The Earth Moved

The world shook yesterday. I was walking through the gardens doing a survey of the new wild flowers that had started to grow. Then I started to hear a rumble, I didn’t think too much about it since there are always rumbles coming from the streets outside the estate walls. I wasn’t concerned till I started to feel dizzy and everything started to sway.  I fell to my knees and thats when I realized that it wasn’t me who was dizzy, the ground beneath my feet was moving. I quickly moved from fear to excitement knowing that I had seen an instrument down in the lab that was designed to measure earthquakes.

I needed to know how big the quake was and I was curious how far it could be felt. After a little searching I found it, a older model of a seismometer designed by James Forbes in 1844. It consisted of an inverted pendulum made from a stiff wire with a movable mass that allowed for the period to be altered. By placing a pencil on the end of the wire I could record the exaggerated movements of the free weight on a paper lined sphere. Here is a sketch of it:

Forbes' seismometer (after Forbes, 1844), movable mass (C), pencil (L), inverted dome (I)

Turns out the quake was about a 5.3 on the Richter scale, a moderate quake around the world, but pretty large here in the British Isles. By this morning the news had already been spread of the quake, and it seems that it was felt from Galway to Glasgow to London. The heaviest of the motion seems to have been felt in Caernarfon, Wales.

Make your seismograph:

Materials: scissors, shoebox with a lid, a heavy weight, masking tape, a pencil with an eraser, a weight for the pencil like nails or washers, playdoh or clay, two paper clips, string and 2(or more) sheets of paper.

  1. Carefully cut a tiny slit in the middle near one end of the shoebox lid.
  2. Place the open box upright, on one end, and put something small and heavy inside to keep it in position.
  3. Tape the lid onto the top of the box forming an upside-down “L” with the slit in the lid facing away from the box.
  4. Attach the weights to the pencil near the sharpened end, make sure not to cover the point. Tape the weights tightly to the pencil. A small piece of clay will keep the weights from slipping off. The weights must be fairly heavy so the seismograph recorder pencil will make good contact with the paper and draw fairly dark drag lines on it.
  5. Open one end of a paper clip and push it securely into the eraser end of the pencil. Tie the string to the unopened end of the clip.
  6. Attach the second paperclip to the other end of the string, and wind the string around the paperclip like you would wind kite string.
  7. Slip the top clip through the slit and adjust the pencil marker so the tip rests on the table, not perfectly straight, but dragging as it moves.
  8. Slip the remaining string under one side of the clasp to fasten the upright pencil into place.
  9. Cut each sheet of paper into thirds lengthwise. These strips will act as your roll paper and record your “earthquake movements”.

Time to record an “earthquake”!

Place a paper strip against the box (below the slit you made in the lid) and slowly pull the strip forward.

  • Notice how straight the drawn line is as you move the strip of paper.

Have someone else bump and shake the table as you pull the paper strips under the dragging pencil marker.

  • Notice how your seismograph makes sideways and up and down movements.

Compare the separate strips of paper.

  • How do the lines differ? how do they show the effects of movement? Could you learn to recognize the difference between a shake and a bump of the table?

So, what’s really happening?

When faults slip during earthquakes, they release energy in the form of a seismic wave. Seismographs capture and record the jolts and shakes from each passing wave. Seismograms (the graph made by a seismograph) are like earthquake fingerprints. Each tells an individual story of an earthquake: how deep it was, which direction the fault moved, and what kinds of rocks the waves passed through.

The strength of an earthquake is determined by the height of the energy waves that are recorded. Seismologists, scientists that study earthquakes, are able to determine when and where the quake happened by determining the times the waves arrive to different locations.

If you think of a fault like a patchwork quilt, each individual patch will have its own energy signature, and that signature will change depending on what direction each patch is moving. So if you think of a large earthquake like causing ripples in a patchwork quilt, you can see how there would be a huge number of overlapping signatures that have to be decoded by seismologists in order to figure out the exact location and cause of an earthquake.

Check out this USGS PDF for a series of 10 articles specifically designed to teach kids about Earthquake science.

Posted By Astrid

One Response to “The Earth Moved”

  1. tie a tie says:

    trying to find you, what’s your twitter?

Leave a Reply