1. Earthquakes
8th Grade Science

2. Stress in the Crust
What is an earthquake?
An Earthquake is the shaking and trembling that results from the movement of rock beneath Earth’s surface.
The movement is caused by stress in Earth’s layers.

3. Stress in the Crust
What is stress?
Stress is a force that acts on the rock to change its shape or volume.
Because stress is a force, it adds energy to the rock.
The energy is stored in the rock until the rock either breaks or changes shape.

4. Types of Stress
Are there different types of stress?
There are three different types of stress that occur in the crust:
Shearing
Tension
Compression

5. Types of Stress
Shearing, tension, and compression work over million years to change the shape and volume of rock.

6. Types of Stress
What is Shearing?
Shearing – Stress that pushes a mass of rock into tow opposite directions.

7. Types of Stress
What is Tension?
Tension – pulls on the crust, stretching rock so that it becomes thinner in the middle.

8. Types of Stress Compression – Squeezes rock until it folds or breaks.
What is Compression?
Compression – Squeezes rock until it folds or breaks.

9. Types of Stress
What is Deformation?
Deformation – A change in the volume or shape of Earth’s crust.
The crust bends, stretches, breaks, tilts, folds and slides.

10. Mountain Building
Over millions of years, fault movement can change a flat plain into a towering mountain range.
Mountains are formed by:
Faulting
Folding
Anticlines and Synclines
Plateaus

11. Earthquakes Earthquakes always begin in rock below the surface
Most earthquakes begin in the lithosphere within 100km of Earth’s surface.

12. Earthquakes
What is the focus and the epicenter of an earthquake?
Focus – is the point beneath Earth’s surface where the rock under stress breaks, triggering an earthquake
Epicenter – The point on the surface directly above the focus.

13. Seismic Waves
What are seismic waves?
Seismic Waves – are vibrations that travel through Earth carrying the energy released during an earthquake.
Seismic waves carry the energy of an earthquake away from the focus, through Earth’s interior and across the surface.

14. Seismic Waves There are three categories of seismic waves: P – waves
What are the types of seismic waves?
There are three categories of seismic waves:
P – waves
S – Waves
Surface waves

15. Seismic Waves
P – waves – are earthquake waves that compress and expand the ground like an accordian.

16. Seismic Waves
S – waves – are earthquake waves that vibrate from side to side as well as up and down.

17. Seismic Waves
Surface waves – move more slowly than P and S waves, but they produce the most sever ground movements.
These wave make the ground roll like an ocean and make buildings shake from one side to the other.

18. Earthquake Damage
The severe shaking produced by seismic waves can cause damage or destroy buildings and bridges, topple utility poles, and fracture gas and water mains.

19. Earthquake Damage
Local Soil Conditions – The loose soil shakes more violently than the surrounding rock. The thicker the layer of soil, the more violent the shaking will be.

20. Earthquake Damage
Liquefaction – occurs when an earthquake’s violent shaking suddenly turns loose, soft soil into liquid mud.
It can trigger mudslides and buildings to sink.

21. Earthquake Damage
Aftershocks – is an earthquake that occurs after a larger earthquake in the same area.

22. Earthquake Damage
Tsunamis – Large waves formed by earthquake displaced water.
Tsunamis spreads out from an earthquake's epicenter and speeds across the ocean.
As they approach shallow water near a coastline, the waves become closer together
The Tsunami grows into a mountain of water that can reach 6 stories high.

23. Earthquakes