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Earthquakes
Table of Contents
Forces in Earth’s Crust
Earthquakes and Seismic Waves
Monitoring Earthquakes
Earthquake Safety
Earthquakes - Forces in Earth’s Crust
Forces in the Earth’s Crust
• The movement of Earth’s plates creates
enormous forces that squeeze or pull the rock
in the crust.
– Stress – force that acts on rock to change its
shape or volume
• Three different kinds of stress can occur in the
crust: tension, compression, and shearing.
– work over millions of years to change the
shape and volume of rock
Earthquakes - Forces in Earth’s Crust
Types of Stress
• Tension – pulls on the crust, stretching
rock so that it becomes thinner in the
middle.
Earthquakes - Forces in Earth’s Crust
Types of Stress
• Compression – stress force that
squeezes rock until it folds or breaks.
Earthquakes - Forces in Earth’s Crust
Types of Stress
• Shearing – stress that pushes a mass of
rock in two opposite directions
Earthquakes - Forces in Earth’s Crust
Kinds of Faults
• Most faults occur along plate
boundaries, where the forces of
plate motion push or pull the
crust so much that the crust
breaks.
– Fault – a break in the rock of
the crust
• There are three main types of
faults: normal faults, reverse
faults, and strike-slip faults.
Earthquakes - Forces in Earth’s Crust
Kinds of Faults
• Tension in Earth’s crust pulls rock apart, causing
normal faults.
• Normal fault – hanging wall falls down below foot
wall. Ex: Rio Grande rift valley, New Mexico
Earthquakes - Forces in Earth’s Crust
Kinds of Faults
• A reverse fault has the same structure as
a normal fault, but the blocks move in the
opposite direction.
• Reverse fault – hanging wall move up above
foot wall Ex: Northern Rocky Mountains
Earthquakes - Forces in Earth’s Crust
Kinds of Faults
• In a strike-slip fault, the rocks on either side
of the fault slip past each other sideways,
with little up and down motion.
• Strike-slip fault – two sides slide past each other
– Ex: San Andreas Fault
Earthquakes - Forces in Earth’s Crust
Changing Earth’s Surface
• Over millions of years, the forces of plate
movement can change a flat plain into
landforms, such as:
– Anticlines and synclines, folded mountains,
fault-block mountains, and plateaus.
Earthquakes - Forces in Earth’s Crust
Changing Earth’s Surface
• Over millions of years, the forces of plate
movement can change a flat plain into
landforms, such as:
– Anticlines and synclines, folded mountains,
fault-block mountains, and plateaus.
Earthquakes - Forces in Earth’s Crust
Folding Earth’s Crust
• Anticline – upward fold in rock
formed by compression of
Earth’s crust.
• Syncline – downward fold in
rock formed by compression
in Earth’s crust.
• Folding produced some of the
world’s largest mountain
ranges
– Ex: Himalayas in Asia and
Alps in Europe
Earthquakes - Forces in Earth’s Crust
Stretching Earth’s Crust
• Fault-Block Mountains – form from two
normal faults, running parallel to each other
– Two hanging walls fall leaving a raised
area in between.
Earthquakes - Forces in Earth’s Crust
Uplifting Earth’s Crust
• Plateau – large area of flat land elevated high
above sea level.
– Form when forces in Earth’s crust push up a
large, flat block of rock
– Ex. Colorado Plateau
Earthquakes - Forces in Earth’s Crust
Links on Faults
• Click the SciLinks button for links on
faults.
Earthquakes - Earthquakes and Seismic Waves
Earthquakes and Seismic Activity
• Earthquake – shaking that results from
the movement of rock beneath Earth’s
surface.
– Occur all the time, usually too small to
notice
– Most begin in the lithosphere (100 km
below surface)
• Focus – point beneath Earth’s surface
where rock breaks under stress and
causes an earthquake.
• Epicenter – point on Earth’s surface
directly above an earthquake’s focus.
Earthquakes - Earthquakes and Seismic Waves
Types of Seismic Waves
• Seismic waves – carry energy from an
earthquake away from the focus, throughout the
Earth
Earthquakes - Earthquakes and Seismic Waves
Types of Seismic Waves
• There are 3 main categories of seismic waves:
P waves, S waves, and surface waves.
• P waves - seismic waves that compress and
expand the ground like an accordion.
Earthquakes - Earthquakes and Seismic Waves
Types of Seismic Waves
• S waves – seismic waves that vibrate from side
to side as well as up and down.
Earthquakes - Earthquakes and Seismic Waves
Types of Seismic Waves
• Surface waves - move more slowly than P
waves and S waves, but they produce the
most severe ground movements.
Earthquakes - Earthquakes and Seismic Waves
Seismic Waves Activity
• Click the Active Art button to open a
browser window and access Active Art
about seismic waves.
Earthquakes - Earthquakes and Seismic Waves
Measuring Earthquakes
• There are three commonly used scales of
measuring earthquakes:
– Mercalli scale, Richter scale, and the moment
magnitude scale.
Earthquakes - Earthquakes and Seismic Waves
Measuring Earthquakes
• Mercalli scale – developed to rate earthquakes
according to the amount of damage at a given
place.
Earthquakes - Earthquakes and Seismic Waves
Measuring Earthquakes
• Richter scale – rates an
earthquake’s magnitude
based on the size of its
seismic waves.
• Magnitude – measure of an
earthquake’s strength based
on seismic waves and
movement along faults.
• Seismograph – device that
records ground movements
caused by seismic waves
Earthquakes - Earthquakes and Seismic Waves
Measuring Earthquakes
• Moment magnitude scale – rates
earthquakes by estimating the total
energy released by an earthquake.
– Determined by studying data from
seismographs
• Each one-point increase in magnitude
represents the release of roughly 32 times
more energy.
Earthquakes - Earthquakes and Seismic Waves
Measuring Earthquakes
• Magnitude below 3 = Scarcely
noticed
• Between 3-5 = little damage
• Between 5-6 = Moderate damage
• Above 6 = Great damage
• Largest Earthquakes ever to be
recorded had a magnitude
measuring above 9
– Chile 1960 (9.5)
– Alaska 1964 (9.2)
Earthquakes
It’s Your Turn!!
Create an Earthquake!!
Earthquakes - Earthquakes and Seismic Waves
Seismic Wave Speeds
• Seismographs at five
observation stations
recorded the arrival
times of the P and S
waves produced by
an earthquake.
These data are
shown in the graph.
Earthquakes - Earthquakes and Seismic Waves
Seismic Wave Speeds
• Reading Graphs:
• What variable is
shown on the x-axis
of the graph? The yaxis?
X-axis––distance
from the epicenter;
y-axis––arrival
time.
Earthquakes - Earthquakes and Seismic Waves
Seismic Wave Speeds
• Reading Graphs:
• How long did it take
the S waves to travel
2,000 km?
7 minutes
Earthquakes - Earthquakes and Seismic Waves
Seismic Wave Speeds
• Estimating:
• How long did it take
the P waves to travel
2,000 km?
4 minutes
Earthquakes - Earthquakes and Seismic Waves
Seismic Wave Speeds
• Calculating:
• What is the difference
in the arrival times of
the P waves and the
S waves at 2,000
km? At 4,000 km?
2,000 = 3 minutes
4,000 = 5 minutes
Earthquakes - Earthquakes and Seismic Waves
Locating the Epicenter
• Geologists use seismic waves to locate
an earthquake’s epicenter.
Earthquakes
Locating the Epicenter
• Seismic waves travel at different speeds.
– P waves travel faster than S waves
– To tell how far the epicenter is from the
seismograph, scientists measure the
difference between the arrival times of the P
waves and S waves.
– The longer the time difference, the further the
epicenter is from the seismograph station.
Earthquakes
Locating the Epicenter
• For scientists to figure out the location of the
epicenter they need P and S wave information
from at least three different locations
– Once the distance is calculated for each
station a circle is drawn on a map (with a
radius = the distance calculated) around
each of the three seismograph stations
– The place where the three circles intercept
should be the earthquake’s epicenter
Earthquakes
Earthquakes - Earthquakes and Seismic Waves
Seismic Waves in the Earth
• Click the Video button to watch a movie
about seismic waves in the earth.
Earthquakes - Monitoring Earthquakes
The Modern Seismograph
• Seismic waves cause the seismograph’s drum to
vibrate. But the suspended weight with the pen
attached moves very little. Therefore, the pen
stays in place and records the drum’s vibrations.
Earthquakes
Reading a Seismogram
• Seismogram – record of an earthquake’s
seismic waves produced by a seismograph.
– The height of the jagged lines are greater for a
more severe earthquake.
Earthquakes - Monitoring Earthquakes
Instruments That Monitor Faults
• Geologists have developed instruments to measure:
– changes in elevation, tilting of the land surface, and
ground movements along faults.
Earthquakes
Instruments That Monitor Faults
• Tiltmeter – measures tilting or raising of
the ground
• Creep meter – uses a wire stretched across
a fault to measure horizontal movement of
the ground
• Laser-ranging device – uses a laser beam to
detect horizontal fault movements
• GPS – (Global Positioning System)
measure tiny movements of markers set up
on the opposite sides of a fault.
Earthquakes
Using Seismograph Data
• Seismographs and fault-monitoring
devices provide data used to map faults
and detect changes along faults.
• Geologists are also trying to use these
data to develop a method of predicting
earthquakes.
Earthquakes
Monitoring Changes Along Faults
• How rocks move along a fault depends on
how much friction there is between the
sides of the fault
• Friction – force that opposes the motion of
one surface as it moves across another
surface.
– Ex: San Andreas Fault, California
Earthquakes - Monitoring Earthquakes
Monitoring Changes Along Faults
• The map shows the probability of a strong
earthquake along the San Andreas fault. A high
percent probability means that a quake is more
likely to occur.
Earthquakes - Monitoring Earthquakes
Links on Earthquake
Measurement
• Click the SciLinks button for links on
earthquake measurement.
Earthquakes - Earthquake Safety
Earthquake Risk
• Geologists can determine earthquake risk
by locating where faults are active and
where past earthquakes have occurred.
Earthquakes
How Earthquakes Cause Damage
• Causes of earthquake damage
include:
– shaking, liquefaction,
aftershocks, and tsunamis.
• Shaking - triggers landslides and
avalanches
– Destroys buildings and bridges,
topples utility poles, and
fractures gas and water mains
• Liquefaction – violent
movements suddenly turn loose
soil into liquid mud.
Earthquakes
How Earthquakes Cause Damage
• Aftershock – earthquake that occurs
after a larger earthquake in the same
area.
– Can sometimes cause the most
damage
• Tsunami – large wave produced by an
earthquake on the ocean floor.
– Ex: December 26, 2004 –
earthquake on the seafloor of the
Indian Ocean caused a tsunami to
hit the shores of Indonesia killing
200,000 people
Earthquakes - Earthquake Safety
How Earthquakes Cause Damage
• Tsunamis spread out from an earthquake's
epicenter and speeds across the ocean.
– The waves are amplified as they approach
the shore.
Earthquakes
Steps to Earthquake Safety
• The best way to protect yourself during an
earthquake is to drop, cover, and hold.
• Before an earthquake
occurs be prepared
– Keep a supply of
canned food, bottled
water, flashlights,
batteries and a
portable radio
Earthquakes - Earthquake Safety
Designing Safer Buildings
• To reduce
earthquake
damage, new
buildings
must be
made
stronger and
more
flexible.
Earthquakes
Designing Safer Buildings
• Base-isolated building –
buildings mounted on
bearings designed to absorb
the energy of an earthquake.
– rests on shock-absorbing
rubber pads or springs
– Flexible joints can be
installed in gas and water
lines to keep them from
breaking.
Earthquakes - Earthquake Safety
Earthquake Damage
• Click the Video button to watch a movie
about earthquake damage.
Earthquakes - Earthquake Safety
More on Earthquake Risk
• Click the PHSchool.com button for an
activity about earthquake risk.