19.2 – Seismic Waves & 19.2 – Seismic Waves & Earth’s InteriorEarth’s Interior

Measuring Seismic WavesMeasuring Seismic Waves

Seismometer = sensitive instruments that Seismometer = sensitive instruments that detect seismic wavesdetect seismic waves Rotating drum covered with paperRotating drum covered with paper Pendulum with writing instrument attachedPendulum with writing instrument attached Mass on pendulum and pen are at rest due to Mass on pendulum and pen are at rest due to

inertiainertia When ground shakes, the pen registers it on paperWhen ground shakes, the pen registers it on paper

Seismogram = record produced by Seismogram = record produced by seismometerseismometer

SeismometerSeismometer

Travel-Time CurveTravel-Time Curve

Seismic waves are recorded at Seismic waves are recorded at various facilitiesvarious facilities

Seismologists construct graph Seismologists construct graph showing arrival of wavesshowing arrival of waves

http://chuma.cas.usf.edu/~juster/A3/travel%20time%20curve.jpg

Distance from epicenterDistance from epicenter

Measure arrival of primary (P), Measure arrival of primary (P), secondary (S), and surface wavessecondary (S), and surface waves

As distance increases, separation As distance increases, separation between waves increasebetween waves increase

Information is used to locate Information is used to locate epicenterepicenter

Clues to Earth’s InteriorClues to Earth’s Interior

Seismic waves help give information Seismic waves help give information about Earth’s interiorabout Earth’s interior

Seismic waves change speed and Seismic waves change speed and direction at boundaries between direction at boundaries between different materialsdifferent materials

Through mantle P and S waves follow Through mantle P and S waves follow direct pathsdirect paths When P waves strike Earth’s core they When P waves strike Earth’s core they

refract (bend)refract (bend)

By creating Travel-time curves and path By creating Travel-time curves and path of each wave, seismologist learn about of each wave, seismologist learn about difference in density and compositiondifference in density and composition

S waves do not travel through liquid, S waves do not travel through liquid, which means they do not travel through which means they do not travel through corecore Means Earth’s core is at least partially liquidMeans Earth’s core is at least partially liquid

Earth’s compositionEarth’s composition

Waves can also change speedWaves can also change speed Speed tells scientists thickness and Speed tells scientists thickness and

composition of Earth’s different composition of Earth’s different regionsregions Upper mantle = peridotiteUpper mantle = peridotite Outer core = mostly liquid iron and Outer core = mostly liquid iron and

nickelnickel Inner core = mostly solid iron and nickelInner core = mostly solid iron and nickel

http://geology.com/rocks/pictures/peridotite.jpg

http://images.encarta.msn.com/xrefmedia/aencmed/targets/illus/ilt/T629167A.gifhttp://images.encarta.msn.com/xrefmedia/aencmed/targets/illus/ilt/T629167A.gif

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