1. Earthquakes
Chapter 8

2. Earthquakes Seismology – study of earthquakes
Seismologist – the scientists that study earthquakes
Caused by a sudden movement between two tectonic plates
Ask students what kind of tectonic plate boundaries might cause earthquakes

3. Earthquakes Happen near the edges of tectonic plates
Because tectonic plates move different ways and at different speeds, they cause faults
Earthquakes occur along faults because of the sliding

5. What causes earthquakes?
Stress builds up as plates push, pull, or slip past each other and causes rock to deform
Deformation – change in the shape of a rock due to stress
The rock in plates moving past each other deforms in one of two ways
Plastic deformation – rock stretches like a rubber band; forces push opposite directions but rock doesn’t break
Elastic deformation – rock stretches, but only so far before it breaks. Energy is released and rock returns to its normal shape.
Elastic rebound – sudden return of elastically deformed rock to its normal shape
Click on “Elastic rebound” to view an animation

6. Faults at Plate Boundaries
Plate Boundary
Major Fault Type
Transform
Strike-Slip
Convergent
Reverse
Divergent
Normal
Hyperlinks for animations for each fault type

7. Faults on Plate Boundaries
Earthquake zones – places where a large number of faults are located
San Andreas Fault Line!
Sometimes earthquakes happen at faults in the middle of tectonic plates
Divergent – blocks of crust pulled away; normal faults
Convergent – blocks of crust pushed together; reverse faults
Transform – blocks of crust slide horizontally; strike-slip faults
1970s – earthquake in Missouri that altered the flow of the Mississippi river. Faults are still active!

8. How Earthquake Waves Travel
Seismic Waves – waves of energy that travel through the Earth after an earthquake
Body waves – seismic waves that travel through Earth’s interior
Surface waves – travel along Earth’s surface

9. How Earthquake Waves Travel
P waves – pressure waves, primary waves; travel through solids, liquids, gases
Travel FASTER than S waves, always the first to be detected
S waves – shear waves, secondary waves; cannot travel through liquids
Travel SLOWER than P waves
Move side to side, shear/stretch rock sideways
P waves – gelatin on a plate; wiggles if it’s tapped, tapping changes the pressure inside the gelatin, cube is momentarily deformed and returns to its normal shape

10. How Earthquake Waves Travel
Surface Waves
Travel along Earth’s surface, mostly in the upper few kilometers
Two types of waves:
Up and down and around
Back and forth
More slowly and cause more destruction

11. Section 8.1 Review!
Page 229
1-11

12. Earthquake Measurement
Section 8.2
Earthquake Measurement

13. Locating Earthquakes
Seismographs – instruments that record seismic waves
Seismogram – tracing of earthquake motion by a seismograph

14. Locating Earthquakes
After an earthquake, different seismograms from around the world compare the arrival times of P and S waves to locate where the earthquake happened
Epicenter – point on Earth’s surface above an earthquake’s starting point
Focus – point inside the Earth where an earthquake starts

15. Locating Earthquakes S-P Time Method
Using different seismograms on the same earthquake, scientists can determine where an earthquake originated
Measure the seconds between the first P wave and the first S wave
The seconds are converted to a distance, which shows how far the epicenter was from the seismograph

17. Earthquake Strength
Magnitude – measure of the strength of an earthquake
Richter scale – created by Charles Richter in 1930s
Measures the ground motion from an earthquake and adjusts for distance to find its strength
For each unit, ground motion is 10x larger

18. Earthquake Strength Magnitude (Richter Scale) Effects 1.0
Detected only by seismographs
2.0
Hanging object swing
3.0
Feels like a truck passing by
4.0
Breaks windows, small things fall
5.0
Furniture moves
6.0
Damage to structures
7.0
Cracks in earth, underground pipes cracked, buildings displaced from foundations
8.0
Bridges destroyed, buildings toppled over
>9.0
Complete damage, waves visible moving through earth

19. Earthquake Strength
Intensity – a measure of how much an earthquake is felt by people and the amount of damage caused by the earthquake
Mercalli Intensity Scale – uses Roman Numerals I through XII to indicate the intensity of an earthquake
XII – total destruction of an area
One earthquake would have different intensities in different areas

20. Earthquakes and society
Section 8.3
Earthquakes and society

21. Earthquake Hazard
Earthquake hazard – how likely an area is to have damaging earthquakes in the future
Determined by past and present seismic activity

22. Earthquake Forecasting
Strength and Frequency
The strength of earthquakes is related to how often they occur!
Worldwide Earthquake Frequency
Magnitude of Earthquake
Average Number per Year
8.0 or higher 1 18
120
800
~6,200
~49,000
~365,000

23. Earthquake Forecasting
Gap hypothesis – sections of active faults that have had few earthquakes in the past are likely to have strong earthquakes in the future
Seismic gap – areas along a fault where few earthquakes have occurred

24. Earthquakes and Buildings
Retrofitting – making older structures more earthquake resistant

25. Retrofitting
Mass damper – weight placed in the room of a building. Motion sensors detect building movement in an earthquake and send information to a computer. The computer moves the weight to counteract the building’s movement.

26. Retrofitting
Cross braces – steel beams placed between floors of a building to counteract pressure that pushes and pulls at the side of a building

27. Retrofitting
Active tendon system – work like the mass damper in the roof; a computer moves a large weight in the bottom of a building to counteract movement

28. Retrofitting
Flexible pipes – prevent waterlines and gas lines from breaking; pipes can twist and bend without breaking

29. Retrofitting
Base isolators – shock absorbers made of layers of rubber and steel

30. Get Prepared! Make an action plan for an earthquake.
What would you do beforehand to prepare for an earthquake?
What would you do during an earthquake to protect yourself?
What would you do after an earthquake to recover?