Chapter 5 Lesson 4

Forces in EarthEarthquakes

Section 1 Forces Inside Earth

A. When rocks break they move along faults.1. Applied forces cause rocks to undergo elastic

deformation2. When elastic limits are passed, rocks break3. Rock on one side of a fault can move up, down,

or sideways in relation to rock on the other side of the fault.

B. Faults occur because forces inside the Earth cause Earth’s plates to move placing stress on or near the plate edge.1. Rocks will bend, compress, stretch, and possible

break

2. Earthquake vibrations produced by breaking rock

a. rocks break, move along the fault, return to original shapes

b. rock on one side of the fault can move over, under, or past each other along fault lines

C. Three types of forces act on rocks- tension, compression, and shear.1. Tension forces; normal fault – caused by rock above the fault moving downward in relation to the rock below the fault

2. Reverse fault- compression forces squeeze rock above the fault up and over the rock below the fault

3. Create by shear forces; strike-slip fault – rocks on either side of the fault move past each other without much upward or downward motion

http://scign.jpl.nasa.gov/learn/plate6.htmfault animation

http://scign.jpl.nasa.gov/learn/plate5.htm

forces animation

Section 2 Features of Earthquakes

A. Seismic waves – waves generated by an earthquake, can move the ground forward and backward, up, and down, and side to side1. Focus- an earthquake’s point of energy release

2. Primary waves (P-waves)- cause particles in rocks to move back and forth in the same direction that the wave is traveling

3. Secondary waves (S-waves) – cause particles in rock to move at right angles to the direction of wave travel

4. Surface waves – move rock particles in a backward, rolling motion and a sideways swaying motion

5. The point on the Earth’s surface directly above the earthquake focus is called the epicenter

B. The different speeds of seismic waves allow scientists to determine the epicenter.1. Primary waves move fastest2. Secondary waves follow3. Surface waves move slowest and arrive at the seismograph station last.

4. Seismograph- measures seismic wavesa. consists of a rotating drum of paper and a pendulum with an attached pen.b. The paper record of the seismic event is called a seismogram

C. Earth’s structure consists of an inner, mostly iron, solid core surrounded by a mostly iron liquid outer core surrounded by the mantle.1. The crust is Earth’s outer layer, about 5-60 km thick

2. A seismic wave’s speed and direction change as the wave moves through different layers with densities.

a. Density generally increase with depth as pressures increase

b. Shadow zones do not receive seismic

waves because the waves are bent or stopped by materials of different density

3. Changes in seismic wave speed allowed detection of boundaries between Earth’s layers

Section 3 People and Earthquakes

A.Although earthquakes are natural geologic events, they kill many people and cause a lot of damage.

1. Seismologist- scientists who study earthquakes

2. Magnitude- measure of energy released by an earthquake; determined by the Richter scale and based on the height of lines on a seismogram

• a. The Richter scale has no upper limit. b. Most earthquakes have magnitudes too low to be felt by humans – 3.0-4.9 on the Richter scale.

3. The modified Mercalli intensity scale describes earthquake intensity based on structural and geologic damage.

Mercalli IntensityEquivalent Richter Magnitude

Witness Observations

I 1.0 to 2.0 Felt by very few people; barely noticeable.

II 2.0 to 3.0 Felt by a few people, especially on upper floors.

III 3.0 to 4.0Noticeable indoors, especially on upper floors, but may not be recognized as an earthquake.

IV 4.0Felt by many indoors, few outdoors. May feel like heavy truck passing by.

V 4.0 to 5.0Felt by almost everyone, some people awakened. Small objects moved. trees and poles may shake.

VI 5.0 to 6.0Felt by everyone. Difficult to stand. Some heavy furniture moved, some plaster falls. Chimneys may be slightly damaged.

VII 6.0Slight to moderate damage in well built, ordinary structures. Considerable damage to poorly built structures. Some walls may fall.

VIII 6.0 to 7.0

Little damage in specially built structures. Considerable damage to ordinary buildings, severe damage to poorly built structures. Some walls collapse.

IX 7.0

Considerable damage to specially built structures, buildings shifted off foundations. Ground cracked noticeably. Wholesale destruction. Landslides.

X 7.0 to 8.0Most masonry and frame structures and their foundations destroyed. Ground badly cracked. Landslides. Wholesale destruction.

XI 8.0Total damage. Few, if any, structures standing. Bridges destroyed. Wide cracks in ground. Waves seen on ground.

XII 8.0 or greaterTotal damage. Waves seen on ground. Objects thrown up into air.

4. Liquefaction- shaking from an earthquake can make wet soil act like a liquid.

5. Ocean waves caused by earthquakes are called tsunamis. a. Caused when a sudden movement of the ocean floor pushes against the water. b. Can travel thousands of kilometers in all directions

B. Earthquakes cannot be reliably predicted. 1. Knowing how and where to plan for earthquakes can help prevent death and damage. 2. Buildings can be constructed to withstand seismic vibrations.

a. Flexible, circular moorings are being placed under buildings; made of alternating layers of rubber and steel.

b. The rubber acts like a cushion to absorb earthquake waves.3. Homes can be protected by careful placement of heavy objects and securing gas appliances.4. During an earthquake, craw under a sturdy table or desk; outdoors; stay away from buildings and power lines.5. After an earthquake, check for water or gas line damage; leave immediately if a gas smell is present.