Guided Notes for Guided Notes for Seismic Waves and Seismic Waves and

Earth’s InteriorEarth’s Interior

Section 19.2Section 19.2

1)1) The study of earthquake waves is The study of earthquake waves is called called seismologyseismology..

2) Define seismometer2) Define seismometer

A seismometer is an instrument used A seismometer is an instrument used to measure horizontal and vertical to measure horizontal and vertical motion during an earthquakemotion during an earthquake

3) How a seismometer records 3) How a seismometer records seismic wavesseismic waves

A frame is anchored to the ground A frame is anchored to the ground and a mass is suspended from a and a mass is suspended from a spring or wire. The frame vibrates spring or wire. The frame vibrates with the ground during an with the ground during an earthquake while the mass is still. earthquake while the mass is still. The movement of the frame is The movement of the frame is transferred to the paper with a transferred to the paper with a recording tool attached to the mass.recording tool attached to the mass.

4) After many years, seismologists 4) After many years, seismologists have been able to construct have been able to construct global global travel-time curves for the initial P-travel-time curves for the initial P-waves and S-waves of an waves and S-waves of an earthquakeearthquake. These general curves . These general curves have provided have provided the average travel the average travel times of all seismic waves for times of all seismic waves for different distances, no matter where different distances, no matter where on Earth an earthquake occurs.on Earth an earthquake occurs.

5) For any distance from the epicenter, 5) For any distance from the epicenter, P-wavesP-waves always arrive first. With always arrive first. With increasing travel distance, the time increasing travel distance, the time separation separation between P-waves and S-between P-waves and S-waves increaseswaves increases. This means that . This means that waves recorded on seismograms waves recorded on seismograms from more distant facilities are from more distant facilities are farther apart than waves recorded on farther apart than waves recorded on instruments closer togetherinstruments closer together..

5) continued….5) continued….

This separation of seismic waves on This separation of seismic waves on seismograms can be used to seismograms can be used to determine the distance from the determine the distance from the epicenter of a quake to the seismic epicenter of a quake to the seismic facility that recorded the facility that recorded the seismogram.seismogram.

6) P-waves that strike the earth’s core 6) P-waves that strike the earth’s core are are refracted, or bentrefracted, or bent, so that , so that beyond a distance of 11,000 km from beyond a distance of 11,000 km from the epicenter, they disappearthe epicenter, they disappear. P-. P-waves re-emerge at a distance of waves re-emerge at a distance of 16,000 km from the epicenter16,000 km from the epicenter. The . The region between these two distances region between these two distances doesn’t receive direct P-wavesdoesn’t receive direct P-waves and is and is known as known as the shadow zonethe shadow zone..

7) S-waves do not enter the Earth’s 7) S-waves do not enter the Earth’s corecore because because they cannot travel they cannot travel through liquidsthrough liquids. The disappearance . The disappearance of S-waves has allowed seismologists of S-waves has allowed seismologists to reason to reason that the earth’s outer core that the earth’s outer core is liquid.is liquid.