Seismographs
How to Hear an Earthquake
Unless otherwise noted the artwork and photographs in this slide show are original and © by Burt Carter.
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The “sensors” we talked about earlier have a name: seismograph. Just like a telegraph is a machine (now rarely
used, if they still even exist) for receiving (and sending) the information in a message, a seismograph is a device
for receiving the information that an earthquake has occurred.
GSW has a seismograph, housed in that half-buried brick building near College Lake.
Actually there are four devices in the vault – the three labeled “detector” here and one off the image (the E-W long
period detector) you can’t see. We have four to give clues about how near and which direction a set of waves
originated, in a very general sense.
The short period detector is a kind of test of the hypothesis that an earthquake has happened. If it is recording
something and the others are not then the event is local and not an earthquake. Examples are: the passing of a train,
increased traffic on Perimeter Drive (right after class), or (in days long past) a rock and roll concert in the Field House.
Photo by Dan Askren
The graphic that shows the effects of waves on the sensors is called a seismogram. (Think “seismograms come
from seismographs just like telegrams come from telegraphs.)
These days the data recorded by the devices is stored digitally and displayed (when it is displayed) on a computer
monitor. The four zones obvious on the screen show the waves as they were recorded by the four seismographs in
the vault.
Photo by Dan Askren
Previously we recorded them on a
drum recorder. How it works will
be explained in the following
slides, but notice that the red
arrow indicates the start of our
recording of an earthquake in
Mexico in 1997.
The lines that go around the drum,
appearing vertical on the picture,
are the trace of a pen that records
the behavior of the seismograph.
The horizontal “lines” are really
just tic marks made by the pen
periodically (every minute) to keep
up with the exact time.
Someone used to change the
paper and reset the clock to the
atomic clock at the Naval
Observatory in Maryland every
day, so we could have a continuous
record and tell within a few
seconds what time a particular
wave arrived.
You never know when an
earthquake will happen
somewhere so you have to watch
all the time.
Photo by Dan Askren
This seismograph will record vertical movements. An obvious modification would make it record horizontal ones.
We’ll look at the parts one-by-one.
1) A drum rotates on a
spindle.
Attached to the drum is a
piece of paper, on which
the seismogram will be
drawn.
As the drum rotates once
in 15 minutes.
A heavy weight with a “pen” attached is suspended by a spring from the top
of a support, and pivoted to the vertical part of the support. This means it
can move up and down and only up and down.
The pen does not contain ink. Instead it is electrified and “draws” on the
heat sensitive paper from a slight distance.
As the drum rotates the pen
is moved in one direction
across it – in this case
downward as indicated by the
arrow.
The rate is carefully
controlled so that the pen
moves from one end of the
drum (top to bottom here) in
one day.
The rate of rotation of the
drum is matched to this so
that it rotates once in 15
minutes.
The pen draws a continuous
line that spirals around the
paper. When we remove the
paper, of course, the
continuity will be broken, but
we know that each line picks
up where the one beside it
leaves off.
Ordinarily both the pen and the drum are
only moving as described above, so the
recording is a more or less straight line
around the paper. (Actually there is
always a little bit of jiggle because the
world is always doing something.)
However, when an earthquake wave
arrives things change, and not the way
you might think, and not exactly the way
the picture implies.
The heavy weight and the way it is
suspended gives that part of the system a
good bit of inertia – a strong tendency in
other words to stay still as it had been
doing.
The drum, in contrast, is rigidly attached
to the shaking ground, so it moves up
and down.
The pen draws the signal on the paper
and when the drum stops that evening
we have a drawn record of how it
unfolded.
Notice that the devices sit on a block of concrete. That pieces of concrete was poured into a mold in a hole about 60’
deep! This was done to try to isolate the machines from local noises like trains and traffic and loud music.
In addition, the hole around the concrete is bigger around than the concrete and backfilled with a very lightweight
mineral called vermiculite (a type of mica that has expanded). This is sort of like filling the hole with styrofoam
peanuts, only vermiculite is a lot more stable. This to damps some of the local events from affecting the instruments.
And even so we can “see” when people drive away from campus …
Photo by Dan Askren