1. Chapter 7 Lesson 2 Earthquakes

2. What is an Earthquake?
A sudden trembling of the ground caused by movement happening in the crust

3. Where Do Earthquakes Happen?
Most earthquakes happen at the edges of plates, where plates meet
About 80 percent of all earthquakes happen along the edge of the Pacific Plate
Along the edges, plates can collide, they may pull apart, or they may slide past each other
These motions can cause the rocks to bend and stretch until they break
As a result, faults form at or below the surface of the crust

4. Tectonic Plates

5. Earthquake Locations

6. What is a Fault?
Fault- huge cracks in
the Earth’s crust

7. Three Kinds of Faults 1. Normal Fault Produced at divergent boundaries
Rocks above the fault
surface move down

8. Three Kinds of Faults 2. Reverse Fault Produced at convergent
boundaries
Rocks above the fault
move upward

9. Three Kinds of Faults 3. Strike Slip Fault Produced at transform
boundaries
Rocks slide past each other in
different directions

10. How Earthquakes Start
Earthquakes start the moment rocks begin to scrape past each other along a fault.

11. How Earthquakes Start Focus
The point where the earthquake starts, where rocks begin to slide past each other
It is usually below the surface
The sudden movement causes vibrations to spread out from the focus
These vibrations travel through the crust in the form of waves

12. How Earthquakes Start Seismic Waves
A vibration that spreads out away from a focus when an earthquake starts
They soon reach the Earth’s surface at a point directly above the focus

13. How Earthquakes Start Epicenter
The point on the Earth’s surface directly above the focus of an earthquake
People can first feel the ground shaking at the epicenter

14. How Earthquakes Start Epicenter This shaking is what causes most
earthquake damage
Damage is usually greatest
at the epicenter because
it is so close to the focus
As waves travel away from
the focus they get weaker

15. Aftershocks
After the first shaking, it may be quiet, and then there are aftershocks
This is the shaking of the Earth’s crust after the initial shaking of an earthquake

16. Earthquake Waves
There are different kinds of seismic waves which travel differently and at different speeds.

17. Earthquake Waves Primary Waves, or P Waves
Accordion-like motion shaking
Rock material squeezes together and spreads apart repeatedly
This motion produces seismic waves that move in the same direction that the rock is shaking
They are the fastest seismic wave- the first to reach any faraway location

18. Earthquake Waves Secondary Waves, or S waves
Shaking like a ruler held off the edge of a desk and “twanged”
This motion in rocks creates seismic waves that move in a perpendicular direction from the vibration
These waves travel slower than primary waves
They are second to arrive at any given faraway location.

19. Earthquake Waves Surface Waves
This shaking causes Earths surface to heave up and down like an ocean wave, or sway from side to side
Tears apart structures built on the surface

21. Earthquake Waves Seismographs
Scientists study and identify waves using a seismograph
A sensitive device that detects the shaking of the Earth’s crust during an earthquake

22. Seismograph

23. Seismogram

24. Tsunamis
If the focus of the earthquake is beneath the sea floor, the seismic waves can travel through the ocean, producing huge ocean waves called tsunamis

25. See Mrs. Balducci’s Website….
…..For more information on earthquakes such as earthquake current events and earthquake simulations.
Look under Chapter 7 Restless Earth LINKS