Earthquakes

Chapter 19

What is an earthquake?

• Natural vibration of the ground

• Caused by movement along fractures or volcanic eruptions

Most earthquakes occur when…

• Rocks fracture or break deep within the Earth.

• Fractures form when STRESS, the forces per unit area acting on a material, exceeds the strength of the rocks involved.

3 Kinds of stress

1. Compression: Pushes together

1. Tension: Pulls apart

1. Shear: Twists

Faults

• Many rocks that make up the crust fail when stress is applied to quickly.

• The result is a system of fractures called FAULTS.

Three Types of Faults

Seismic Waves

• Vibrations of the ground during an earthquake are called Seismic Waves (there are 3 types).

1. Primary Waves (P Waves)

• Squeeze and pull rocks in the same direction along the wave direction.

• The first wave to arrive at an earthquake

http://daphne.meccahosting.com/~a0000e89/insideearth2.htm

Secondary Waves (S Waves)

• Cause rocks to move at right angles in relation to the direction of the wave.

http://daphne.meccahosting.com/~a0000e89/insideearth2.htm

Surface Waves

• Move in 2 directions as they pass through rock.

• Produces motion in the upper crust– Motion can be up and down– Motion can be around– Motion can be back and forth

• Travel more slowly than S and P waves• More destructive

Focus – point inside the Earth where an earthquake begins

Epicenter – point on Earth’s surface above focus

Seismology: the study of earthquakes

How Seismographs (seismometers) Work

the pendulum remains fixed as the ground moves beneath it

Typical Seismogram

http://isu.indstate.edu/jspeer/Earth&Sky/EarthCh11.ppt

How do scientists calculate how far a location is from the epicenter of an earthquake?

• Scientists calculate the difference between arrival times of the P waves and S waves

• The further away an earthquake is, the greater the time between the arrival of the P waves and the S waves

How are Earthquakes Measured? Richter Scale: The size of the largest

seismic wave in magnitude.

How are Earthquakes Measured? Mercalli Intensity Scale

Most earthquakes…. Are associated with tectonic plate

boundaries.

Almost 80% of all earthquakes occur in the Circum-Pacific Belt!

Destruction to buildings

“Pancaking”

• Where structural buildings fall and debris resembles a stack of pancakes.

Tsunamis

http://www.uwgb.edu/dutchs/EarthSC-102VisualsIndex.HTM

Formation of a tsunami

http://isu.indstate.edu/jspeer/Earth&Sky/EarthCh11.ppt

Tsunami Warning System

http://isu.indstate.edu/jspeer/Earth&Sky/EarthCh11.ppt