An Illustrated Guide to Reading a Seismogram
This video provides a tutorial for anyone interested in interpreting the seismic records on public webicorder displays.
Location Taken: Menlo Park, CA, US
Ever wondered how to read the data on a seismogram?
The webicorder is the digital version of the
paper records used by scientists in days of
Seismometers measure vibrations.
More vibration… more wiggle.
Some seismometers measure only up and down.
Sometimes, they shake too much and data are
Some seismometers record in all three dimensions.
Many webicorders display 15 minutes per line,
alternating colors as time passes.
Plots are automatically updated every few
Scientists use webicorders for a quick look
at current activity, but webicorders are really
just a blurry picture of the actual data To
really understand what’s going on, they
look at the real data, which is far more informative.
Here is a small earthquake.
It begins as a sharp crack as the rock breaks,
and then quickly decays with time.
In this plot, the data are clipped to allow
viewing of other data.
That can be very useful when there are lots
of earthquakes, as in this seismic swarm,
with multiple earthquakes of magnitude 2 to
Seismometers are sensitive enough to pick
up vibrations from earthquakes around the
world – if they’re big.
This is what the giant 2011 Japan earthquake
looked like on a seismometer in Wyoming, 5000
Compared to the P and S waves that travel
through the earth, surface waves are slower
and arrive later, but can continue to be recorded
for hours as they propagate around the earth.
Seismometers record not only earthquakes but
anything that can make the ground vibrate:
Helicopters, Ocean waves, Traffic, Animals
Wind can be seen as noisy periods.
Here, the wind picks up each day and drops
off at night.
Here, traffic noise shows up on a seismic
station located a quarter mile from a road.
Seismometers are sensitive electronic instruments
often deployed in harsh wilderness environment.
They can and do break.
Sometimes they just stop working, but sometimes
they just become noisy and unreliable.
Here’s a record from a three channel instrument,
one of which is broken – guess which one?
Nearby equipment and machinery can also create
Worried about something on a webicorder?
Then check readings from nearby instruments.
Anything that shows up on only a single seismometer
is either noise or earthquake activity too
small to worry about.
You’ve done your homework and you’re ready
to graduate from the “Acme School of Webicorder
Arts and Sciences.”
You’re now prepared to start cruising data
on the internet.
Have fun out there, mind your s’s and p’s,
and please keep in mind, if you see an ominous
signal at Mount Scarrymore, don’t hesitate
to seek the opinion of a professional seismologist.
They have many years of training, and on-the-job
And they all have their favorite story for
that funny signal no one can explain.