What Are Earthquakes?7-1

Key Concept:Sudden motions along breaks in

Earth’s crust can release energy in the form of seismic waves.

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.

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.

Faults at Tectonic Plate Boundaries

• An example of 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.

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.

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.1) Between the two plates

2) Inside the down going plate.

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.

Fault Zones

• An area along a plate boundary, where there are many connected faults is called a fault zone.

• A fault zone can have different depths, length and cut thru the lithosphere in different directions.

Why Earthquakes Happen• As plates move, stress is placed on the edges. • In response to stress, rocks deform. • Rocks deform in two ways:

1) Plastic deformation: like a piece of clay being molded.

2) 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.

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.

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 the amount of energy released during elastic rebound.

Earthquake Waves• Earthquakes are the

result of energy movement as seismic waves.

• There are 3 types of seismic waves:

1) P wave2) S wave3) Surface wave • Each wave travels thru

Earth differently.

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.

S Waves

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

after P waves.

Surface waves

• Only move along the Earth’s 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:

1) Move up and down.

2) Move back and forth.