What Are Earthquakes?

1
What Are Earthquakes?

• Key Concept
• Sudden motions along breaks in Earths crust can
  release energy in the form of seismic waves.

2
Where Earthquakes Happen?

• The majority of earthquakes happen near the
  boundaries of tectonic plates.
• Large earthquakes have occurred in the interior
  of tectonic plates.
• Earthquakes happen along faults.

3
Faults at Tectonic Plate Boundaries

• Divergent Boundaries
• Plates pull away from each other.
• Tension causes plates to break up into fault
  blocks.
• Some of the fault blocks drop down relative to
  the others.
• Earthquakes happen as the fault block moves.

4
Faults at Tectonic Plate Boundaries

• An example of a divergent boundary is a mid-ocean
  ridge.
• There the lithosphere is thin.
• Earthquakes that happen at the mid-ocean ridge
  are shallow.
• At divergent boundaries, earthquakes occur at
  depths less than 20 km below the ocean floor.

5
Earthquakes at convergent Boundaries

• When two plates collide with each other.
• Two things may happen,
• 1) Two plates crumple up and make a mountain.
• 2) One plate subducts under the other.
• Compression occurs as the two plates collide.

6
Earthquakes at convergent Boundaries

• Compression causes the lithosphere to break up
  into fault blocks.
• These blocks thrust over one another as the
  plates move.
• Two types of earthquakes may occur.
• Between the two plates
• Inside the down going plate.

7
Earthquakes at Transform Boundaries

• As two plates move past each other, stress causes
  rocks to shear.
• Earthquakes happen along these strike-slip
  faults as the plates move.
• Most transform boundaries occur between oceanic
  lithosphere.
• Earthquakes occur at shallow depths.

8
Fault Zones

• An area along a plate boundary, where there are
  many connected faults is called a fault zone.
• At a fault zone can have different depths, length
  and cut through the lithosphere in different
  directions.

9
Why Earthquakes Happen

• As plates move, stress is placed on the edges.
• In response to stress, rocks deform.
• Rocks deform in two ways
• Plastic deformation like a piece of clay being
  molded.
• Elastic deformation stretching like a rubber
  band.
• Like a rubber band, plates will break if
  stretched too far.
• The break releases energy as the rocks return to
  their original form.

10
Elastic Rebound

• The sudden return of deformed rock to its
  original shape is called elastic rebound
• Elastic rebound occurs when the stress on a rock
  becomes so great, that it breaks.

11
Elastic Rebound

• This causes rocks on both sides of the fault to
  slide past each other.
• In this sudden motion, stress is released in the
  form of energy waves.
• Energy waves traveling through rock are called
  seismic waves.
• The strength of an earthquake is determined by
  amount of energy released during elastic rebound.

12
Earthquake Waves

• Earthquakes are the result of energy movement as
  seismic waves.
• There are 3 types of seismic waves
• P wave
• S wave
• Surface wave
• Each wave travels thru Earth differently.

13
P Waves

• Also called pressure waves.
• Are the fastest waves
• Also called primary waves, because they are the
  first to be detected.
• P waves can move thru liquids, solids and gasses.

14
S Waves

• Also called shear waves.
• Second fastest wave.
• Move side to side.
• Can not travel through liquids.
• Called secondary waves because always arrives
  after P waves.

15
Surface waves

• Only move along the Earths crust.
• Produce motions along the top of the crust.
• Since these waves focus on the surface, they
  cause the most damage.
• Two type of surface waves
• Move up and down.
• Move back and forth.