1. EARTHQUAKES

2. Section 19.1Forces within Earth
Most earthquakes happen because the Earth’s crust moves along plate boundaries
Usually the movement is very slight because the rocks resist the movement
This causes stress to build up

3. Stress Is the total force acting on crustal rocks per unit of area
Movement occurs when stress overcomes the strength of the rocks
There are three kinds of stress
Compression is the stress that decreases the volume of a material
Tension is the stress that pulls a material apart
Shear is the stress that twists a material

4. Strain The deformation of a material in response to the stress
Rocks can take bending, twisting and stretching to a point. That critical point is when the rocks break releasing energy (earthquake)

5. Elastic Deformation
Caused when a material is compressed, bent or stretched
Generally is temporary
Once stress is reduced to zero, materials return to normal

6. Plastic Deformation Caused when stress builds up past a critical point
Rocks are permanently deformed
Most materials will display both elastic and plastic deformation behavior—the amount of stress and strain determine which

7. Faults
Any fracture or system of fractures along which the Earth moves.
The surface along which the earth moves is called the fault plane.
There are several types of fault: reverse, normal and strike slip

8. San Andreas Fault Line

9. Reverse Faults Form due to horizontal and vertical compression
Squeezes the rock and causes a shortening of the crust
Rock on one side pushes up relative to the other side

11. Normal Faults Movement is partly horizontal and partly vertical
Horizontal movement pulls rocks apart and stretches the crust
Vertical movement occurs as stretching causes rock on one side to move down relative to other side

13. Strike Slip Fault Caused by horizontal shear
Movement is mainly horizontal and in opposite directions

15. Earthquake Waves
Seismic waves are the vibrations of the ground that occurs during an earthquake
Three main types of seismic waves
Primary waves
Secondary waves
Surface waves

17. Primary Waves Also called P waves
Squeeze and push rocks in the direction along which the waves are traveling
Movement is similar to that of a coiled wire—the movement is back and forth movement

18. Secondary Waves Also called S waves
Slower than P waves so they are felt second hence the name
Their motion cause rocks to move a right angles in direction of waves

20. Surface Waves Third type and slowest of waves
Travel only along Earth’s surface
Can cause ground to move sideways and up and down like ocean waves
Cause the most destruction because they take the longest time to pass and cause the most movement of the ground

22. Focus and Epicenter
The focus is the where the waves originate; usually several km below Earth’s surface
The epicenter is the point on Earth’s surface above the focus
Surface waves originate from the epicenter and spread out

23. Learning Targets Define stress and strain as they apply to rocks.
Distinguish among the three types of movement of faults.
Contrast the three types of seismic waves.

24. 19.2 Seismic Waves and Earth’s Interior
Most vibrations of Earth can’t be felt far away from the epicenter
But a seismometer can detect even slight movement
A seismometer is a sensitive instrument that measures Earth motion
Produces a seismogram a paper or computer graph of the movement

26. Clues to Earth’s Interior
Seismic waves change speed and direction at the boundaries between different materials
P waves and S waves initially travel through the mantle following direct paths
P waves move through mantle and are refracted when they strike the core (they bend)
S waves will not travel through liquid and do not travel through the core

28. Learning Targets Describe how a seismometer works.
Explain how seismic waves have been used to determine the structure and composition of Earth.

29. 19.3 Measuring and Locating Earthquakes
Richter scale
Moment magnitude scale
Modified Mercalli scale

33. Learning Targets
Compare and contrast earthquake magnitude and intensity and the scales to measure each.