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Polk Community College Lakeland, Fl |
Seismic wave types:
There are two basic types of seismic waves: body waves and surface
waves. Body waves travel into the body of the Earth, often going
deep into the Earth. Body waves are used to locate earthquakes
and investigate the interior of the Earth. Surface waves travel
along the surface of the Earth. Surface waves do most of the destruction
from earthquakes. There are 2 types of body waves: p-wave and
s-waves. In p waves, also known as compressional waves, the particle
motion is parallel to the wave motion. In s waves, also known
as shear waves, the particle motion is perpendicular to the wave
motion. Seismic waves are caused by earthquakes, landslides, and
explosions. They can be detected and measured by seismometers,
devices that measure ground motion.
Physical demonstration of p waves and
s waves:
P waves and s waves travel through the solid Earth by the vibration
of the rock that makes up the Earth. The particles that vibrate
as the seismic waves pass are the atoms and molecules of rock.
P waves and s waves can be demonstrated on an extended slinky.
In this demonstration it is the coils of the slinky that act as
the particles that vibrate as the seismic waves travel. The motion
of the coils as the p waves pass involves compression, coils closer
together, and rarefaction, coils farther apart. The motion of
the coils as the s waves pass involves positive (motion "upward")
and negative (motion "downward") displacement of the
coils.
Computer seismic wave demonstrator:
This computer demonstration represents the motion of slinky coils
for p waves and s waves. The slashes, the "/"s, represent
the coils of a slinky. In order to make it easier to see the motion
of a single coil two of the "/"s have been replaced
by pluses, "+"s. Watch the motion of the "+"s
to see how one coil moves as the seismic wave passes.
To run the demonstration for p waves:
1) Place the cursor at the x on the left side of the screen below.
2) Keep it there, and stationary, until the slashes stop moving.
The motion will disappear to the right.
3) Move the cursor slowly toward the right (compressing the slinky
coils) and then back to its original position.
4) Watch the wave, the compressions and rarefactions, move to
the right.
5) Watch the "+"s to see the particle motion. This shows
the motion of a single "coil" of the slinky.
The motion of the "/"s as the waves pass involves compression,
"/"s closer together, and rarefaction, "/"s
farther apart. This type of motion is shown below.
To run the demonstration for s waves:
1) Place the cursor at the x on the left side of the screen below.
2) Keep it there, and stationary, until the slashes stop moving.
The motion will disappear to the right.
3) Move the cursor upward and downward returning to its original
position. This makes a wave with just positive displacement, as
shown at the right below.
4) Or move the cursor upward and downward going well past its
original position then returning to its original position. This
makes a wave with positive and negative displacement, as shown
at the left below.
5) Watch the movement of the "+"s to follow the motion
of a single "coil" of the slinky.
The motion of the "/"s as the waves pass may involve
positive and negative displacement of the "/"s. This
type of motion is shown below.
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