1. What are Earthquakes?
The shaking or trembling caused by the sudden release of energy
Usually associated with faulting or breaking of rocks
Continuing adjustment of position results in aftershocks
Aftershock: small earthquakes following a large earthquake

2. What is the Elastic Rebound Theory?
Explains how energy is stored in rocks
Rocks bend until the strength of the rock is exceeded
Rupture occurs and the rocks quickly rebound to an undeformed shape
Energy is released in waves that radiate outward from the fault

3. What is the Elastic Rebound Theory?
Due to motion along a fault
Fault is a broken rock formation
Opposite sides stretch
Force applied to the rocks eventually exceeds the strength
Force is called stress
The rock moves
Strain is the movement of the rocks
Strain occurs along plate boundaries
Caused by explosions or volcanic eruptions
System returns to unstrained conditions (Rebound)

4. The Focus and Epicenter of an Earthquake
The first rupture occurs at the focus…deep in the Earth
The point of the Earth’s surface above this rupture is called the epicenter
Deep focuses cause little damage
Beneath 700 km the rock is too hot and plastic to break….it flows

5. Seismographs record earthquake events
Seismograms are the records that are produced

6. Types of Faults: Dip-slip (vertical motion)
Normal fault
Hanging wall moves down compared to footwall
Reverse Fault
Hanging wall moves up compared to footwall

7. Types of Faults: Strike-slip (horizontal motion)
Lateral right
Motion is to the right
Lateral left
Motion is to the left

8. What are Seismic Waves?
Response of material to the arrival of energy fronts released by rupture
Two types:
Body waves
P and S
Surface waves
R and L

9. Body Waves: P and S waves
P or primary waves
fastest waves
travel through solids, liquids, or gases
compressional wave, material movement is in the same direction as wave movement
S or secondary waves
slower than P waves
travel through solids only
shear waves - move material perpendicular to wave movement

10. Surface Waves: R and L waves
Travel just below or along the ground’s surface
Slower than body waves; rolling and side-to-side movement
Especially damaging to buildings

11. How is an Earthquake’s Epicenter Located?
Seismic wave behavior
P waves arrive first, then S waves, then L and R
Average speeds for all these waves is known
After an earthquake, the difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter.

12. How is an Earthquake’s Epicenter Located?
Time-distance graph showing the average travel times for P- and S-waves. The farther away a seismograph is from the focus of an earthquake, the longer the interval between the arrivals of the P- and S- waves

13. How is an Earthquake’s Epicenter Located?
Three seismograph stations are needed to locate the epicenter of an earthquake
A circle where the radius equals the distance to the epicenter is drawn
The intersection of the circles locates the epicenter

14. Where Do Earthquakes Occur and How Often?
~80% of all earthquakes occur in the circum-Pacific belt
most of these result from convergent margin activity
~15% occur in the Mediterranean-Asiatic belt
remaining 5% occur in the interiors of plates and on spreading ridge centers
more than 150,000 quakes strong enough to be felt are recorded each year

15. How are the Size and Strength of an Earthquake Measured?
Intensity
subjective measure of the kind of damage done and people’s reactions to it
Based on geology, construction, and perception
Modified Mercalli Intensity Map
1994 Northridge, CA earthquake, magnitude 6.7

16. Mercalli Scale

17. How are the Size and Strength of an Earthquake Measured?
Magnitude
Richter scale measures total amount of energy released by an earthquake;
Maximum amplitude
Correct to a standard seisometer
Correct for distance from epicenter
Logarithmic in microns

18. Richter Scale

19. What are the Destructive Effects of Earthquakes?
Ground Shaking
amplitude, duration, and damage increases in poorly consolidated rocks

20. Can Earthquakes be Predicted?
Earthquake Precursors
changes in elevation or tilting of land surface, fluctuations in groundwater levels, magnetic field, electrical resistance of the ground

21. Can Earthquakes be Predicted?
Earthquake Prediction Programs (Statistical models)
Based on uniformitarianism
Recurrence levels
Fault history

22. Can Earthquakes be Predicted?
Earthquake Prediction Programs (Geophysical models)
Ground tilt
Magnetism
Resistivity
Seismic velocity
Radon emission
Foreshocks
Stress changes

23. Can Earthquakes be Predicted?
Earthquake Prediction Programs
Seismic Gaps: active faults that are not broken
Animal Behavior
Predictability based on magnitude, occurrence, rupture length and displacement

24. Can Earthquakes be Controlled?
Graph showing the relationship between the amount of waste injected into wells per month and the average number of Denver earthquakes per month
Some have suggested that pumping fluids into seismic gaps will cause small earthquakes while preventing large ones

25. Earthquake Hazards Ground shaking Surface ruptures
Changes in ground level
Landslides
Liquefaction
Fires
Tsunamis

26. Ground Shaking What most people think about when you say earthquake
Lessens with distance

27. Surface Ruptures
Very rare
Occurs in the movies

28. Changes in Ground Level
Large areas of land moved up or down

29. Landslides Unstable ground loosens and moves
Can include snow, rock, debris, and ice
Depends on the slope, vegetation, and rainfall
Can be very deadly

30. Liquefaction
Saturated ground “liquefies” and can no longer hold the weight
Buildings sink

31. Fires Indirect damage of an earthquake Ruptured gas lines
Downed power lines
Ruptured water lines
All problems created
by civilization

32. Tsunamis
Vertical displacement of a large amount of water

33. The Economics and Societal Impacts of EQs
Damage in Oakland, CA, 1989
Building collapse
Fire
Tsunami
Ground failure