Chapter 12
Section 1 How and Where Earthquakes Happen
There are over 3,000,000 earthquakes each year. That means that there is about 1
every 10 seconds.
Earthquakes _________________________________. They are the shaking of the
earth's crust caused by the release of energy. The plates move on convection
currents in the mantle - stress is built up, must be released, a shaking of the earth
happens. A minor cause of earthquakes is _____________________.
Why Earthquakes Happen
When the rocks are pressed together, friction will build up. The immobile state creates
more of a build up of pressure. They each then have a sudden release of that force
causing them to slip past each other causing an earthquake.
Elastic rebound theory
Anatomy of an Earthquake
The focus ____________________________________; the place where the earthquake
happened. Even thought the depth of the focus varies, most continental quakes are
classified as shallow; about 70km below, more than 70km is intermediate; 300-650 deep.
The ones that cause the most surface damage are the ___________ones since the
energy is closer to the surface.
Epicenter - ____________________________________________________
Seismic waves
Seismic waves ________________________________________________________.
They travel in all directions from the focus. They look like when you drop a pebble into a
There are two types of waves:
Body Waves: ___________________________________________________________
Surface Waves: _________________________________________________________
Body Waves
They travel through the interior of the earth. The rate that a wave travels depends
on what they are traveling through. They bend depending on the density of the layer,
similar to the refraction of light waves. Body waves are the first arriving tremors and
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Surface Waves
They move similar to water waves and travel just under the surface. When P & S
waves hit the surface sometimes areas very far away from the epicenter will be
affected. These waves cause considerable damage because of their low
frequency, long duration and large amplitude.
Love Waves (L waves)
Move side to side perpendicular to the waves travel direction. Looks
like a snake traveling (horizontal shearing)
Speed approx 90% that of S waves
Rayleigh Waves
Move more slowly; 70% slower then S waves
Move matter elliptically (like pond ripples)
Also called ground roll,
Seismic Waves and Earth’s Interior
Seismic waves are useful tools for scientists to explore. The type of
material the waves travel through affects the speed and direction of their
movement. By studying the speeds and directions of the waves, scientists
can learn more about what makes up the interior of the Earth.
Earth’s Internal Layers
3 Compositional layers
5 Mechanical
Shadow Zone
Shadow Zone - is an area of the earth that receives no seismic waves; because of the
refraction of the waves going through different densities of materials.
Refraction - the bending of waves because of density differences
Studying Earthquakes gave us the inferred layers of the earth. S waves can’t go through
liquid, so we concluded a layer must be liquid since no S waves would occur in the
shadow zone.
Earthquakes and Plate Tectonics
Most of the earthquakes will occur in 3 main tectonic areas, Convergent Oceanic,
Divergent Oceanic and Continental. These areas (environments) are usually in areas on
or near plate boundaries.
Convergent Oceanic Environments
When either an oceanic and oceanic plate collides one will subduct. An earthquake will occur. The
same is true when an oceanic plate collides with a continental plate, an earthquake will also occur.
Divergent Oceanic Environments
The Mid-Ocean Ridge has earthquakes occurring all of the time because the plates are pulling away
from each other.
Continental Environments
When two continental plates converge, diverge or move horizontally away from each other, they can
build up pressure which will cause quakes to happen and even mountains to be built.
Fault Zones
This is an area with numerous, closely spaced faults. They form at plate boundaries.
Earthquakes Away From Plate Boundaries
Not all quakes result from movement along plate boundaries. In 181-1812 a series of quakes occurred
near New Madrid, Missouri. The waves were so strong that they were felt as far away is the Eastern
Coast and New England.
Section 2 Studying Earthquakes
Seismology – __________________________________________________________
Recording Earthquakes
Since P waves are the fastest moving waves, they are the first to appear on the
seismograph. The S waves would be the second to arrive and the last would be the
surface waves.
Locating the Earthquake
To determine the distance to an epicenter, scientists analyze the arrival times of the P and S waves.
The longer the lag time between them, the further away the actual earthquake occurred.
Time Travel graph
It shows the relationship between the arrival times of the P & S waves and the
distance to the epicenter.
How to locate the epicenter of an earthquake: You need three seismographic stations to
record the earthquake so you can determine through triangulation.
Locating an earthquake epicenter if I know the difference in arrival times of P & S waves I
can determine the distance to the epicenter
Earthquake Measurement
Scientists study the amount of energy release and the change that they caused. This is
the study of the magnitude and intensity of an earthquake.
It is the measure of the strength of earthquake. It is determined by measuring the
amount of ground motion caused by the earthquake.
Richter scale
It measures the ground motion from an Earthquake.
earthquakes now.
It is used for smaller
Movement Magnitude
It measures the strength based on the size of the area of the fault that moves, the
average distance that the fault block moves and the rigidity of the rocks in the fault zone.
The larger the number, the stronger the earthquake is.
Intensity is the measure of the effect of an earthquake.
Modified Mercalli Scale:
Expresses the intensity in Roman numeral from I to XI and provides a description
of the effects of each Earthquake Intensity.
Section 3 Earthquakes and Society
Most injuries from earthquakes are a result from the collapse of buildings and other
structures along with other objects crashing about. Other dangers can be
explosions, floods, landslides and fires.
A tsunami is huge ocean waves that are formed y submarine earthquakes, volcanos or
landslides. The speed depends on depth of water; at 4500 meters (usual ocean
dept) can travel 750km/hr. At the shore it slows down and reaches enormous
Destruction of Building and Property
Foundation failure will happen due to the up and down and side to side motion of the
The ground will become liquefied due to the friction and moisture content of the
ground. This will make the ground unstable for foundations.
and unsafe.
After Shocks:
They are smaller earthquakes that occur after larger ones. They can cause further
damage to areas like fires from damaged gas lines and power outages.
Earthquake Safety
Earthquakes can happen anywhere. Some places will get more of them and some
areas more destructive one because of their proximity to boundaries. By
following safety measures some damages may not be as bad.
Before an Earthquake
Make sure you have a plan to meet family members after the disaster. Learn how and
where to report damage. Make sure you have supplies and flashlights in an
area that you can get to in case of an emergency.
During the Earthquake
Stay Calm.
Take cover in a strong building under something sturdy. If you are outside, move into
an open area. Stay away from electrical wires and gas and pipe lines. Don’t
move in an area that is prone to have them. Make sure you build structures that
can withstand them. If you are in a car, stop the car in an area that is not by
buildings or on bridges.
After an Earthquake
Watch out for electrical problems or wires lying around. Don’t go back into your home
unless you are given an all clear to do so. Make sure that your area is safe and
free from aftershocks.
Earthquake Warning and Forcasts
Scientists have studied earthquakes for a long time trying to predict when they will
occur. There is no reliable way to forecast them. Today scientists are looking for
small changes in the earth’s crust to see if they can predict an earthquake.
Seismic Gaps
A seismic gap is an area along a fault that experienced only a few
earthquakes recently, but where strong ones have occurred in the
Foreshocks are little earthquakes that precede an earthquake by a few seconds. Not a
reliable source for earthquake prediction.
Changes in Rocks
Scientists look for stress and strain in rocks. They look for magnetic changes, natural
gas seepage and other related cracks in the rocks in an area.
Reliability of Earthquake Forecasts
Very unreliable, they can only continue to study them.
Some historical quakes of the past:
1906 1964 1960 2008 -
San Francisco 8.3
Alaska, unrecorded because of the severity, anywhere from ox 8.5 to a 9.0
800 km of fault affected, vibrations continued for 18 months,
Over 10,000 aftershocks
Chilean earthquake, 9.5
China earthquake, 7.9 devastated the country just before the Olympics
Earthquake Effects
Frequency of
Less than 2.0
Micro earthquakes, not felt.
About 8,000 per
Generally not felt, but recorded.
About 1,000 per
Often felt, but rarely causes damage.
49,000 per year
Noticeable shaking of indoor items, rattling noises.
Significant damage unlikely.
6,200 per year
Can cause major damage to poorly constructed
buildings over small regions. At most slight damage
to well-designed buildings.
800 per year
Can be destructive in areas up to about 100 miles
across in populated areas.
120 per year
Can cause serious damage over larger areas.
18 per year
Can cause serious damage in areas several hundred
1 per year
miles across.
Devastating in areas several thousand miles across. 1 per 20 years
Never recorded; see below for equivalent seismic
energy yield.
Extremely rare