1. Earthquakes
Chapter 19

2. Stress and strain Earthquakes occur when rocks fracture
Fractures occur when stress exceeds strain
Stress: forces per unit area acting on a material
Three types
COMPRESSION
TENSION
SHEAR
Strain: deformation of materials in response to stress

3. Rock behavior ELASTIC BEHAVIOR: return to original shape after stress
DUCTILE BEHAVIOR: rocks are deformed or change shape
Ductile deformation is permanent
Material stays deformed even if stress is reduced to zero
Material can break or fail depending on its strength

4. Earthquakes http://www.youtube.com/watch?v=VSgB1IWr6O4
Produced by the release of elastic energy stored in rock that has been subjected to great forces
When the strength of the rock is exceeded by these forces, it breaks.
Causes vibrations of an earthquake

5. faults
FAULT: resulting fracture or system of fractures where movement occurs
Types of Faults
1. Reverse Fault
Horizontal compression
2. Normal Fault
Horizontal tension
3. Strike-slip Fault
Horizontal shear

6. Earthquake waves
The vibrations of the ground during an earthquake are called SEISMIC WAVES.
THREE TYPES
1. Primary waves: pull rocks in same direction
2. Secondary waves: rocks move at right angles
3. Surface waves: rocks move in two directions, up and down or side to side

7. Earthquake waves Surface waves: travel along Earth’s surface
Body waves: travel through Earth’s interior
P-waves, S-waves
Body waves spread out from the point of failure of rocks.

8. Earthquake waves Focus: Where earthquake originates
Epicenter: point on Earth’s surface directly above the focus

9. vibrations Vibrations sent out by earthquakes shake the entire globe.
Seismometers: detect and record these vibrations in the form of a seismogram

10. How does a seismometer work?
Take a piece of tape and attach a piece of paper to the desk.
One student hold the pencil to the paper while another slowly pulls the table.
What do you see?
Now, the third student should begin to shake the table opposite the direction it is being pulled.
Now what do you notice?

11. Seismometer questions
1. Explain how a seismometer works.
2. What is a seismogram?
3. Describe how each type of wave (P-waves, S-waves, and surface waves) causes rock particles to move.
[You can use your book to help answer these questions]

12. Earthquake magnitude and intensity
Magnitude: amount of energy released
Measured on the Richter scale
Based on the size of the largest seismic wave
Each point is 10x larger than previous
How much larger is a magnitude-8 earthquake than a magnitude-7? What about magnitude-6?
Moment magnitude scale: takes into account the size of the fault rupture, amount of movement along the fault, rock’s stiffness

13. Earthquake intensity
Intensity: the amount of damage done to structures involved
Measured on the modified Mercalli scale
Rates the types of damage and other effects of an earthquake as noted by observers
Uses roman numerals I-XII
I: not felt except under unusual conditions
XII: Damage is total. Objects are thrown upward into the air.

14. Structural failure
Pancaking: supporting walls of the ground floor fail and cause the upper floors to fall and collapse
Building height: intermediate buildings (5- 15 stories) collapse more readily than shorter or higher buildings due to the frequency of the Earthquake waves.

15. Tall or Small? Which is Safer? It depends on resonance!!
Small buildings are more affected, or shaken, by high- frequency waves (short and frequent). For example, a small boat sailing in the ocean will not be greatly affected by a low-frequency swell where the waves are far apart. On the other hand several small waves in quick succession can overturn, or capsize, the boat.
Large structures are more affected by low-frequency, or slow shaking. Ex: An ocean liner will experience little disturbance by short waves in quick succession. However, a low-frequency swell will significantly affect the ship.

16. Earthquake Hazards Land and soil failure
Can trigger massive landslides
Subsurface material in fluid-saturated sand may liquefy and form quicksand
Soft soils can actually amplify the waves
Why do you think this is?

17. Fault scarps
Great vertical offset where the fault intersects the ground surface

18. tsunami
A large ocean wave generated by vertical motions of the seafloor during an earthquake

19. Predicting an earthquake
We use two factors to estimate the probability of an earthquake
1. Earthquake History
Seismic gaps: sections of active faults that haven’t experienced significant earthquakes for a long period of time
Lasers
2. Strain Accumulation
The rate at which strain builds up in rocks
Strain accumulated in a particular location of a fault
Strain released during last earthquake
Amount of time passed since last earthquake