1. Chapter 19 - Earthquakes

2. EARTHQUAKES CAN BE VERY POWERFUL. FORTUNATELY ONLY
AROUND 75 MAJOR EARTHQUAKES
HAPPEN EACH YEAR. THEY ARE CAUSED WHEN ROCKS FRACTURE OR BREAK.

3. What causes an earthquake?
A. Rocks break or fracture because of stress When the stress overcomes the strength of the rock, the rock breaks.

4. Rocks act like a wooden stick being bent
Rocks act like a wooden stick being bent. The rocks bend until they reach a breaking point, and then they move, or break, when they cannot take the stress any longer.

5. What causes an earthquake?
Rocks break or fracture because of stress. When the stress overcomes the strength of the rock, the rock breaks.
There are 3 types of stress:
B. Tensional stress = forces that pull rocks apart

6. What causes an earthquake?
C. Compressional stress = forces that push rocks together
D. Shear stress: forces that twist the shape of a rock
Strain: Rocks becoming deformed because of stress.

7. What is a fault?
A. The place (a crack) where the rock fractures and moves.
B. This movement releases the energy feel as an earthquake.

8. TYPES OF FAULTS
Faults are cracks in the Earth’s crust, along which movement has taken place. There are 3 major types (shown on the next slides).

9. C. Normal faults - Caused by TENSION - The crust becomes lengthened.

10. Reverse faults
- Caused by COMPRESSION
- The crust becomes shortened.

11. Strike-slip fault
- Caused when rocks move past one another horizontally.
- Caused by SHEAR stress
- Example is the San Andreas Fault.

12. Earthquake waves
A. Energy is released through vibrations of Earth
B. Energy is released in waves, called seismic waves, and travel outward in ALL directions.

13. Types of Waves
P Waves - compression waves (squeezed and pull rock in same direction) fastest move through solids and liquids

14. Types of Waves
S Waves - “slinky waves,” rock moves up and down - slower - move only through solids

15. Types of Waves
E. Surface Waves- - on the surface of the Earth only move up, down and sideways - cause the most damage - slowest

16. Focus - The point INSIDE the Earth where the earthquake begins.

17. G. Epicenter. - The point on Earth’s SURFACE
G. Epicenter - The point on Earth’s SURFACE where the earthquake begins It is directly above the focus.

18. So…why do earthquakes occur??
TO RELEASE BUILT UP STRESS IN THE ROCKS!!