Download Name: ___________________________ Chapter 6 Notes: Earthquakes Stress 

Forces in Earth’s Crust
Name: ___________________________
Chapter 6 Notes: Earthquakes
 Stress
 Stress: A force that acts on rocks to change its shape or volume.
 Stress pushes, pulls, or twists the rock in Earth’s crust
 Types of Stress
 Tension: a force that pulls on the crust.
 It stretches the rock so it is thinner in the middle.
 Occurs at Divergent Plate Boundaries
 Compression: the stress force that squeezes rock until it folds or breaks
 Occurs at Convergent Plate Boundaries
 Landforms from Compression
 Mountain
 Shearing: a stress that pushes rock in opposite directions
 Occurs at Transform Plate Boundaries
 Landforms from tension
 Valley
 Faults
 Fault: is a break in the rock of the crust where rock surfaces slip past each other.
 Faults occurs along plate boundaries where forces of the plate motion push or pull the
crust so much that the crust breaks.
 Hanging wall: block of rock that lies above the fault.
 Footwall: rock that lies below the fault.
 Types of Faults
 Normal Faults: fault in which the hanging wall has moved
downward relative to the footwall
 Occur where plates diverge
 Ex: Along the Rio Grande (rift valley)
 Reverse Faults: Faults in which the hanging wall moves
upward relative to the footwall

Opposite of normal fault

Occurs where plates converge
 Ex: in Canada & Northern Rocky Mtns
 Strike-Slip Fault: fault in which the rocks slide past each other
without up and down motion
 Occurs at transform boundaries
 Changing Earth’s Surface
 The forces of plate movement have added mountains, anticlines, synclines, and plateaus.

Folded Mountains: plate movement and collision of crust
cause Earth’s crust to fold
 Anticline: the fold in the rock that bends upward

Syncline: the fold in the rock that bends downward
 Examples of Folded Mountains
 Central Appalachian Mountains
 Himalayas in Asia
 Alps in Switzerland

Fault Block Mountains: forms when 2 Normal Faults cut
through a block of rock

Plateau: large area of flat land elevated high above sea level
 Formed when forces uplift some areas without much folding
Earthquakes and Seismic Waves:
 Earthquake
 Earthquake: shaking and trembling that results from the movement of rock beneath
earth’s surface
 Caused by forces of plate movement.
 The movement causes stress in the crust
 At the fault
 Stress increases until the rock breaks
 Releases an enormous amount of energy

Focus: the area beneath Earth’s surface where rock is under stress and breaks.
 Triggers an earthquake
 Usually about 100 km below the surface
 Epicenter: The point on Earth’s surface directly above the focus.
 Seismic Waves
 Seismic Waves: vibrations that travel through Earth carrying the energy released during
an earthquake
 Types of Seismic waves
 P waves: the first wave to arrive
 Primary waves
 They compress and expand
 Like an accordion

S



waves: come after the P waves
Secondary waves
Vibrate side to side and up and down
Cannot move through liquids

Surface Waves: P waves and S waves that reach the surface
 They move slower but still cause severe ground movement because it has loose
soil, sand, gravel, mud, and small rocks that can move, shift and slide.
Measuring Earthquakes
 Scientists use 3 main methods to measure Earthquakes
 Mercalli Scale: rates earthquakes according to damage
 The same earthquake can have different Mercalli ratings
 Different amounts of ground motion at different places
 Richter Scale: rates earthquakes according to the magnitude of the seismic waves.
 Magnitude: is the number assigned to the earthquake based on the size.
 The magnitude tells how much energy was released by an earthquake.
 Moment Magnitude Scale: rating that estimates the energy released by an earthquake.
 Scientists study seismographs
 Seismograph: instrument that records and measures seismic waves
 These show the kinds of waves present and how strong they were.
 Comparing Magnitude
 For each point increased in magnitude, 32 times as much energy is released
 Earthquakes below a 3 are barely noticed.
 Magnitudes between 5-6 causes moderate damage.
 Magnitude 8 or greater causes great damage.
 Locating the Epicenter
 Geologists use the seismic waves to locate an earthquake’s epicenter
Monitoring Earthquakes
 Geologists us seismograph
 Seismogram: the pattern of zigzag lines on paper as a record of an earthquake’s seismic
waves
 Instruments used to monitor faults
 Tiltmeters
 Creep Meters
 Laser-Ranging Devices
 GPS Satellites
Earthquake Safety
 Damage from Earthquakes
 Shaking
 Can cause landslides and avalanches
 Damage or destroy buildings, bridges, utilities, pipes
 S-waves and Surface waves can tear buildings apart
 Loose soil shakes more than solid rock
 Liquefaction
 Liquefaction: the result of the Earth’s shaking that turns soil to liquid mud.
 Aftershocks
 Aftershock: an earthquake that occurs after a larger earthquake in the same area.
 May strike hours, days, or months later
 Tsunamis
 Waves produced from earthquake under water
 Steps to Safety
 Drop, cover & hold

Indoors:
 Go under sturdy table or inside corner
 Avoid windows, mirrors, wall hangings, furniture

Outdoors:
 Go to open area
 Avoid vehicles, power lines, trees, building