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.


2.2 Earthquakes release energy.

I.Energy from earthquakes travels through Earth.

†††† A. Earthquake energy travels outward in all directions.

B. Seismic waves- vibrations caused by earthquakes; earthquake energy travels as seismic waves.

C. All earthquakes start beneath the surface in the lithosphere.

1. The focus of the earthquake is the point underground where rocks first begin to move.

2. The epicenter is the point on Earthís surface directly above the focus.

3. Earthquakes are often named after the city where the epicenter is closest.

4. The shallower the focus, the stronger the earthquake.

D. Waves and Energy

††††† 1. All waves carry energy from place to place.

2. Seismic waves can travel completely through the earth in 20 minutes.

3. Scientists can learn about the Earthís layers by studying the paths and speeds of seismic waves traveling through the Earth.

4. Earthquakes produce 3 types of waves.

††††† a. Primary (P) waves

i. The fastest of the seismic waves, these are the first waves to reach any particular location after an earthquake occurs.

ii. They travel through the crust at an average speed of about 3 mi/s.

iii. Can travel through solids, liquids or gases.

iv. Primary waves cause particles of the material that they are passing through to be pushed together and pulled apart.

v. May cause buildings to push then pull back.

b. Secondary (S) waves

i. The second seismic waves to reach a location after an earthquake, though they start at the same time.

ii. Travel through the Earthís interior at half the speed of P waves.

iii. As they pass through a material, they cause the materialís particles to be shaken up and down or side to side.

iv. Cause buildings to rock back and forth.

v. Can travel through rock, but not liquids or gases, since they do not have a definite shape (particles do not move back to their original positions).

vi. This is how scientists figured out that the outer core was not solid.

c. Surface waves

††††† i. Seismic waves that move along Earthís surface.

ii. They make the ground roll up and down or side to side.

iii. Cause the largest ground movement and the most damage.

iv. Travel more slowly than the other seismic wave types.

II. Seismic waves can be measured.

†††† A. Seismic stations are places where ground movements are measured.

B. A seismograph is an instrument that constantly records ground movements.

C. Two types of seismographs: one records side-to-side movements (like the animation above) and one tracks up and down movements.

D. Seismographs can detect ground movements as small as one hundred-millionth of a centimeter.

E. The recording produced by a seismograph is called a seismogram.

III. Locating an Earthquake

A. To locate the epicenter of an earthquake, scientists must have seismograms from at least 3 seismic stations.

B. Three-step procedure:

1. Scientists find the difference between arrival times of primary and secondary waves at each of 3 stations.

2. Time difference is used to determine the distance of the epicenter from each station.The greater the difference in time, the farther away the epicenter is.

3. A circle is drawn around each station, with a radius corresponding to the epicenterís distance from that station.The point where the three circles meet is the epicenter.

C. Scientists can also use seismograms to locate the depth of the focus of an earthquake.