• Earthquakes: vibrations through earth’s crust– Occur when rocks under stress slip or shift along a

fault

Normally, rocks are pressed together tightly = not movingOr “locked”

Elastic Rebound Theory

• Rocks along each side of a fault are moving slowly.

• If the rock is “locked”, then stress will increase• When the rock is stressed beyond its capacity,

the rocks separate, or fracture, at the weakest point, and then they spring back to their original position– As rocks move they create vibrations, called Seismic

Waves

• Aftershocks: series of tremors caused by other areas adjusting to stress of the initial earthquake

Focus: place where slip first occurs

Epicenter: The point on Earth’s surface directly above the focus.

Seismic waves radiate out in all directions from the focus.

Major Earthquake Zones

Where do earthquakes occur?

• Along or near the edges of lithospheric plates

Journal:Why do think that earthquakes occur in these

areas? Explain the cause of the earthquakes!

Fault Zones

• Groups of interconnected faults along a plate boundary

6.2 Recording Earthquakes

• Seismograph: instrument that detects and records seismic waves

Types of Waves

• The different types of energy waves shake the ground in different ways and also travel through the earth at different velocities.

Types of Waves

• P wave: The fastest wave, and therefore the first to arrive at a given location. • compression wave, alternately compresses and

expands material in the same direction it is traveling.

• S wave : is slower than the P wave and arrives next• shakes the ground up and down and back and forth

perpendicular to the direction it is traveling.

• Surface waves: follow the P and S waves.

Locating an Earthquake

• Scientists analyze the difference between the arrival of P and S waves .

Journal:

• What instrument measures earthquakes???

Earthquake Measurement

• Magnitude: Measure of energy released– Described as ground motion

– Measured using the RICHTER SCALE

• Mercalli Scale: describes intensity, or amount of damage an earthquake causes. – Scale I – XII

Journal:

• Describe the difference between the Richter Scale and the Mercalli Scale.

• Reading an Earthquake Wave Graph IN CLASS