1. Earthquakes

2. Learning Targets. I can:
Forces Within Earth
Learning Targets. I can:
Define stress and strain as they apply to rocks.
Distinguish among the three types of faults.
Contrast three types of seismic waves.
Vocabulary
stress
strain
fault
primary wave
secondary wave
surface wave
focus
epicenter

3. Stress – pressure that breaks rock. It is the Force per unit Area
Forces Within Earth
I. Forces Within Earth
Stress – pressure that breaks rock. It is the Force per unit Area
Compression
Tension
Shear

4. Compression is stress that decreases the volume of a material.
Forces Within Earth
Compression is stress that decreases the volume of a material.
Tension is stress that pulls a material apart.
Shear is stress that causes a material to twist.

5. B. Strain – how the rock responds to stresses.
Forces Within Earth
B. Strain – how the rock responds to stresses.
C. Deformation of rock
1. Elastic deformation; rock can bounce back
2. Ductile deformation; permanent
3. Failure; rock breaks. This is called a fault

6. Forces Within Earth
Stress and Strain
There is a distinct relationship between stress and strain that can be plotted as a stress-strain curve.
A stress-strain curve usually has two segments: a straight segment and a curved segment.
Low stresses produce the straight segment, which represents the elastic strain of a material.
If the elastic strain is reduced to zero, the deformation disappears.

7. Stress and Strain Ductile Deformation
Forces Within Earth
Stress and Strain
Ductile Deformation
When stress exceeds a certain value, a material undergoes deformation, shown by the curved segment of the graph.
This type of strain produces permanent deformation, which means that the material stays deformed even if the stress is reduced to zero.

8. 1. Reverse fault – result of compression forces
Forces Within Earth
D. Faults
1. Reverse fault – result of compression forces
2. Normal fault – result of tensional forces
3. Strike-slip fault – result of shear forces

9. Forces Within Earth
Earthquake Waves
Most earthquakes are caused by movements along faults.
Irregular surfaces in rocks can snag and lock, causing stress to build in the rocks.
When the rocks reach their elastic limit they break, and this produces an earthquake.

10. E. Types of Seismic (Earthquake) Waves
Forces Within Earth
E. Types of Seismic (Earthquake) Waves
1. Primary waves, or P-waves, squeeze and pull rocks in the same direction the waves are traveling. Fastest seismic waves.

11. Forces Within Earth
2. Secondary waves, or S-waves, cause rocks to move at right angles to the direction of the waves. Travel at ½ the speed of P-waves. Do not travel through liquids.
3. Surface waves travel along Earth’s surface, moving in two directions as they pass through rock.
Wave Simulator

12. The focus is the point underground where the rock actually breaks
Forces Within Earth
P-waves and S-waves, also called body waves, pass through Earth’s interior.
The focus is the point underground where the rock actually breaks
The epicenter of an earthquake is on Earth’s surface directly above the focus.