what is seismology ?
What Is Seismology?
Seismology is
the study of earthquakes and seismic waves that move through and around the
earth. A seismologist is a scientist who studies earthquakes
and seismic waves.
What Are Seismic Waves?
Seismic waves are
the waves of energy caused by the sudden breaking of rock within the earth or
an explosion. They are the energy that travels through the earth and is
recorded on seismographs.
Types of Seismic Waves
There are several different kinds of seismic waves, and they all
move in different ways. The two main types of waves are body waves and surface
waves. Body waves can travel through the earth's inner layers, but surface
waves can only move along the surface of the planet like ripples on water.
Earthquakes radiate seismic energy as both body and surface waves.
BODY WAVES
Traveling through the interior of the
earth, body waves arrive before the surface waves emitted by
an earthquake. These waves are of a higher frequency than surface waves.
P WAVES
The first kind of body wave is the P
wave or primary wave. This is the fastest kind of seismic
wave, and, consequently, the first to 'arrive' at a seismic station. The P wave
can move through solid rock and fluids, like water or the liquid layers of the
earth. It pushes and pulls the rock it moves through just like sound waves push
and pull the air. Have you ever heard a big clap of thunder and heard the
windows rattle at the same time? The windows rattle because the sound waves
were pushing and pulling on the window glass much like P waves push and pull on
rock. Sometimes animals can hear the P waves of an earthquake. Dogs, for
instance, commonly begin barking hysterically just before an earthquake 'hits'
(or more specifically, before the surface waves arrive). Usually people can
only feel the bump and rattle of these waves.
P waves are also known as compressional waves, because of the pushing and pulling they do. Subjected to a P wave, particles move in the same direction that the the wave is moving in, which is the direction that the energy is traveling in, and is sometimes called the 'direction of wave propagation'. .
P waves are also known as compressional waves, because of the pushing and pulling they do. Subjected to a P wave, particles move in the same direction that the the wave is moving in, which is the direction that the energy is traveling in, and is sometimes called the 'direction of wave propagation'. .
FIGURE 1 - A P WAVE TRAVELS THROUGH A MEDIUM BY MEANS OF COMPRESSION AND DILATION. PARTICLES ARE REPRESENTED BY CUBES IN THIS MODEL. IMAGE ©2000-2006 LAWRENCE BRAILE, USED WITH PERMISSION.
S WAVES
The second type of body wave is the S
wave or secondary wave, which is the second wave you feel
in an earthquake. An S wave is slower than a P wave and can only move through
solid rock, not through any liquid medium. It is this property of S waves that
led seismologists to conclude that the Earth's outer core is a
liquid. S waves move rock particles up and down, or side-to-side--perpendicular
to the direction that the wave is traveling in (the direction of wave
propagation). .
FIGURE 2 - AN S WAVE TRAVELS THROUGH A MEDIUM. PARTICLES ARE REPRESENTED BY CUBES IN THIS MODEL. IMAGE ©2000-2006 LAWRENCE BRAILE, USED WITH PERMISSION.
SURFACE WAVES
Travelling only through the crust, surface
waves are of a lower frequency than body waves, and are easily
distinguished on a seismogram as a result. Though they arrive after body waves,
it is surface waves that are almost enitrely responsible for the damage and
destruction associated with earthquakes. This damage and the strength of the
surface waves are reduced in deeper earthquakes.
LOVE WAVES
The first kind of surface wave is called a Love
wave, named after A.E.H. Love, a British mathematician who worked out the
mathematical model for this kind of wave in 1911. It's the fastest surface wave
and moves the ground from side-to-side. Confined to the surface of the crust,
Love waves produce entirely horizontal motion.
FIGURE 3 - A LOVE WAVE TRAVELS THROUGH A MEDIUM. PARTICLES ARE REPRESENTED BY CUBES IN THIS MODEL. IMAGE ©2000-2006 LAWRENCE BRAILE, USED WITH PERMISSION.
RAYLEIGH WAVES
The other kind of surface wave is the Rayleigh
wave, named for John William Strutt, Lord Rayleigh, who mathematically
predicted the existence of this kind of wave in 1885. A Rayleigh wave rolls
along the ground just like a wave rolls across a lake or an ocean. Because it
rolls, it moves the ground up and down, and side-to-side in the same direction
that the wave is moving. Most of the shaking felt from an earthquake is due to
the Rayleigh wave, which can be much larger than the other waves.
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