Chapter 5 Lesson 4
Forces in Earth
Earthquakes
Section 1 Forces Inside Earth
A.When rocks break they move along faults.
1.Applied forces cause rocks to undergo elasticdeformation
2.When elastic limits are passed, rocks break
3.Rock on one side of a fault can move up, down,or sideways in relation to rock on the other sideof the fault.
B. Faults occur because forces inside the Earthcause Earth’s plates to move placing stress onor near the plate edge.
1. Rocks will bend, compress, stretch, andpossible
break
2. Earthquakevibrations producedby breaking rock
a.rocks break, movealong the fault,return to originalshapes
b. rock on one side ofthe fault can moveover, under, or pasteach other alongfault lines
C. Three types of forces act on rocks- tension,compression, and shear.
1. Tension forces; normal fault – caused byrock above the fault moving downward inrelation to the rock below the fault
2. Reverse fault- compression forces squeezerock above the fault up and over the rockbelow the fault
3. Create by shear forces; strike-slip fault –rocks on either side of the fault move pasteach other without much upward ordownward motion
fault animation
forces animation
Section 2 Features of Earthquakes
A.Seismic waves – waves generated by anearthquake, can move the ground forwardand backward, up, and down, and side toside
1. Focus- an earthquake’s point of energyrelease
2. Primary waves (P-waves)- causeparticles in rocks to move back and forthin the same direction that the wave istraveling
3. Secondary waves (S-waves) – causeparticles in rock to move at right angles tothe direction of wave travel
4. Surface waves – move rock particles in abackward, rolling motion and a sidewaysswaying motion
5. The point on the Earth’s surface directlyabove the earthquake focus is called theepicenter
B. The different speeds of seismic waves allowscientists to determine the epicenter.
1. Primary waves move fastest
2. Secondary waves follow
3. Surface waves move slowest and arrive atthe seismograph
station last.
4. Seismograph- measures seismic waves
a. consists of a rotating drum of paperand a pendulum with an attachedpen.
b. The paper record of the seismic event iscalled a seismogram
C. Earth’s structure consists of an inner, mostlyiron, solid core surrounded by a mostly ironliquid outer core surrounded by the mantle.
1. The crust is Earth’s outer layer, about 5-60km thick
2. A seismic wave’s speed and directionchange as the wave moves through differentlayers with densities.
a. Density generally increase with depthas pressures increase
b. Shadow zones
do not receive seismic
waves because the waves
are bent or stopped by
materials of different
density
3. Changes inseismic wavespeed alloweddetection ofboundariesbetween Earth’slayers
Section 3 People and Earthquakes
A.Although earthquakes are natural geologicevents, they kill many people and cause alot of damage.
1. Seismologist- scientists who studyearthquakes
2. Magnitude- measure of energyreleased by an earthquake; determined by theRichter scale and based on the height oflines on a seismogram
• a. The Richter scale has no upper limit.
b. Most earthquakes have magnitudes too lowto be felt by humans – 3.0-4.9 on the Richterscale.
3. The modified Mercalli intensity scaledescribes earthquake intensity based onstructural and geologic damage.
Modified Mercalli Intensity Scale
Mercalli Intensity
Equivalent RichterMagnitude
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 likeheavy 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 heavyfurniture moved, some plaster falls. Chimneysmay be slightly damaged.
VII
6.0
Slight to moderate damage in well built, ordinarystructures. Considerable damage to poorly builtstructures. 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. Somewalls collapse.
IX
7.0
Considerable damage to specially builtstructures, buildings shifted off foundations.Ground cracked noticeably. Wholesaledestruction. Landslides.
X
7.0 to 8.0
Most masonry and frame structures and theirfoundations destroyed. Ground badly cracked.Landslides. Wholesale destruction.
XI
8.0
Total damage. Few, if any, structures standing.Bridges destroyed. Wide cracks in ground. Wavesseen on ground.
XII
8.0 or greater
Total damage. Waves seen on ground. Objectsthrown up into air.
4. Liquefaction- shaking from an earthquake canmake wet soil act like a liquid.
5. Ocean waves caused by earthquakes arecalled tsunamis.
a. Caused when a sudden movement of theocean floor pushes against the water.
b. Can travel thousands of kilometers in alldirections
B. Earthquakes cannot bereliably predicted.
1. Knowing how and where toplan for earthquakes can helpprevent death and damage.
2. Buildings can beconstructed to withstandseismic vibrations.
a. Flexible, circularmoorings are being placedunder buildings; made ofalternating layers of rubber andsteel.
b. The rubber acts like a cushion to absorbearthquake waves.
3. Homes can be protected by careful placementof heavy objects and securing gas appliances.
4. During an earthquake, craw under a sturdytable or desk; outdoors; stay away from buildingsand power lines.
5. After an earthquake,
check for water or gas line
damage; leave
immediately if a gas smell
is present.