Earthquakes

And Earth’s Structure

Earth’s Structure

Origin of the Earth

• Meteors and Asteroids bombarded the Earth

• Gravitational compressionGravitational compression

• Density Stratified planetDensity Stratified planet

Earth’s Interior• CoreCore

– densedense– Iron and NickelIron and Nickel– Inner CoreInner Core - solid - solid– Outer CoreOuter Core - liquid - liquid

– Less dense than coreLess dense than core– Iron and Magnesium silicatesIron and Magnesium silicates– Mostly solidMostly solid– Upper mantle is partially moltenUpper mantle is partially molten

• MantleMantle

– Outermost layerOutermost layer– Very thin and rigidVery thin and rigid– ContinentalContinental – granite – granite

– Density = 2.8 g/cmDensity = 2.8 g/cm33

– OceanicOceanic – basalt – basalt– Density = 3.0 g/cmDensity = 3.0 g/cm33

• CrustCrust

Evidence of Internal Structure

Granite• DensityDensity

– calculate density of Earthcalculate density of Earth– Speculate on probable compositionsSpeculate on probable compositions

Basalt

• MeteoritesMeteorites– Use composition and age to Use composition and age to

determine composition and determine composition and age of Earthage of Earth

• Seismic wavesSeismic waves– Travel times and direction Travel times and direction

give indication of internal give indication of internal structure of Earth structure of Earth

Types of Seismic Waves

• S wavesS waves

• P wavesP waves– Primary wavesPrimary waves

– Push and pull movementPush and pull movement

– Travel fastest (Travel fastest (~ 6 km/sec)~ 6 km/sec)

– Travel thru solids and liquidsTravel thru solids and liquids

– Secondary wavesSecondary waves

– Move side-to-sideMove side-to-side

– Slower (~ 4 km/sec)Slower (~ 4 km/sec)

– Travel thru solids onlyTravel thru solids only

Seismic Waves Through Earth

What is an Earthquake?

• The vibration of Earth produced by the rapid release of energy

Elastic Rebound Theory

• The crust will first bend• When the stress exceeds strength of the rock, it will

“snap” into a new position• In the process of breaking, “seismic waves are released

Parts of an Earthquake

•Focus

•Epicenter

•Energy released radiates in all directions from the focus•Energy is in the form of waves – “seismic waves”

– Location on surface directly above the focus

–place within earth where EQ originate

How Do We Locate an Epicenter?

• Instruments around the world record EQ

•Record Earth movements by stationary mass on rotating drum

•Use seismogram to:– Locate an epicenter– Determine magnitude

Seismograph

Seismogram

•Measure the distance between P- and S-waves– This is the time difference in arrival times

•Help determine the epicenter

Locating the Epicenter

• Plot the time difference on y-axis (time interval)

• Trace plot to blue line to determine distance to epicenter

Locating an Epicenter Triangulation

• A minimum of 3 seismic stations are needed to determine the epicenter

• Measure that distance around the seismic station

• The epicenter may be located anywhere on that line

Determining Magnitude

• Measure the amplitude of the strongest wave

•The amplitude is the height on paper

•Plot distance between p- and s-wave

•Plot amplitude

• Connect plots to determine magnitude

You are now a Seismologist!!

Virtual Earthquake Introduction:http://vcourseware5.calstatela.edu/VirtualEarthquake/VQuakeExecute/htm

Virtual Earthquake:

http://sciencecourseware.org