1. Faults and Earthquakes

2. Take-Away Points
Earthquakes generate waves that travel through the earth
Earthquakes occur when rocks slip along faults
Faults are classified by the kinds of movement that occur along them
Earthquakes don’t kill people, collapsing buildings and flooding kill people
Magnitude and Intensity
Seismic waves are used to map the earth’s interior
Predicting earthquakes is not yet possible

3. 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

4. The point directly above the focus on the surface is the epicenter
The Focus and Epicenter of an Earthquake
The point within Earth where faulting begins is the focus, or hypocenter
The point directly above the focus on the surface is the epicenter

5. Some Important Earthquakes
San Francisco
Killed 500 (later studies, possibly 2,500)
First Revealed Importance of Faults
1923 – Tokyo - Killed 140,000 in firestorm
Alaska
Killed about 200
Wrecked Anchorage.
Tsunamis on West Coast.
Tangshan, China
Hit an Urban Area of Ten Million People
Killed 650,000

6. 1. Earthquakes generate waves that travel through the earth
How Seismographs Work
1. Earthquakes generate waves that travel through the earth

7. 1. Earthquakes generate waves that travel through the earth
Seismic Waves
1. Earthquakes generate waves that travel through the earth

8. 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

9. 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

10. 2. Earthquakes occur when rocks slip along faults
Elastic Rebound
2. Earthquakes occur when rocks slip along faults

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. Major Hazards of Earthquakes
Building Collapse
Landslides
Fire
Tsunamis (Not Tidal Waves!)
4. Earthquakes don’t kill people, buildings kill people

16. Safest & Most Dangerous Buildings
Small, Wood-frame House - Safest
Steel-Frame
Reinforced Concrete
Unreinforced Masonry
Adobe - Most Dangerous
4. Earthquakes don’t kill people, buildings kill people

17. 4. Earthquakes don’t kill people, buildings kill people
Tsunamis
Probably Caused by Submarine Landslides
Travel about 400 M.p.h.
Pass Unnoticed at Sea, Cause Damage on Shore
Warning Network Around Pacific Can Forecast Arrival
Whether or Not Damage Occurs Depends on:
Direction of Travel
Harbor Shape
Bottom
Tide & Weather
4. Earthquakes don’t kill people, buildings kill people

18. Magnitude and Intensity
How Strong Earthquake Feels to Observer
Magnitude
Related to Energy Release
Determined from Seismic Records
Rough correlation between the two for shallow earthquakes
5. Magnitude and Intensity

19. 5. Magnitude and Intensity
How Strong Earthquake Feels to Observer
Depends On:
Distance to Quake
Geology
Type of Building
Observer!
5. Magnitude and Intensity

20. Magnitude - Determined from Seismic Records
Richter Scale:
Related to Energy Release
Exponential
No Upper or Lower Bounds
Largest Quakes about Mag. 8.7
Magnitude-Energy Relation
4 - 1
5 - 30 :
1 Megaton = about 7
7 - 27,000
,000
5. Magnitude and Intensity

21. 5. Magnitude and Intensity
Magnitude and Energy
Magnitude
Energy
Explosive Power
Example 9
U.S. Energy Use for a month
Alaska 1964 Indonesia 2004 8
U.S. Energy Use for a day
San Francisco, 1906 7
One Megaton
World Series Earthquake, 1989 6
U.S. Energy Use for a minute
Large Thunderstorm 5
One Kiloton 4 3
One ton of explosives
World Trade Center Collapse
5. Magnitude and Intensity

22. 5. Magnitude and Intensity
Magnitude and Energy
Magnitude
Energy
Explosive Power
Example 3
One ton of Explosives
World Trade Center Collapse 2 1
Topple 50-meter tree
One kilogram of explosives
Head-on colision at 60 mph
Drop a car 10 meters
Half stick of dynamite
Very bad day skydiving -1
Impact of bullet
One gram of explosives -2
Hammer blow -3
Dribbling a basketball
5. Magnitude and Intensity

23. Take-Away Points
Earthquakes generate waves that travel through the earth
Earthquakes occur when rocks slip along faults
Faults are classified by the kinds of movement that occur along them
Earthquakes don’t kill people, buildings kill people
Magnitude and Intensity
Seismic waves are used to map the earth’s interior
Predicting earthquakes is not yet possible