Ch 12-Earthquakes

• Objectives– Elastic rebound– Compare body waves and surface waves– Explain how the structure of Earth’s interior affects seismic

waves– Explain why earthquakes generally occur at plate boundaries– Describe the instrument used to measure and record

earthquakes– Summarize the method scientists use to locate an eipcenter– Describe the scales used to measure the magnitude and

intensity of earthquakes– Discuss the relationship between tsunamis and earthquakes– Describe possible effects of major earthquake on buildings– List safety techniques to prevent injury caused by earthquake

activity– Identify methods scientists use to forecast earthquake risks

Earthquakes

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• Movement or trembling of the ground that is caused by sudden release of energy when rocks along a fault move

• What happens?– Rocks under stress suddenly shift along a fault.

What is a fault?• Break in body of rock along which one block slides

relative to another

• Why do earthquakes happen?

• What do you think elastic rebound means?– Sudden return of elastically deformed rock to its

undeformed shape• Anatomy of Earthquake

– Focus-location w/in Earth along fault at which first motion of earthquake occurs

– Epicenter-point on Earth’s surface directly above an earthquake’s starting pt

• Earthquakes that have most damage=shallow foci

• Seismic Waves– As rocks slip, rocks release energy in form of

vibrations, travel in all directions– 2 types-body and surface waves

• Concepts in Motion

Seismic Waves

• Body Waves-travel through the body of a medium– P waves-travel fastest and 1st , causes

particles to move back and forth parallel to direction that wave is traveling. Can travel through solids, liquids, and gases

– S waves-shear waves, 2nd fastest, cause particles to move side to side, perpendicular to direction of wave. Only travel through solid material

Surface Waves

• Form from motion along shallow fault or from the conversion of energy when P waves and S waves reach Earth’s surface– Slowest, most damage– 2 types-Love and Rayleigh– Love-rock side to side and perpendicular to

waves– Rayleigh-ground to move w/ elliptical, rolling

motion

Seismic Waves and Earth’s Interior

• What are Earth’s internal layers?• What are the mechanical layers?

– Lithosphere, Asthenosphere, mesosphere, outer core, inner core

• Composition of material determines the speed and direction of the waves

• What are shadow zones?– Area on Earth’s surface where no direct seismic

waves from particular earthquake can be detected. Why are there shadow zones?

Earthquakes and Plate Tectonics

• What causes earthquakes?– Stresses in the Earth’s lithosphere

• Earthquakes happen at 3 main tectonic environments– Convergent Oceanic Environment-plates move

toward each other and collide• 2 oceanic plates or oceanic and continental

– Divergent Oceanic Environment-plates moving away from each other• Mid ocean ridges

– Continental Environment-2 continental plates converge, diverge or move horizontally in opposite directions• Stress may cause frequent earthquakes or mts to form

Fault Zones

• What is a fault zone? Region of numerous, closely spaced faults

• Form at plate boundaries. Why?– Intense stress that results when plates

separate• Not all earthquakes occur from movement

along plate boundaries– New Madrid, Missouri– Mississippi River region

Sec 2-Studying Earthquakes

• Seismology-study of earthquakes and seismic waves

• What instrument is used to measure and record earthquakes?– Seismograph-instrument that records

vibrations in ground, record motion by tracing wave shaped lines on paper or by translating the motion into electronic signals

– What waves are recorded first?

• Locating an Epicenter– Analyze arrival times of P waves and S waves Longer

lag time between waves, the farther the earthquake is– Use computers- to perform triangulations based on data

from seismograph stations • http://glencoe.com/sec/science/earthscience

/2007/concept_motion/animated_art/PandSwaves19_9.swf

• Earthquake measurement– Study magnitude and intensity and amount of damage

• Magnitude-measure of strength of an earthquake– Measure the amount of ground motion caused by

earthquakes– Richter scale– Moment magnitude-measurement of earthquake

strength based on size of area of fault that moves– More accurate for large earthquakes

• Intensity-amount of damage caused by an earthquake– Mercalli scale-provides description of effects

of each earthquake• Intensity depends on earthquake’s

magnitude, distance between epicenter and affected area, local geology, earthquake’s duration and human infrastructure

1906-San Francisco

1906-San Francisco

1906-San Francisco

Sec 3-Earthquakes and Society

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• http://glencoe.com/sec/science/earthscience/2007/concept_motion/animated_art/Tsunami19_23.swf

• What is a tsunamis? – Giant ocean wave that forms after volcanic eruption, submarine

earthquake, or landslide– Waves increase in height as they near shore

• Destruction of earthquakes– Tall buildings, buildings build on loose soil and rock

• How to prevent injury by earthquake activity– Be prepared, supply of food and water, flashlights, batteries, portable radio– Stay calm, move to safer position, stay away from windows and heavy

furniture– Be cautious, check for fire or other hazards, wear shoes, avoid downed

power lines

Earthquake Warnings and Forecasts

• Study past earthquakes, nothing reliable to predict when and where

• Studying Earth’s crust to detect changes• Seismic gaps-area along fault where relatively

few earthquakes have occurred recently• Foreshocks-little earthquakes that precede

actual earthquakes• Changes in rock

– Use sensors to detect slight tilting of ground and to identify the strain and cracks in rocks caused by stress that builds up in fault zones

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