Section 1: Earth’s Crust in Motion
How Do Stress Forces Affect
Rock?
 The movement of earth’s
plates creates powerful forces
that squeeze or pull the rock in the crust – these forces are
examples of stress

Stress – a force that acts on rock to change its shape or
volume
 An
earthquake is the shaking and trembling that
results from the movement of rock beneath earth’s surface
How Does Stress Effect the
Earth’s Crust?
Deformation – any change in
the volume or shape of earth’s crust
 Three kinds of stress in the crust:
–Shearing – stress that pushes a mass
of rock in two opposite directions
–Tension – pulls on the crust, stretching
rock so that it becomes thinner in the middle like
warm bubble gum
–Compression – squeezes rock
until it folds or breaks like a giant trash compactor
What Is a Fault?
Fault – a
break in earth’s
crust where slabs
of crust slip past each
other; These usually
occur at plate
boundaries
What Kind of Faults Are
There?
 Three Kinds:
–Strike-slip faults
–Normal Faults
–Reverse Faults
What Are Strike-slip Faults?
 Strike-slip faults
–Shearing forces cause
rocks to slip past each other
sideways with little up
and down Motion;
– Ex. San Andreas fault in California
What Are Normal Faults?
 Normal faults
–Tension forces cause the rocks
to form the fault at an angle
– One block is above the fault
–Hanging wall – the half of
the fault that lies above fault angle
–Footwall – the half of the
fault that lies below
– Ex. Rio Grande rift valley
What Are Reverse Faults?
 Reverse faults
–compression forces
cause the rocks to move
towards each other
– Same structure as normal fault but the
blocks move in opposite direction;
hanging wall move up
– Ex. Appalachian Mountains and
Mount Gould in Glacier National Park
A miner walks on the foot wall and looks up at the hanging wall!
B
A
Hanging wall
moves down
Hanging wall
moves up
What type of fault?
What type of fault?
Normal Fault
Reverse Fault
How Are Mountains Effected
by These Forces?
Fault-block mountains – normal
faults uplift a block of rock
Folding – bends in the rock that form
when compression shortens and thickens part
of the earth’s crust. Ex. Himalayas
How Are Mountains Effected
by These Forces? (Continued)
Anticlines – a fold
upward into an arch
Syncline – a fold
downward into an arch
Plateaus – a large area of
flat land elevated high above
sea level
Section 2: Measuring Quakes
How Does the Energy of an
Earthquake Travel Through Earth?
Earthquakes –
most begin in the lithosphere
Focus – the point
beneath the earth’s surface
where rock that is under
stress breaks, triggering an
earthquake
Epicenter – the point
on the earth’s surface directly
above the focus
What Are Seismic Waves?
Seismic Waves –
vibrations that travel through Earth
carrying the energy released during
an earthquake
– They move like ripples on a pond
– They carry the energy of an earthquake
away from the focus, through Earth’s
interior, and across the surface
– The energy is greatest at the
Epicenter
What Are the Different Kinds of
Seismic Waves?
 Three categories:
–P waves
–S waves
–Surface waves
 P waves and S waves are sent out
focus
from the
; Surface waves
develop when the waves reach the
surface
What Are P Waves?
primary waves
– The first waves to arrive
 P waves are
– Earthquake waves that
compress and expand the
ground like an accordion
– Cause buildings to contract and expand
What Are S Waves?
 S waves are
secondary
waves
– Earthquake waves that vibrate from side
to side as well as up and down
– These waves shake the ground
back and forth
– Shake structures violently
–Cannot move through liquids
What Are Surface Waves?
 When P waves and S waves reach the
surface some are transformed into
surface waves
– Surface waves move more
slowly than P waves and S waves
severe
– Produce the most
ground movements
– Can make the ground roll like ocean
waves or shake buildings from side to
side
How Do Scientists Detect
Seismic Waves?
Seismograph – records
movements
the ground
caused by seismic waves as they
move through the Earth
How Do Scientists Measure
Earthquakes?
 There are at least 20 different measures for rating
earthquakes, three are:
–Mercalli
–Richter
–Moment Magnitude
Magnitude – a measurement of
earthquake strength based on seismic waves
What Is the Mercalli Scale?
 Rated earthquakes according to
intensity
–Intensity: strength of
their
ground motion in a given place
 Not a precise measurement
 Describes how earthquakes
affect people, buildings, and the
land surface
What Is the Richter Scale?
 A rating of the
size of
seismic waves as
measured by a particular type of
seismograph
 Accurate measurements for
small, nearby earthquakes not
large, distant earthquakes
What Is the Moment
Magnitude?
rating system that
estimates the total energy
A
released by an earthquake
 Can be used to rate earthquakes
of all sizes, near or far
 Below
5.0 – little damage
 Above 5.0 – great destruction
How Do Scientists Locate the
Epicenter?
 Geologists use
seismic waves
– P waves arrive first
– S waves arrive close behind
– Scientist measure the
difference in arrival times
• The farther away an earthquake is the
greater the time between their arrival
– Scientists draw three circles using data
from seismographs set at different
stations to see where they intersect – the
epicenter