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 occurs 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