1. EARTHQUAKES

2. Rocks move along faults…
A fault is a fracture or break in the Earth’s lithosphere where blocks of rock move past each other.
Along some parts of the fault, the rock on either side may slide along slowly and constantly.
Along other parts of the fault, the rocks may stick or lock together.
Rocks bend as stress is put on them.
As the stress increases, the rocks finally break free.

3. Stress in Lithosphere = Earthquake
A sudden release of stress in the lithosphere causes an earthquake.
An earthquake is a shaking of the ground caused by sudden movement of large blocks of rock along the fault.
Most faults are located along plate boundaries, so most earthquakes occur in these areas.

4. About 80% of the earthquakes occur in the belt around the edges of the Pacific Ocean.

5. What is the best known fault in this belt in the United States?
San Andreas Fault in California.
It forms part of the boundary between the North American Plate and the Pacific Plate.
Unlike many other faults, parts of the San Andreas Fault can be seen on the surface of the ground.

7. Strength of an earthquake
The strength of an earthquake depends in part on…
How much stress builds up before the rocks move
The distance the rocks move along the fault

8. Reverse or Thrust Faults
3 Types of Faults
Normal Faults
Reverse or Thrust Faults
Strike-Slip Faults
The blocks of rock along different types of faults move in different directions, depending on the kinds of stress they are under.
More than one type of fault may be present along the same plate boundary.

9. Example: Great Rift Valley of Africa
Normal Faults
Earthquakes along normal faults are common near boundaries where tectonic plates are moving apart. (Divergent Boundaries)
Example: Great Rift Valley of Africa
Pull
Pull
Direction rocks move

10. Reverse or Thrust Faults
Stress that presses rock together causes reverse faults. (Convergent Boundaries)
The Himalaya Mountains, which rise in the area where the Indian Plate is pushing into the Eurasian Plate, have may earthquakes along reverse faults.
Direction rocks move
Push

11. Strike-Slip Faults
These faults can occur where plates scrape past each other. (Transform Boundaries)
The San Andreas Fault is a strike-slip fault.

12. Earthquakes Release Energy
Earthquake energy travels outward in all directions – up, down, and to the sides.
The energy travels as seismic waves, which are vibrations caused by earthquakes.
All earthquakes start beneath Earth’s surface.
The focus of an earthquake is the point underground where the rock first begins to move.
Seismic waves travel outward from the earthquakes focus.
The epicenter is the point on the Earth’s surface directly above the focus.

14. Earthquakes produce 3 types of seismic waves…
Primary Waves
Secondary Waves
Surface Waves
Each type moves through material differently.
Scientists learn about Earth’s layers by studying the paths and speeds of seismic waves traveling through Earth.

15. Primary Waves, P Waves
These are the fastest waves that are the first to reach any particular location after an earthquake occurs.
Primary waves can travel through solids, liquids, and gases.
Primary waves = P waves

16. Secondary Waves, S Waves
Secondary waves travel through Earth’s interior at the same time as primary waves, but at about half the speed.
Therefore, they are the second waves to arrive at any particular location after an earthquake.
Secondary waves can travel through rock, but unlike primary waves they cannot travel through liquids or gases.
Secondary waves = S waves

17. Surface Waves
Surface waves move along Earth’s surface, not through its interior.
They make the ground roll up and down or shake from side to side.
Surface waves cause the largest ground movements and the most damage.
They travel more slowly than the other types of seismic waves.

19. Seismic waves can be measured…
There are seismic stations all over the world where ground movements are measured.
A seismograph is an instrument that constantly records ground movement.
The recording produced by a seismograph is called a seismogram.
By studying seismograms, scientists can determine the locations and strengths of earthquakes.

20. Seismograph and Seismogram
The height of the wiggles on the seismogram indicates the amount of ground movement produced by seismic waves at the seismograph’s location.

21. Interesting Facts…
Most injuries and deaths due to earthquakes are not directly caused by the movement of the ground.
They are caused by collapsing buildings and other structures and by fires.
After an earthquake, fires may start due to broken natural-gas lines, broken electrical power lines, or overturned stoves.

22. How do we measure an earthquakes magnitude?
The Richter Scale : An earthquake’s magnitude is based on how fast the ground moves at the seismic station.
The Modified Mercalli Intensity Scale: The intensity scale consists of a series of certain key responses such as people awakening, movement of furniture, damage to chimneys, and finally - total destruction.

24. http://elearning.niu.edu/simulations/images/S_portfolio/ Mercalli/Mercalli_Scale.swf

25. Review Questions… What is a fault?
How does an earthquake occur in the lithosphere?
What are the 3 types of faults? How are they different?
What are seismic waves?
What are the 3 types of seismic waves? How are they the same and how are they different?
How are seismic waves measured?
How do we measure an earthquakes magnitude?