19.2 – Seismic Waves & Earth ’ s Interior

19.2 – Seismic Waves & Earth’s Interior

Measuring Seismic Waves • Seismometer = sensitive instruments that detect seismic waves • Rotating drum covered with paper • Pendulum with writing instrument attached • Mass on pendulum and pen are at rest due to inertia • When ground shakes, the pen registers it on paper • Seismogram = record produced by seismometer

Seismometer

Travel-Time Curve • Seismic waves are recorded at various facilities • Seismologists construct graph showing arrival of waves

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

Distance from epicenter • Measure arrival of primary (P), secondary (S), and surface waves • As distance increases, separation between waves increase • Information is used to locate epicenter

Clues to Earth’s Interior • Seismic waves help give information about Earth’s interior • Seismic waves change speed and direction at boundaries between different materials • Through mantle P and S waves follow direct paths • When P waves strike Earth’s core they refract (bend)

By creating Travel-time curves and path of each wave, seismologist learn about difference in density and composition • S waves do not travel through liquid, which means they do not travel through core • Means Earth’s core is at least partially liquid

http://geology.com/rocks/pictures/peridotite.jpg Earth’s composition • Waves can also change speed • Speed tells scientists thickness and composition of Earth’s different regions • Upper mantle = peridotite • Outer core = mostly liquid iron and nickel • Inner core = mostly solid iron and nickel

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In-Class Assignment/Homework

19.2 – Seismic Waves & Earth ’ s Interior