Transcript Cause of Earthquakes

Cause of Earthquakes
What is an Earthquake?
 An earthquake is the vibration of Earth produced by the
sudden, rapid release of energy.
Seismology
 When an earthquake occurs, the elastic energy is released
and sends out vibrations that travel throughout the Earth.
These vibrations are called seismic waves. The study of how
seismic waves behave within Earth is called seismology.
Reference:
Pages
Focus and Epicenter
Earthquake Focus and Epicenter?
Focus:
 The exact location within Earth were seismic waves are generated by
sudden release of stored elastic energy. Most often located on a preexisting fault.
Epicenter:
 The point on the surface of Earth directly above the focus.
Earth’s Surface
Epicenter
Fault
Plane
Focus
Cause of Earthquakes
 Earthquakes occur when energy stored in elastically strained
rocks is suddenly released.
 This release of energy causes intense ground shaking in the
area near the source of the earthquake (Focus) and sends
waves of elastic energy, called seismic waves, in all
directions throughout Earth.
 Earthquakes can be generated by bomb blasts, volcanic
eruptions, and sudden slippage along faults.
 Earthquakes are definitely a geologic hazard for people
living in earthquake regions, but the seismic waves
generated by earthquakes are invaluable for studying the
interior of the Earth.
Cause of Earthquakes
How Does Earthquakes Originate?
 Movement in areas along the fault plane stops (fault sticks).
 Elastic energy is stored in the rock as the rock becomes
deformed and bends, much like a bent stick.
 When the elastic strain built up along the fault exceeds the
elastic limit, the rock will break or slip at its weakest point
which we call the the focus.
 This slippage along the fault allows the rock to “snap back”
and the vibrations sends out waves of energy in all directions
called seismic waves, or earthquake waves.
 The springing back of the rock is called “elastic rebound”.
Elastic Rebound Theory
 The mechanism that cause earthquakes was not understood
until H. F. Reid proposed his idea of “Elastic rebound.”
 Reid suggested that most natural earthquakes are caused by
sudden slippage along a fault zone.
Original position of
rocks before any
strain.
Elastic Rebound Theory
 The elastic rebound theory suggests that if movement
along a fault gets stuck, elastic strain energy builds up
deforming rocks on either side of the fault.
A feature across the
fault bends as the
rocks on both sides of
the fault pushes in
opposite directions
and elastic strain
builds up.
Fault
Elastic Rebound Theory
 When the rocks along the fault can no longer hold the strain
slippage occurs at the weakest point along a fault (focus) which
causes rock to start moving on both sides of the fault.
 Energy is released in all directions causing an earthquake.
Fault
The fault ruptures
releasing elastic
energy.
Energy Released
Elastic Rebound Theory
 When the elastic strain is released the rocks on both
sides of the fault will “snap-back” to their unstrained
positions.
Fault
Elastic strain on
rocks released.
Sample Problem
Earthquakes commonly occur at plate boundaries, with
reference to elastic rebound, what causes an earthquake?
Answer:
There is a build up of stress (energy) within the tectonic
plates, where it reaches the elastic limit. The plates then
rupture (snap) and the stored energy is released. Plates
then move (snap) back to unstressed positions releasing
the stored energy as seismic waves. This is what we call
an earthquake.