A seismograph is a machine used by scientists to measure movement in the Earth. The readout from this machine is called a seismogram. It can reveal valuable information such as the location or severity of an earthquake.
The results on a seismogram are displayed by lines. When there is no activity, the readout will reveal a long, straight line. When movement is detected, a long row of lines that go up and down will be seen. These lines represent movement in the Earth, also known as seismic waves. Two kinds of waves that are studied on a seismogram are P and S waves.
P waves, also known as push waves, result from forward movement. Torsion waves, often called S waves, represent the spiraling motion of particles twisting between inner structures. P waves are usually the first to be recorded on a seismogram because they travel the fastest. S waves usually have more height, or amplitude, than P waves. The amplitude of the waves can help to reveal information about the magnitude of an earthquake.
A seismogram can reveal how far the seismic waves occurred from the seismograph. This is often shown when the readout has P waves but there are no S waves. This generally means the movement occurred far away. A more accurate indication of distance may be available when the time is calculated between the recording of P and S waves. In many instances, several seismograms that originated from various locations are compared for this purpose.
A seismogram often reveals how close an earthquake occurs to the surface. This is made apparent by the recording of surface waves, which are slower than S and P waves. These can be identified because there is usually more space between them than there is between the other types of waves. They are also often the largest marks on the seismogram.
Some of the marks on a seismogram may not be related to an earthquake. Some marks may be microseisms. This is motion that a seismograph registers, but which is caused by other factors such as explosions or ocean waves.
Seismograms have changed over time. These readouts were once recorded using pen or pencil and paper or a light beam and photosensitive paper. Modern seismograms are most commonly produced digitally. The results are often transmitted long distances from the point where they are recorded to another point where a professional analyzes them.