1. Some information from: www. floyd. k12. va
Some information from:
Earthquakes!

2. Seismicity & Distribution of Earthquakes
There are thousands of small earthquakes every day “Strong” earthquakes (~M7) occur once a month. >M8 occur about once/year.
Where are the deepest earthquakes?
This figure shows the “baseball-stitching” pattern of earthquakes around the globe. This represents only a small percentage of the earthquakes that would typically occur in a year. Over a million earthquakes of Magnitude 2 and lower occur every year. And 1,500 Magnitude 5 occur every year.
RESOURCE: For a one-page flier on “How often do earthquakes occur?” go to
With some imagination you can see some of the gross continental boundaries.
Worldwide, strong earthquakes happen more than once per month. Smaller earthquakes, such as magnitude 2 earthquakes, occur several hundred times a day.
The deepest earthquakes are where one plate is subducting beneath another plate.
INTERACTIVE FLASH rollovers—This and the next slides are available from : DynamicPlanet-Earthquakes & Volcanoes
ACTIVITIES: World Tectonic Map activity link on page:
Exploring Rates of Earthquake Occurance:
For earthquakes of the past 2 weeks, go to

3. World Seismicity & Plate Tectonics
Notice that the earthquakes coincide with plate boundaries, and the deepest quakes (blue) are in subduction zones Question: Where would you expect to see volcanoes?
This slide shows the relationship between plate tectonics and earthquake location. There are thousands more earthquakes every year than shown here.
Many are in the middle of plates. Why would there be earthquakes in the middle of plates? Remember that the plates are sliding over a round globe and they crack, crumple, and creak as they adjust to the new position. All these adjustments cause earthquakes.
For earthquakes of last 2 weeks, go to
INTERACTIVE FLASH rollovers : ﾊﾊﾊﾊDynamicPlanet-Earthquakes & Volcanoes
Create your own maps at
Modified from USGS Graphics

4. 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

5. What is the Elastic Rebound Theory?
Explains how energy is stored in rocks
Everything has an elastic limit
Rocks bend until the strength of the rock is exceeded
Rupture occurs and the rocks quickly rebound to an undeformed shape
Video!
Area where break and movement occurs is fault
Energy is released in waves that radiate outward from the fault

6. What causes faults? Stress between two plates Volcanism
Type of stress changes type of fault
Volcanism
Cavern collapse/ subsidence
Contemporary Physical Geology, H.L. Levin, Saunders Publ.

7. Types of Fault
Normal
Reverse
Strike-slip

8. Types of fault: Normal
Hanging wall moves down
Due to tension
Wasatch

9. Types of fault: Reverse
Hanging wall moves up
Due to compression
Reverse_fault_480.mov - YouTube

10. Types of fault: Strike-slip
Rocks move past one another
Due to shear
Strike-slip-fault_480.mov - YouTube

11. A lecture from IRIS
4.Fault Models Lecture - YouTube

12. 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

13. Seismographs record earthquake events

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

15. 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

16. 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
Very damaging to buildings

17. How is an Earthquake’s Epicenter Located?
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.

18. Where Do Earthquakes Occur and How Often?
~80% of all earthquakes occur in the circum-Pacific belt (The Ring of Fire)
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

19. At convergent boundaries, focal depth increases along a dipping seismic zone called a Benioff zone

20. 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

21. 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

22. 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
isoseismal lines identify areas of equal intensity
Modified Mercalli Intensity Map
1994 Northridge, CA earthquake, magnitude 6.7

23. How are the Size and Strength of an Earthquake Measured?
Magnitude
Richter scale measures total amount of energy released by an earthquake; independent of intensity
Amplitude of the largest wave produced is corrected for distance and assigned a value on an open-ended logarithmic scale

24. Structure of Earth Behavior of waves tells us about the material
Earth's Interior
Interactives . Dynamic Earth . Intro
Structure of the Earth

25. Delaughter et al, found that at the undergraduate level, only 5% of students responded that scientists knew about the Earth’s interior based on information from earthquakes.

26. Basic Structure of Earth

27. Basic Structure of Earth
Speed and paths of seismic waves change as they travel through materials with different densities
Earth is made up of layers of different materials
P and S waves do specific things in the mantle, outer core and inner core

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

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

30. 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
seismic dilatancy model
seismic gaps

31. Can Earthquakes be Predicted?
Earthquake Prediction Programs
include laboratory and field studies of rocks before, during, and after earthquakes
monitor activity along major faults
produce risk assessments

32. 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