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EARTHQUAKES:
WHY? AND HOW?
EARTHQUAKES
sudden movement or shaking of the Earth
• Caused by plate tectonic stresses
releases stored
Energy!
• Located at plate boundaries
• Resulting in breakage of the Earth’s brittle crust
Elastic Rebound Theory
• Stresses build on both sides of a fault.
• When stresses become too great, the
rocks break and move.
• This releases the built up energy and
creates an earthquake.
• Video Clip
3 Types of Stress
• Compression- occurs at
convergent boundaries
(plates move toward each
other)
• Tension- occurs at divergent
boundaries (plates move
away from each other)
• Shear- occurs at transform
boundaries (plates are sliding
past each other)
PLATE TECTONIC STRESSES
•
Plate boundaries and faults (= cracks where plate sections
are moving in different directions) cause friction as plates move
•
Plates in a fault zone have
STICK-SLIP motion
– Periods of no movement
(stick)
and fast movement (slip)
– Energy stored as plates stick
– Energy released as plates
slip
3 Responses to Increasing
Stress
• Elastic Deformation- rock returns to
original shape when stress is removed
• Plastic Deformation- rock does not return
to original shape when stress is removed
• Fracture- rock breaks
3 Types of Faults
–Normal Fault
–Reverse Fault
–Strike-Slip Fault
Normal Fault
• Result of plates moving apart.
–The hanging wall moves downward
in relation to the footwall.
Normal Fault: Tetons in
Wyoming
• Plate boundaries are always faults, but not all faults are plate
boundaries.
Reverse Fault
• Result of plates moving together.
–The hanging wall moves upward in
relation to the footwall.
Reverse Fault: Sierra Madre
Strike-Slip Fault
• Result of plates sliding past each
other.
–The displacement is horizontal
Strike-Slip: San Andreas Fault
Parts of a Wave
• Energy is transmitted in waves
EARTHQUAKE WAVES
• FOCUS = place deep within the Earth and along the fault where
rupture occurs
• EPICENTER = geographic point
on surface directly above focus
• SEISMIC WAVES produced by the release of energy
– move out in circles from the point of rupture (focus)
2 types of Seismic Waves:
surface & body (travel inside & through earth’s layers)
2 Types of Body Waves
• Body waves travel faster than surface waves.
• P waves: back and forth movement of rock; travel thru solid, liquid,
gas
• S waves: sideways movement of rock; travel thru solids only
2 Types of Surface Waves
• Travel along the
ground, outward
from an
earthquake’s
epicenter.
• Slowest of the
seismic waves
• Do most of the
damage
• Two Types of
Surface Waves:
– Love Wave
– Rayleigh Wave
Tsunami
• Deadly ocean waves most often caused
by undersea earthquakes.
• Tsunami Awareness
• NOAA DART