*Source: Inside Earth by Prentice Hall Publishers, Science Explorer
What is stress?
- Stress is a force that acts on a rock to change its shape or volume.
- Tension - stretches rock so that it becomes thinner in the middle
- Compression - pushes rock together until it folds or breaks
- Shearing - pushes a mass of rock in two opposite directions
- A fault is a break in the rock of the crust.
- Most faults occur along plate boundaries, where the forces of plate motion push or pull the crust so much that the crust breaks. There are three main types of faults.
- Normal Faults - tension in Earth's crust pulls rock apart
- Reverse Faults - compression pushes the rock of the crust together
- Strike-Slip Faults - the rock on either side of the fault slip past each other
What is an earthquake?
Focus vs. Epicenter
•The focus is the place inside the earth where an earthquake begins.
The epicenter is the place on the surface of the earth directly above the focus, where an earthquake is first felt.
Check out these web sites for moving animation of seismic waves.
L-waves or Surface waves
What Causes Earthquakes?
Three Major Earthquake Zones