Section 21.2

WHAT IS AN EARTHQUAKE?

An earthquake is a movement of Earth’s crust caused by a release of energy  

WHAT CAUSES AN EARTHQUAKE?

Eruption of a volcano

Collapse of a cavern or building

Major cause – stress between two plates

Friction builds up between two plates The stress causes the plates to deform or

‘stretch’ Eventually, the stress becomes great

enough that it overcomes friction and the plates move

The plates ‘snap’ back into their original shape and begin building up friction again

Point on the fault (or plates) where movement first occurs

Where an earthquake starts 3 types

Shallow (0-70km) Intermediate (71-300km) Deep (301km and higher)

Point on the Earth’s surface directly above the focus

What is reported as the location of an earthquake

P-waves: primary waves; travel through solids, liquids, and gases; back and forth motion

P waves are ______________ waves.

S-waves: secondary waves; travel through solids only; side to side motion (snake)

S waves are ___________ waves

L-waves: surface waves

Combination of up and down and back and forth motion

Cause the most damage due to their rolling action and long wavelengths

Seismograph: an instrument that detects and records earthquake waves

P-waves always arrive first, then S-waves, then L-waves

P and S Waves Graph – pg.767 – Under Graphing Skills

As distance increases, the travel time for seismic waves increases also.

To locate the exact location of an epicenter, you must have points from three separate seismograph locations around the world

Richter Scale Developed by Charles Richter in the 1940’s Measures the amount of energy released

by the earthquake itself Scale from 1-10 (weakest - strongest) Each number is 32 times stronger than the

next lower number (Ex: ‘6’ is 32 times stronger than a ‘5’ and 1024 times stronger than a ‘4’)

We cannot predict earthquakes other than giving periods of time in which we think they will occur.

Earthquakes are most common along sliding plate boundaries, but they can occur in other places.

http://earthquake.usgs.gov/learn/kids/