocks are in the core of the syncline, they
will erode to form a valley, but if the inner layers are harder, erosion carves them into a peak instead.
Think of it as a simle
(2) Anticline
Occurs at a convergent boundary.
An anticline is a fold that arches upward and looks like a bowl that is turned over.
Think of it as a frown
The oldest rocks are found at the center of an anticline and the youngest ones are draped over them at the
top of the structure.
(3) - Monocline
Occurs at a divergent boundary.
Is a fold in which is a simple bend in the rock layers so that they are no longer horizontal but are inclined.
Monoclines form when one part of Earth’s crust moves up or down relative to another part.
In a monocline, the oldest rocks are at the bottom and the youngest are at the top
Sizes of Folds
Folds vary greatly in size. Some folds are small enough to be contained in a hand-held rock specimen.
Other folds cover thousands of square miles can be seen only from the air.
Strike-Slip
Lesson 2 - Faults
Stress on rock can also cause rock to break.
Breaks in rock along which there is no movement of the surrounding rock is called a fracture.
A break along which the surrounding rock moves is called a fault.
A fault is a break in a body of rock along which one block slides relative to another.
Anatomy of a Fault
The hanging wall is the rock above the fault plane.
The footwall is the rock below the fault plane.
Three Types of Faults - (Picture see lesson 2)
There are three types of faults
(1) Normal Faults
(2) Reverse (Thrust) Fault
(3) Strike-Slip Faults
1) Normal Faults
A normal fault is a dip-slip fault in which the hanging wall moves downward relative to the footwall(Think of it
at an extentional fault).
Normal faults commonly form at divergent boundaries, where the crust is being pulled apart by tension.
Example’s of normal faulting include the Teton Range, Wyoming ( Rockies).
2) Reverse Faults & Thrust Faults
When compression causes the hanging wall to move upward relative to the footwall, a reverse fault forms.
A thrust fault is a special type of reverse fault in which the fault plane is at a low angle or is nearly horizontal.
Reverse faults & thrust faults are common in convergent boundaries and responsible for steep mountain
ranges, such as the Himalayas and the Alps.
Stresses that raised up the Rocky Mountains caused a block of ancient Precambrian crust to be thrust more
than 50 miles over much younger Cretaceous rocks.
Result - upper rocks are more than 1 billion years older than the lower rocks.
3) Strike-Slip Faults
In a strike-slip fault, the rock on either side of the fault plane slides horizontally (or by each other) in
response to shear stress.
Strike-slip faults got their name because they slide, or slip, parallel to the direction of the length, or strike, of
the fault.
Strike-slip faults commonly occur at transform boundaries like our own San Andreas Fault.
Sizes of Faults
Like folds, faults vary greatly in size. Some faults are so small that they affect only a few layers of rock in a
small region.
Other faults are thousands of miles long and may extend several miles below Earth’s surface.
The San Andreas fault is an example of a large fault system.
Lesson 3 - Why Earthquakes Happen
Earthquakes are a movement or trembling of the ground that is caused by a sudden release of energy when
rocks along a fault move.
Earthquakes usually occur when rocks under stress suddenly shift along a fault.
The shift is a response to the stress that has built up at a fault, deforming the locked up crust.
Geologists believe earthquakes occur as a result of elastic rebound.
Elastic Rebound
Elastic rebound is the sudden return of elastically deformed rock to its un-deformed shape.
Just like a rubber-band.
This occurs when rocks are stressed past the point at which they can maintain their integrity.
At this point they fracture then separate at their weakest point along the fault and rebound, or spring back to
their original shape.
Anatomy of an Earthquake
The point on Earth’s surface above an earthquake’s starting point is called the epicenter.
The focus is the starting point within Earth (underground) at which the first motion of an earthquake
occurs.
Although the focus depths of earthquakes vary, 90% of continental earthquakes have a shallow focus.
Earthquakes that cause the most damage usually have shallow foci (less than ~ 40 miles deep).
Seismic Waves
When an earthquake occurs it gives of energy in the form of seismic waves.
Seismic waves are the waves of energy caused by the sudden breaking of rock within the earth or an
explosion.
Each type of wave travels outward from the focus and their speed and direction is effected by Earth’s
interior.
The energy released by an earthquake (seismic waves) are recorded on seismographs.
Earthquakes and Plate Boundaries
The diagram below shows the different tectonic boundaries where earthquakes typically occur.
The three main types of tectonic settings that earthquakes occur at are:
(1) Convergent oceanic environments,
(2) Divergent oceanic environments, and
(3) Continental environments.
(1) Convergent Oceanic Environments
Convergent oceanic boundaries can occur between two oceanic plates or between one oceanic plate and
one continental plate.
(2) Divergent Oceanic Environments
Earthquakes can occur along mid-ocean ridges because oceanic lithosphere is pulling away from both sides
of the ridge.
(3) Continental Environments
Earthquakes also occur at locations where two continental plates converge, diverge, or move horizontally in
opposite directions at transform boundaries.
Lesson 4 – Types of Seismic Waves
Two Types of Seismic Waves
(1) Body waves are seismic sound waves that travel through the interior of Earth.
(2) Surface waves are seismic sound waves that are formed by the interaction of P and S waves with
Earth's surface.
Body Wave #1 - P waves (Primary Waves)
The fastest kind of body wave is the P-wave or primary wave.
P-waves are also referred to as compression waves because the rock it moves through it pushes and pulls
just like sound waves push and pull the air.
Example: thunder
Not only do you hear it but it also is able to rattle the windows at the same time because the sound waves
were pushing and pulling on the window glass much like P waves push and pull on rock.
The P-wave will be the first to 'arrive' at a seismic station.
It can travel through any type of material, including fluids, and can travel at nearly twice the speed of S
waves.
In air, they take the form of sound waves, hence they travel at the speed of sound (768mph).
Sometimes animals can hear the P-waves of an earthquake.
Dogs, for instance, commonly begin barking hysterically just before an earthquake 'hits' (or more specifically,
before the surface waves arrive).
Body Wave #2 - S waves (Secondary Waves)
The second type of body wave is the S-wave or secondary wave.
S-waves move rock particles up and down, or side-to-side--perpendicular to the direction that the wave is
traveling in.
An S-wave can be up to 50% slower then a P-wave depending on the rock it is going through.
They can only move through solid rock, but not through any liquid medium which includes our liquid core.
It is this property of S-waves that led seismologists to conclude that the Earth's outer core has the
consistency of liquid.
Seismic Waves and Earth’s Interior
By studying seismic waves, scientists have discovered Earth’s five structural layers:
The lithosphere,
The asthenosphere,
The mesosphere,
The outer core,
The inner core.
Shadow Zones
The properties of body waves also create a shadow zone.
A shadow zone is an area where no direct seismic waves from an earthquake can be detected.
Surface Waves
Events with magnitudes greater than 4.5 are strong enough to be recorded by a seismograph anywhere in
the world, so long as its sensors are not located in the earthquake's shadow.
The shadow zone results from the S-waves being stopped entirely by the liquid core and P-waves being
bent (refracted) by the liquid core.
Surface Waves
When a body wave gets to the surface it creates two distinct surface waves, which move slower and at a
lower frequency making them easily distinguishable on a seismogram.
Because of their long duration, and large amplitude, they can be the most destructive type of seismic wave.
Surface waves are a lot like water waves.
Two Types of Surface Waves are – 1) Love
Named after A.E.H. Love, a British mathematician, these waves move in side-to-side, perpendicular motion.
They are about 90% of the S-wave velocity, and have the largest amplitude.
Love = Hugs
Two Types of Surface Waves are – 2) Rayleigh
The other kind of surface wave is the Rayleigh wave, named for another mathmatician Lord Rayleigh.
Rayleigh waves are surface waves that cause the ground to move with an elliptical, rolling motion, like riding
a bike / skateboard of hills.
Lesson #5 Studying Earthquakes
The study of earthquakes and seismic waves is called seismology.
Seismologist can detect and record vibrations in the ground using an instrument called a seismograph.
Seismograph
Seismographs record three types of ground motion—vertical, east-west, and north-south.
Because they are the fastest, P waves are the first seismic waves to be recorded by a seismograph.
S waves are the second seismic waves to be recorded, and surface waves are the last to be recorded by a
seismograph.
Locating an Earthquake
To determine the distance to an epicenter, scientists can use the simple formula of (distance = velocity x
time (d=rt).
In order to save time a lag-time graph was created.
By analyzing the difference in the arrival times of the P & S waves seismologist can determine the distance
to the epicenter, using this graph.
Scientists use the triangulation method which means they need information from at least 3 seismograph
stations to locate the earthquake.
Lesson #6 History of Earthquake Measurement
Intensity measures the strength of shaking produced by the earthquake at a certain location.
Earthquakes were originally measured based on intensity or damage caused.
Mercalli Scale
The modified Mercalli scale expresses intensity in Roman numerals from I to XII (12 being most) and
provides a description of the intensity effects of an earthquake on humans at the surface.
Evolving Technology
However, comparing earthquakes is complicated because some are gentle (roll), others are violent (jolt).
Intensity also varies the farther from the epicenter of the earthquake.
Richter Scale
The Richter magnitude scale (Local Magnitude) was developed in 1935 by Charles F. Richter of Caltech to
compare the size and magnitude of earthquakes.
The magnitude of an earthquake is based on the amount of seismic energy released and recorded by a
seismograph.
Richter Scale Simplified Everything
It is calculated by measuring the amplitude of the s- wave (a.k.a.- the distance the seismograph needle
moved).
A graphical device (a nomogram) can be used to simplify the process and to estimate the magnitude based
on the distance and amplitude measurements.
Richter had to create an adjustment factor to allow for near versus distant quakes.
His logarithmic scale assigns values from 1-10 to the magnitude of any earthquake – “10” being the most
powerful.
Earthquake Magnitude
Each time the magnitude increases by one unit, the measured ground motion (amplitude) becomes 10
times larger.
For example, a 5.0 on the Richter scale will produce 10 times as much ground motion as an earthquake
with a magnitude of 4.0.
Figure out the amplitude difference between a magnitude 6.0 and 4.0.
Answer
100 times - A magnitude 6.0 will produce 100 times as much ground motion (amplitude) (10 × 10) as an
earthquake with a magnitude of 4.0.
To determine how much power the factor is about 30x per increment.
So difference is =
900 more time powerful.
Most Powerful Earthquakes Ever Recorded
• 1960 Chile 9.5
• 1964 Alaska 9.2
• 2004 Indian Ocean 9.1
• 2011 Japan Ocean 9.1
Richter Scale Failures
The main drawback to the Richter scale is its accuracy.
It is not accurate at estimating earthquake magnitudes where the epicenter is too far away (~350 miles)
or where the earthquake magnitude was greater than 7.0.
While the Richter scale was widely used for most of the 20th century, scientists now prefer to use the
moment magnitude scale because it is more precise.
Moment Magnitude Takes Over (1979)
Like the Richter Scale, Moment Magnitude scale also measures magnitude.
To make it user friendly it keeps consistency in the numbers.
Example: A medium-sized earthquakes or 5.0 on the Richter Scale is also a 5.0 on the Moment Magnitude
Scale.
Accuracy of Moment Magnitude
Instead of using averages, the Moment Magnitude becomes more accurate by factoring in:
1) Average amount of movement that happened
2) Area of the fault that moved
3) Rigidity of the rock
Drawback of Moment Magnitude
Finding an earthquake fault's length, depth, and its slip can take several days or weeks after a big
earthquake.
So seismologist /media report the Richter Magnitude initially until these calculations are complete.
Lesson #8 Tsunamis
A tsunami is a series of water waves caused by the displacement of a large volume of a body of water
typically caused by earthquake.
Tsunami waves resemble a rapidly rising tide.
Tsunamis generally consist of a series of waves with periods ranging from minutes to hours.
Although limited to coastal areas, their destructive power can be enormous –
2004 Indian Ocean tsunami killed over 230,000 people.
2011 Tohoku tsunami of Japan killed over 19,000 people, damaged estimated at $235 billion.
Tsunami can be generated when the sea floor abruptly deforms and vertically displaces the overlying water.
This will occur at a convergent plate boundary where there is a thrust or mega-thrust fault.
80% of all Tsunamis occur along the Ring of Fire and until recently tsunami’s where rare (5 every century).
California is not under any threat of a tsunami because we are not on the Ring of Fire.
However we are surrounded by it and the nearest threat is the Juan de Fuca plate.
Earthquakes and Society
Most earthquake injuries result from the collapse of buildings and or from falling objects.
Other dangers include a tsunami, landslides, explosions caused by broken electric and gas lines, and
floodwaters released from collapsing dams.
Destruction to Buildings and Property
We will tend to see more destruction to buildings that are designed poorly.
Nowadays most cities have earthquake codes including Los Angeles.
Predicting Earthquakes
While some claim to predict earthquakes scientist have not found anything or anyone that is reliable yet.
Pat Regan – believes that earthquakes are related to the preponderance of UFO sightings prior to seismic
activity.
Predicting Earthquakes- Seismic Gap Theory
One way seismologist can try and predict earthquakes is by using the Seismic gap theory.
Seismic Gap theory states that, over long periods of time, the displacement on any segment must be equal
to that experienced by all the other parts of the fault.
Seismic Gap
Scientist know of several seismic gaps that exist along the San Andreas Fault zone may be sites of major
earthquakes in the future.
Other Theories- Solar and Lunar Activity
Some scientist have theorized that a combination of solar activity and an extreme lunar cycle can cause
earthquakes.
On March 9th, 2011, NASA reported a powerful solar flare, which they said would cause ‘global
disturbances’ and on March 10th, Space.com posed the question “Will March 19 ‘Supermoon’ Trigger
Natural Disasters?”
The very next day, the third largest earthquake ever recorded struck Japan.
Many scientist have speculated that a “Supermoon”, or just a fullmoon can also lead to earthquakes.
Pole Shift
As the magnetic pole continues to wander rapidly toward Russia, it has led to speculation that this might be
leading to the record number of earthquakes seen in 2010, as well as the recent mega-quakes.
According to proponents of this theory, pole shift is a geomagnetic event that exerts pressure on the Earth’s
tectonic plates.
Climate Change
Some climatologists believe that shifting ice caps can have an effect on tectonic activity because the
massive weight of the ice is changing locations as well as additional water added to the seas.