SEISMIC WAVESMaterials:
Procedure:
This activity is designed to introduce you to the different types of seismic waves in an environment you can control: an interactive, online wave generator. This wave generator requires a browser that is capable of displaying frames and layers, both. If your browser does not support these features, you will not be able to use this activity at all. We apologize for the inconvenience; there is no alternate page available.
The purpose of this activity is for you to familiarize yourself with the properties of seismic waves through the use of an interactive wave generator that will simulate the motions of the four main types of seismic waves: P waves, S waves, Love waves, and Rayleigh waves. You will also be able to choose a fifth option -- a combination of P and S waves -- that crudely simulates the wave motion experienced during an earthquake.
The general workings of this online wave generator are as follows
(more detailed descriptions are available on the
explanation page):
Buttons on the "control panel" at the top of the page allow you to select the type of seismic wave simulation you wish to run, as well as the speed of the propagating waves and the direction of the initial displacement (the first motion) which can be "up" or "down". When you click on a button labelled "GO!", the waves will begin to propagate, according to your specifications, in the "wave block" below. You will actually see a small "packet" of waves every time -- three wavelengths, to be exact. For the P+S option, there will be three wavelengths each of P waves and S waves.
Atop and within the wave block, which represents a three-dimensional section of the Earth's crust (its top is the surface of the Earth), are three small icons. These represent seismic instruments. Two sit atop the surface of the block and produce virtual seismograms based upon the motion they experience. The third does not produce seismograms, but it lies within the interior of the block, and its location can be altered with controls found on the "control panel". By directly observing the motion of the three icons, as well as the two sets of seismograms, you should be able to determine the exact kind of ground motion each type of seismic wave produces. You will also be able to see what happens during a simulated earthquake.
That summarizes the use of the interactive wave generator. Feel free to try as many different kinds of simulations as you can; the more variations you try, the more likely you are to understand the distinctions between the different types of seismic waves.
Before you begin working with the wave simulations, you should acquaint yourself with the layout and features of this activity. Please go now to the explanation page and read through the descriptions of these features, how to use them, and what to expect while working with the wave generator. Then when you are ready, use the link at the bottom of that page to start the interactive wave generator. When you have run simulations of every type of seismic wave scenario given, and made careful observations of them all, return to this page (by quitting the online activity) and answer the questions below. Or you may wish to open this page in a new window so that you can read through the questions below while working with the wave generator.
Did the three seismograms (each representing
a different component of ground motion) help you to see the
differences in the motion of the four types of waves?
Had the three components been combined into just one seismogram
for the four single waves (as was the case for the "P+S" scenario),
would you have been able to distinguish the waves based upon the
seismograms alone?
The S wave, as you saw it in this simulation, shook the wave
block with a vertical motion as it moved from left to right. This
is a fairly limited view of the S wave; its motion can more correctly
be described as a shearing motion in any direction perpendicular
to the direction of propagation. In our simulation, we chose to
have the S wave shear the wave block in a vertical orientation
to distinguish it as much as possible from the Love wave. Love
waves also move with a shearing motion perpendicular to the
direction of propagation, but this motion is limited to the horizontal
plane.
Would Love waves ever show up on a seismogram recorded from
the vertical component of a seismometer?
Aside from the vertical vs. horizontal distinction between the
shearing motions of the S and Love waves, you should have noticed
another difference in the way they affected the wave block as
they propagated through it. (Think about the motion of
the underground sensor.)
Which of the other waves, if any, had a similar limitation
on its motion?
Love and Rayleigh waves belong to a class of seismic waves
called surface waves, because they propagate only along
the surface of the Earth, their motion lessening with depth.
P and S waves, on the other hand, travel within the body
of the Earth. What name, do you think, is given to the class of
seismic waves that these two waves comprise?
Another limitation of this simulation is that the direction
of propagation for every wave was parallel to the horizontal
(and aligned due west to east). In real life, seismic waves
can, of course, originate from any direction, and they do
not always propagate in a manner parallel to the horizontal.
If a body wave were travelling up toward the Earth's surface
at some significantly large angle to the horizontal, what do you
think might happen, other than shaking, when it reached that
boundary? (Think about the behavior of other types of waves
when they encounter a boundary between different materials.)
Surface waves can actually be thought of as polarized body
waves, produced when body waves encounter the Earth's surface.
Which type of surface wave do you think is a polarized form
of the P wave? Of the S wave?
The explanation page tells you
to picture the seismic waves in the wave block as having originated
far to the "west" of the western end block.
Could you figure out the distance to the source of the
waves by carefully observing the P+S wave simulation?
What implications for earthquake research might this have?
Return
to the Text