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Seismic waves
and the layers of the earth
Three hundred years ago the famous scientist Isaac Newton
calculated, from his studies of planets and the force of gravity, that the
average density of the Earth is twice that of surface rocks and therefore that
the Earth's interior must be composed of much denser material.
We now know that the earth is made up of 4 layers:
CRUST --The thin, outermost layer of the earth is called
the crust. It makes up only one percent of the earth's mass. This consists of
the continents and ocean basins.
The
crust has varying thickness, ranging between 35-70 km thick in the continents
and 5-10 km thick in the ocean basins.
Within
the crust, intricate patterns are created when rocks are redistributed and
deposited in layers through the geologic processes. The crust is composed
mainly of alumino-silicates.
MANTLE -- The mantle is a dense, hot layer of semi-solid
rock approximately 2,900 km thick and is composed mainly of ferro-magnesium
silicates.
This
is where most of the internal heat of the Earth is located. Large convective
cells in the mantle circulate heat and may drive plate tectonic
processes.
CORE - Below the mantle is the core. It makes up nearly one
third the mass of the earth.
The
Earth's core is actually made up of two distinct parts: a 2,200 km-thick liquid
outer core and a 1,250 km-thick solid inner core. The outer core is made of
iron and is very dense.
As
the Earth rotates, the liquid outer core spins, creating the Earth's magnetic
field.
The
inner core is made of solid iron and nickel. Many scientists believe it is kept
in the solid state because of the extreme pressure from the other layers.
How
do we know about the different layers of the earth?
Information today comes from
studies of the paths and characteristics of seismic waves from earthquake waves
traveling through the Earth, as well as from laboratory experiments on surface
minerals and rocks at high pressure and temperature and studies of the Earth's
motions in the Solar System, its gravity and magnetic fields, and the flow of
heat from inside the Earth.
Timing
and strength of seismic waves gives us a picture of the interior of the earth.
Seismic waves gradually bend and
change speed as the density of rock changes.
About Seismic Waves
What
are the different types of Seismic Waves?
Seismic waves are waves of energy
that travel through the earth, for example as a result of an earthquake,
explosion, or some other process that imparts low-frequency acoustic energy.
Seismic wave studies have allowed
scientists to construct a model of the earth's interior.
There are two types of seismic
waves, body wave and surface waves.
The
fastest wave, and therefore the first to arrive at a given location, is called
the P wave.
The
P wave, or compressional wave, alternately compresses and expands material in
the same direction it is traveling. Think of a slinky
being compressed.
P waves propagate through the
Earth with a speed of about 15,000 miles per hour and are the first waves to
cause vibration of a building.
The
S wave is slower than the P wave and arrives next, shaking the ground up and
down and back and forth perpendicular to the direction it is traveling.
S waves arrive next and cause a
structure to vibrate from side to side. They are the most damaging waves,
because buildings are more easily damaged from horizontal motion than from
vertical motion.
S-waves are transverse waves -- see video
SURFACE
WAVES
Surface waves follow the P and S
waves. A surface wave is a seismic seismic wave that is trapped near the
surface of the earth.
Surface waves are analogous to
water waves and travel along the Earth's surface. They travel slower than body
waves.
Because
of their low frequency, long duration, and large amplitude, they can be the
most destructive type of seismic wave.
There
are two types of surface waves: Rayleigh waves and Love waves.
Surface waves -- sometimes called
long waves, or simply L waves -- are responsible for most of the damage
associated with earthquakes, because they cause the most intense vibrations.
Surface
waves stem from body waves that reach the surface.
Energy that reaches the surface
of the earth generates waves that travel outwar from the epicenter.
These
waves are call surface waves and move by giving particles an elliptical motion
as well as a back and froth swaying motion. These are the waves that cause most
of the destruction from an earthquake.
Rayleigh waves, also called ground roll, are surface waves
that travel as ripples with motions that are similar to those of waves on the
surface of water
Love waves are surface waves that cause circular
shearing of the ground.
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