EXPLANATION of the online "Seismic Waves" activity
This page should help you familiarize yourself with the purpose
and operation of this interactive wave generator.
When you start the activity, you will see three frames created
in your browser window. The image below shows you the main
features of each frame, discussed in detail in the text that follows.
Clicking on the name of a frame or feature will take you directly to
a description of it. Clicking the arrow next to each text entry
will bring you back to the imagemap below.
There is only one "active" part of the title frame,
a button labelled Quit! (see below).
This button is active at all times.
Clicking the Quit button will always take you back to the
introduction page for this activity.
The animated wave generation takes place entirely within the Action Frame.
In terms of controls, however, this is a "passive" frame.
There is only one active button: the help button.
This button, and the animated features you'll observe in this frame
and listed in detail below.
The main feature of the Action Frame and
the key to this activity is the wave block, an elongated rectangular
prism representing a portion of the Earth's crust through which
the seismic waves generated in this simulation will propagate,
always from left to right.
The outer surfaces of the wave block are marked with a grid
of squares to help you judge relative distances. The upper surface
of the wave block represents the ground surface and is colored
green. Atop it sit two seismic monitoring stations.
These stations will each generate virtual
below. In addition, there is an underground sensor
located within the wave block, adjacent to its near surface so that
you can see it in this cut-away view. The depth of this sensor
The wave block has a fixed and defined orientation, shown by the three
axis labels on the end blocks
that bound its left and right edges. As the arrows indicate, the long
axis of the wave block is the east-west axis, and north is "into"
the computer screen. The wave block represents a section of the crust
longer than is shown, which is why waves will seem to spontaneously
appear from the western end block
and stop at the eastern end block.
Imagine instead that the source of the waves is far beyond the
western end block and that they continue to
propagate through the crust after they pass the eastern
When seismic waves propagate through the wave block, you will see the
entire block deform in response. (The two stations
and the underground sensor will move, too.)
Observe this propagating disturbance;
it, along with the seismograms, will help you
identify the different types of waves.
The wave block is bounded on its western (left)
and eastern (right) edges by two end blocks. The seismic waves you
generate in this activity will appear to emerge from the western end
block and disappear into the eastern end block. These blocks are there
to signify that the wave block is actually
longer (in its east-west direction) than what you see on the screen;
think of them as the arrowheads on the graphical representation of
an infinitely long line. The end blocks are also home to the
direction axis labels and the
Two small houses on the wave block
represent the locations of the seismic monitoring
equipment ("stations") that produce the virtual
seismograms at the bottom of the frame.
The left, orange-tinted house is Station 1;
the bluish house on the right (to the east, according
to the axis labels) is Station 2.
These icons will move with the wave motion as the seismic waves pass
beneath them. A good way to understand the type of motion produced by
each kind of wave is to watch the motion of these "seismic stations",
including the underground sensor.
A small icon shaped like the cross-section of a tunnel represents
the underground sensor within the wave block.
Unlike the two stations on the surface of
the wave block, the underground sensor does
not produce seismograms you can study. You can, however, see
whether motion is different at depth than it is at the surface
by watching the motion of this sensor as the waves pass by.
It is possible to adjust the depth of the underground sensor
using the underground elevator panel
located next to its station label
To talk about the motion caused by a certain kind of seismic wave,
it helps to have a directional frame of reference. In our wave
simulator, this is provided by the three axis labels on the
The direction toward the top of your computer screen can be thought
of as "up"; the bottom of your screen is "down". This is the vertical
axis, labelled Z here. The east-west axis runs right to
left across the screen, with east on your right (when facing the
screen). This horizontal axis is labelled E. The other
horizontal axis is labelled N. It represents the north-south
direction, with north being "into" your screen (shown in two dimensions
as an axis running diagonally up and to your right), and south pointing
"out" of it (and toward your lower left).
As mentioned before, all the waves propagating through the
wave block in this simulation will move
from left to right, meaning they will travel from the west and
toward the east.
As the simulated seismic waves move through the
wave block, they will disturb the two
seismic stations (and the
underground sensor). The motions of the
two stations will be translated by virtual
seismographs into graphical representations -- seismograms --
similar to those recorded by actual seismographs. A black line
drawn on a white background will show the displacement
experienced by each station.
To make these virtual seismograms more helpful, they will
be split into three separate components
of motion, aligned along the principal axes
of the wave block: Z (vertical),
N (north-south), and E (east-west). Thus, each
of the three seismograms will show a record of the motion the
station experienced in a particular
orientation. If the ground motion is entirely vertical, for
example, the two horizontal components
will show no motion whatsoever -- their seismograms will be
There is one exception to the three-component configuration of the
virtual seismograms: when the P+S option is chosen,
the three components will be combined into a single seismogram,
labelled ALL. Also note that all the seismograms will be
reset each time the Go! button is clicked.
The seismograms from the two
stations are divided into three categories,
labelled Z, N, and E. These are
the three components of each virtual seismometer at those
stations. The Z component shows motion in the vertical
sense, with "up" on the seismogram representing motion upward at
the station. The N component represents motion along a
north-south axis, and "up" on the seismogram translates to motion
in a northward direction. The E component shows motion
along an east-west axis, with east as the "up" direction on the
seismogram. These axes are shown by the axis labels
on the end blocks.
For the P+S option, which runs two different kinds of waves simultaneously,
the three components are reduced to just one, labelled ALL.
This seismogram, then, represents the sum total of the three
components of ground motion at that station.
Should you wish to refer to these instructions while you are
running the wave generator, click on the help button located
on the eastern end block. This will
open a new browser window containing this page of information
about the activity.
Within the frame at upper right, called the Control Panel, are all
the controls you will need to run the interactive wave generator.
Indeed, the only two "active" parts of this program not
found in the Control Panel are the Quit button
and the help button.
The items found within the Control Panel are detailed below.
In the Action Frame is the main animated
feature of this simulator, the wave block,
which represents a rectangular prism of ground material, gridded
and with its upper surface (the virtual ground surface) in green.
On top of this wave block sit two house-shaped
stations (Stations 1 & 2), and under the
surface the block is an underground sensor.
These three icons are identified in the upper left-hand side of the
In the upper right-hand side of the wave block using this sliding bar.
Just click and hold down on the bar and then drag it sideways in either
direction to increase or decrease the wave speed. This control is
functional at all times -- even while the waves are moving in the
Action Frame below.
Underground Elevator Panel
Next to the station label for the
underground sensor is a small panel
with two arrows: a green one pointing up, and a red one pointing
down. This is the underground elevator panel. By clicking on
the arrows (they light up when you move over them with your mouse
pointer), you can raise and lower the position of the
underground sensor. The sensor can
move only by integer numbers of wave-block
grid squares, and cannot be moved while seismic waves are
running in the Action Frame.
There are five clickable buttons on the Control Panel
that allow you to choose which type of seismic wave you will generate
in the wave block of the
Action Frame. They are labelled P,
S, LOVE, RAYLEIGH, and P+S.
The first four button each select a different type of
seismic wave. The purpose of this activity is to observe the
differences in the motion produced by these waves.
The last button actually selects a combination of two of the
previous waves, the P and the S, running them together at
their appropriate relative speeds. This option simulates, very
basically, the type of motion you would expect to witness during
One caution: only the P+S option shows the waves propagating at their
appropriate relative velocities. All the single wave simulations
run at the same velocity (assuming you do not adjust the
speed scale). This is not an accurate
representation of the behavior of actual seismic waves, which
propagate with different velocities.
This switch allows you to select the directionality of the first
wave arrival, which can be either "up" (in the direction of the
component name, be it Z (vertically up), N (north),
or E (east)) or "down" (down for Z, south for N, and west for E).
When you have selected all the details of your simulation -- the
type of wave, the speed,
the direction of the first arrival, and
the position of the underground sensor --
it is time to start the wave generator. To do this, simply
click the round green button labelled GO! This will start
a pulse of seismic waves running the length of the
wave block in the Action Frame
below. Most controls on the Control Panel
(including the Go! button) will not function while the wave
simulation is running. The exception is the speed scale;
you can change the speed of the waves at any time.
If you've read through the instructions above,
you should have a good idea of how to work the interactive
seismic wave generator. Click here to load
this activity now!
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page of Activity #2