Chapter 2: Earthquakes


Earthquakes release stress that has built up in rocks.


2.1 Earthquakes occur along faults.

2.2 Earthquakes release energy.

2.3 Earthquake damage can be reduced.


2.1 Earthquakes occur along faults.

I. Rocks move along faults.


A. Fault- a fracture, or break, in Earth’s lithosphere, along which blocks of rock move past each other.


B. Rocks bend as stress is put on them.

1. Stress- the force exerted when an object presses on, pulls on, or pushes against another object.

2. Along some parts of a fault, the rock on either side may slide along slowly and constantly.

3. Along other parts the rocks may stick or lock together, putting stress on them.

4. As stress increases, the rocks break free causing an earthquake.


C. Earthquakes

1. An earthquake is a shaking of the ground caused by the sudden movement of large blocks of rock along a fault.

2. Are there earthquakes in Pennsylvania?


D. Most earthquakes occur along plate boundaries.

      1. This is where faults occur.

2. Remember, the blocks of rock that move during an earthquake are much smaller than the tectonic plates.

3. The strength of an earthquake depends on

      a. how much stress builds up before the rocks move.

      b. the distance the rocks move along the fault.

4. 80% of all earthquakes occur in a belt along the edges of the Pacific Ocean. 

5. The best known U.S. fault in this belt is the San Andreas Fault.

      a. Located in California.

b. Forms part of the boundary between the North American Plate and the Pacific Plate.

c. Parts can be seen above ground, unlike many other faults.

6. A small percentage of earthquakes occur along faults within plates (see above link to PA earthquakes).


E. All earthquakes occur in the lithosphere. 

      1. The lithosphere is cold and rigid.

      2. In contrast, the asthenosphere is warm and fluid.


II. Faults are classified by how rocks move.


A. Scientists classify faults according to the way the rocks on one side move with respect to the rocks on the other side.


B. There are three main types of faults: normal, reverse and strike-slip.


C. The type of fault is determined by whether the plates are pulling apart, pushing together, or scraping past each other.

      1. Normal Faults

a. Here, the block of rock above the fault plane slides down relative to the other block due to the stress that pulls the rocks apart.

b. Earthquakes along these faults are common near boundaries where plates are moving apart.

2. Reverse Faults

a. Here, the block of rock above the fault plane moves up relative to the other block due to the stress that presses the rocks together.

b. These faults can occur near collision-zone boundaries between plates.

c. The Himalayas, which form in the area where the Indian Plate is pushing into the Eurasian plate, have many earthquakes along reverse faults.

3. Strike-Slip Faults

a. Here, blocks of rock move sideways on either side of the fault plane due to stresses that push blocks of rock horizontally.

b. Occur where plates slide past each other.

c. The San Andreas Fault is a strike-slip fault.