10.4 Studying Earth’s 10.4 Studying Earth’s InteriorInteriorKey Idea:Key Idea:

Scientists use data from Scientists use data from seismic waves to learn about seismic waves to learn about

the structure of Earth’s the structure of Earth’s interior.interior.

Remember:Remember: The velocity of the seismic waves The velocity of the seismic waves

increases as the density of the rocks increases as the density of the rocks increases. (The more compact the rock, increases. (The more compact the rock, the higher the velocity of P and S waves)the higher the velocity of P and S waves)

The P waves travel through solids (rocks), The P waves travel through solids (rocks), and fluids (magma, or water). The P waves and fluids (magma, or water). The P waves move faster than the S waves.move faster than the S waves.

The S waves can only travel through The S waves can only travel through solids. When the S are not received at solids. When the S are not received at some point, it is a sign that they traveled some point, it is a sign that they traveled through a fluid material.through a fluid material.

Changes in Behavior of Seismic Changes in Behavior of Seismic WavesWaves

Scientists observed that:Scientists observed that: P waves are greatly slowed around 2900 P waves are greatly slowed around 2900

Km below the surface of the earth.Km below the surface of the earth. S waves are stopped at the same depth.S waves are stopped at the same depth.Knowing that the velocity of seismic waves Knowing that the velocity of seismic waves

changes with the density of the rock, and changes with the density of the rock, and that S waves do not travel through liquids, that S waves do not travel through liquids, then the scientists concluded that the then the scientists concluded that the material below 2900 Km, the outer core, material below 2900 Km, the outer core, must be liquid. must be liquid.

Then…Then… P wave velocity increases again at a P wave velocity increases again at a

depth of about 5200 Km. This depth of about 5200 Km. This increase in speed suggests that the increase in speed suggests that the inner core is solid. inner core is solid.

The Shadow ZoneThe Shadow Zone

Seismograph stations in what is Seismograph stations in what is called the earthquake’s “shadow called the earthquake’s “shadow zone” cannot detect P or S waves zone” cannot detect P or S waves from the earthquake. from the earthquake.

The Shadow zone is wide belt around The Shadow zone is wide belt around the side of Earth opposite the focus the side of Earth opposite the focus of the earthquake. of the earthquake.

The Shadow ZoneThe Shadow Zone

Why Does the Shadow Zone Why Does the Shadow Zone Exists?Exists?

P waves passing through the mantle are refracted P waves passing through the mantle are refracted (bent) in smooth arcs back to the surface. (bent) in smooth arcs back to the surface.

When P waves travel deep enough to enter the outer When P waves travel deep enough to enter the outer core, they are refracted again as they enter the outer core, they are refracted again as they enter the outer core and yet again when they reenter the mantle.core and yet again when they reenter the mantle.

As a result, seismograph stations that are between As a result, seismograph stations that are between the earthquakes epicenter and the shadow zone the earthquakes epicenter and the shadow zone receive both P and S waves. receive both P and S waves.

Stations within the shadow zone receive neither P Stations within the shadow zone receive neither P nor S waves, because P waves have been refracted nor S waves, because P waves have been refracted away and S waves do not pass through the liquid away and S waves do not pass through the liquid outer core.outer core.

Stations Beyond the shadow zone, on the opposite Stations Beyond the shadow zone, on the opposite side of the Earth receive only P waves.side of the Earth receive only P waves.

Why Do Shadow Zones Exist?Why Do Shadow Zones Exist? Notice the Notice the refraction of the refraction of the P waves and S P waves and S waves. S waves waves. S waves do not pass do not pass through the corethrough the core

The Moho DiscontinuityThe Moho Discontinuity An increase in the velocities of P waves An increase in the velocities of P waves

and S waves occurs between the crust and and S waves occurs between the crust and the mantle. This transition zone, which is the mantle. This transition zone, which is denser then the crust was discovered by a denser then the crust was discovered by a Croatian scientist, Andreja Mohorovicic.Croatian scientist, Andreja Mohorovicic.

The discontinuity is named after him The discontinuity is named after him (today people use the short name: Moho)(today people use the short name: Moho)

The Moho discontinuity is about 32 Km The Moho discontinuity is about 32 Km under the continents, and between 5-10 under the continents, and between 5-10 km under the oceans. km under the oceans.

Who Was Mohorovichic?Who Was Mohorovichic? As a boy of 15 he spoke Italian, French, and English as well As a boy of 15 he spoke Italian, French, and English as well

as his native Croatian, later added German, Czech, Latin, as his native Croatian, later added German, Czech, Latin, and old Greek. He studied physics at the University of and old Greek. He studied physics at the University of Prague under some famous professors including E. Mach Prague under some famous professors including E. Mach and did his graduate work at the University of Zagreb, from and did his graduate work at the University of Zagreb, from which he obtained a Ph.D. In 1894 Dr. Mohorovicic became which he obtained a Ph.D. In 1894 Dr. Mohorovicic became Director of the Institute for Meteorology and Geodynamics Director of the Institute for Meteorology and Geodynamics and Professor at the University of Zagreb in 1897, where he and Professor at the University of Zagreb in 1897, where he remained until his retirement in 1921. His special interest remained until his retirement in 1921. His special interest was the precise measurement of time for both astronomical was the precise measurement of time for both astronomical and seismical events, but his reputation mainly rests on his and seismical events, but his reputation mainly rests on his classic paper in the field of seismology, The Earthquake of classic paper in the field of seismology, The Earthquake of October 8, 1909, which contains the news of his discovery October 8, 1909, which contains the news of his discovery of a major discontinuity at a depth of 55 kilometers. This of a major discontinuity at a depth of 55 kilometers. This discontinuity, now generally known as the Moho in his discontinuity, now generally known as the Moho in his honor, defines the crust of the earth. Professor Mohorovicic honor, defines the crust of the earth. Professor Mohorovicic died in 1936 in circumstances approaching poverty.died in 1936 in circumstances approaching poverty.

The Transition ZoneThe Transition Zone The P waves and S waves increase their The P waves and S waves increase their

velocities at depths between 400 and 670 velocities at depths between 400 and 670 Km. These depths mark a region in the Km. These depths mark a region in the middle of the mantle, which scientists call middle of the mantle, which scientists call “the transition zone”. “the transition zone”.

This is because the material deeper within This is because the material deeper within the mantle is under greater pressure from the mantle is under greater pressure from the overlaying material; the material is the overlaying material; the material is more compressed, the particles are closer more compressed, the particles are closer packed together; in other words the packed together; in other words the material is more dense, and the P and S material is more dense, and the P and S waves travel more quickly through it. waves travel more quickly through it.

The Inner Structure of the EarthThe Inner Structure of the Earth

The direct scientific investigation of the deep The direct scientific investigation of the deep layers of the Earth is basically impossible; layers of the Earth is basically impossible; the scientists use indirect methods explore the scientists use indirect methods explore the depth of our planet. the depth of our planet.

Based on the behavior of the seismic Based on the behavior of the seismic waves, the scientists were able to identify waves, the scientists were able to identify certain discontinuities which mark the certain discontinuities which mark the transition from one inner layer to another. transition from one inner layer to another.

Each layer has different chemical Each layer has different chemical composition, density, and state. composition, density, and state.

The Inner Structure of the EarthThe Inner Structure of the Earth

The Inner Structure of the EarthThe Inner Structure of the Earth

The Inner Structure of the EarthThe Inner Structure of the Earth