Earthquakes

Earthquakes

Earthquakes are natural vibrations of the ground, caused by: Fractures in earth’s crust Volcanic eruptions

Stress and Strain

Stress causes fractures in rock

Three types of stress: Compression Tension Shear

Deformation caused by stress is known as strain

Compression

Compression is stress that decreases the volume of material

Tension

Tension is stress that pulls a material apart

Shear

Shear is stress that causes a material to twist

Effects of Strain and Stress

Stress and strain can cause deformation

Low stresses cause elastic strain Elastic strain causes a material

to bend and stretch Can return to normal, like a

rubber band High stresses cause ductile

deformation Material is permanently

deformed Can cause breaks or failure

Faults

When stress causes rock to fail, a fault occurso Fault – Fracture(s) along

which movement occurs

Three main types of faults: Reverse Normal Strike-Slip (Transform)

Reverse Fault

Reverse Faults are fractures that result from horizontal compression

Shortens crust horizontally

Normal Fault

Normal Faults are fractures caused by horizontal tension

Lengthens crust horizontally

Strike-Slip Fault

Strike-Slip Faults are fractures caused by horizontal shear

Motion is mainly horizontal

Earthquake Waves

Vibrations of the ground during earthquakes are called Seismic Waves

Three types: Primary Wave (P-Wave) Secondary Wave (S-Wave) Surface Waves

P-Waves

P-Waves squeeze and pull rocks in the same direction as the wave travels

S-Waves

S-Waves cause rocks to move at right angles to the direction of the wave

Surface Waves

Surface Waves move rocks up and down, as well as side-to-side

Motion is similar to an ocean wave

Focus

Earthquakes originate at a point called a Focus

Focus is usually below the surface The Epicenter is the point on Earth’s surface

directly above the focus