Forces in Earth Earthquakes Chapter 5 Lesson 4 Forces in Earth Earthquakes

Section 1 Forces Inside Earth When rocks break they move along faults. Applied forces cause rocks to undergo elastic deformation When elastic limits are passed, rocks break 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 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 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/plate5.htm forces animation http://scign.jpl.nasa.gov/learn/plate6.htm fault animation

Section 2 Features of Earthquakes Seismic waves – waves generated by an earthquake, can move the ground forward and backward, up, and down, and side to side 1. 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 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 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 fastest 2. Secondary waves follow 3. Surface waves move slowest and arrive at the seismograph station last.

4. Seismograph- measures seismic waves a. 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

Modified Mercalli Intensity Scale Equivalent 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 4.0 Noticeable indoors, especially on upper floors, but may not be recognized as an earthquake. IV 4.0 Felt by many indoors, few outdoors. May feel like heavy truck passing by. V 4.0 to 5.0 Felt by almost everyone, some people awakened. Small objects moved. trees and poles may shake. VI 5.0 to 6.0 Felt by everyone. Difficult to stand. Some heavy furniture moved, some plaster falls. Chimneys may be slightly damaged. VII 6.0 Slight 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 8.0 Most masonry and frame structures and their foundations destroyed. Ground badly cracked. Landslides. Wholesale destruction. XI 8.0 Total damage. Few, if any, structures standing. Bridges destroyed. Wide cracks in ground. Waves seen on ground. XII 8.0 or greater Total damage. Waves seen on ground. Objects thrown up into air.