Seismogram measuring the earths movement

When the earth trembles, as it does during an earthquake, a seismograph takes a reading and the resulting measurements and readings is called the seismogram. Many years ago, a seismogram used to be a pencil attached to a revolving drum where the lines would be drawn to represent the activity going on during the earthquake or sometimes an explosion.

 Looking at a seismogram, you will first notice all the squiggly lines running across the paper. These represent the seismic waves that the seismograph has picked up during an earthquake or some type of movement below the earths surface. Microseisms are so small that no one ever feels them, they don’t cause any damage and they will not hurt anyone.

These small ‘tremors’ can be anything from waves hitting the beach or heavy traffic near the seismograph. These microseisms can also be caused by the wind or any number of ordinary activities in the atmosphere or man made such as construction drilling near the site of the seismograph.

Among the wiggly lines, the first set, after the long, plain and straight line which represents no activity, would be the P lines and this represents the P wave and is much bigger than all the smaller, microseisms wavy lines. Next on the seismograph will be the S waves which are bigger than the P waves. This indicates the next wave of damaging earth movements.

The absences of S waves on your seismogram means that the earthquake probably happened on the other side of the planet and it was not able to be felt or picked up by your seismograph. S waves can’t travel through the liquid layers of the earth, therefore, these S waves never made it to your seismogram.

Often the larger, longer waves marked on the seismogram are the surface waves, sometimes called the Rayleigh or Love waves. These waves have a lower frequency which means the up and down lines on the seismogram are spread out more. Traveling a little slower than S waves, these surface waves arrive directly after the S waves.

If the earthquake was a shallow one, meaning it was close to the surface of the earth, the surface waves will be the largest waves to be seen on the seismogram.  

Seismographs and their information – seismogram – can help save lives when they are read by a team of scientist know as seismologists who can predict when the earthquakes are going to be coming based on the latest data. Once they see the first wave of lines, the P lines, then they can basically predict within minutes when the S lines and surface lines, if there are going to be any, will be coming and warn the public through the proper channels, such as radio or television.  

Today, a lot of the seismograms have come a long way since the drum and paper and pencil kind of the first earthquake predictors. Today, there are many seismograms that rely on the technology of computers and various stakes and antennas placed in the ground and around the seismograms to help gather the information to help save the public.