Download 10.4 Studying Earth`s Interior

10.4 Studying Earth’s
Interior
Key Idea:
Scientists use data from
seismic waves to learn about
the structure of Earth’s
interior.
Remember:
The velocity of the seismic waves
increases as the density of the rocks
increases. (The more compact the rock,
the higher the velocity of P and S waves)
 The P waves travel through solids (rocks),
and fluids (magma, or water). The P
waves move faster than the S waves.
 The S waves can only travel through
solids. When the S are not received at
some point, it is a sign that they traveled
through a fluid material.

Changes in Behavior of Seismic
Waves
Scientists observed that:
 P waves are greatly slowed around 2900
Km below the surface of the earth.
 S waves are stopped at the same depth.
Knowing that the velocity of seismic waves
changes with the density of the rock, and
that S waves do not travel through liquids,
then the scientists concluded that the
material below 2900 Km, the outer core,
must be liquid.
Then…
P
wave velocity increases again at a
depth of about 5200 Km. This
increase in speed suggests that the
inner core is solid.
The Shadow Zone
The Shadow Zone
 Seismograph
stations in what is
called the earthquake’s “shadow
zone” cannot detect P or S waves
from the earthquake.
 The Shadow zone is wide belt around
the side of Earth opposite the focus
of the earthquake.
Why Does the Shadow Zone
Exists?





P waves passing through the mantle are refracted
(bent) in smooth arcs back to the surface.
When P waves travel deep enough to enter the
outer core, they are refracted again as they enter
the outer core and yet again when they reenter
the mantle.
As a result, seismograph stations that are
between the earthquakes epicenter and the
shadow zone receive both P and S waves.
Stations within the shadow zone receive neither P
nor S waves, because P waves have been
refracted away and S waves do not pass through
the liquid outer core.
Stations Beyond the shadow zone, on the
opposite side of the Earth receive only P waves.
Why Do Shadow Zones Exist?

of the
and S
waves
pass
Notice the
refraction
P waves
waves. S
do not
The Moho Discontinuity
An increase in the velocities of P waves
and S waves occurs between the crust and
the mantle. This transition zone, which is
denser then the crust was discovered by a
Croatian scientist, Andreja Mohorovicic.
 The discontinuity is named after him
(today people use the short name: Moho)
 The Moho discontinuity is about 32 Km
under the continents, and between 5-10
km under the oceans.

Who Was Mohorovichic?

As a boy of 15 he spoke Italian, French, and English as well
as his native Croatian, later added German, Czech, Latin,
and old Greek. He studied physics at the University of
Prague under some famous professors including E. Mach
and did his graduate work at the University of Zagreb, from
which he obtained a Ph.D. In 1894 Dr. Mohorovicic became
Director of the Institute for Meteorology and Geodynamics
and Professor at the University of Zagreb in 1897, where he
remained until his retirement in 1921. His special interest
was the precise measurement of time for both astronomical
and seismical events, but his reputation mainly rests on his
classic paper in the field of seismology, The Earthquake of
October 8, 1909, which contains the news of his discovery
of a major discontinuity at a depth of 55 kilometers. This
discontinuity, now generally known as the Moho in his
honor, defines the crust of the earth. Professor Mohorovicic
died in 1936 in circumstances approaching poverty.
The Transition Zone
The P waves and S waves increase their
velocities at depths between 400 and 670
Km. These depths mark a region in the
middle of the mantle, which scientists call
“the transition zone”.
 This is because the material deeper within
the mantle is under greater pressure from
the overlaying material; the material is
more compressed, the particles are closer
packed together; in other words the
material is more dense, and the P and S
waves travel more quickly through it.

The Inner Structure of the Earth
The direct scientific investigation of the deep
layers of the Earth is basically impossible;
the scientists use indirect methods explore
the depth of our planet.
 Based on the behavior of the seismic
waves, the scientists were able to identify
certain discontinuities which mark the
transition from one inner layer to another.
 Each layer has different chemical
composition, density, and state.
The Inner Structure of the Earth
The Inner Structure of the Earth
The Inner Structure of the Earth