Earthquakes are measured by seismographs or seismometers. These machines create seismograms, which are the records of waves and strength of waves during an earthquake. Alterations in results must be made, however, because the strength of seismic waves decreases based on the distance from the earthquake. Seismometers can record earthquakes from extreme distances, because seismic waves are able to travel and register throughout Earth's entire interior.
Seismometers hold a suspended reader above a fixed base that allows the reader to stay still as the ground moves. The movement from the ground to the suspension measures the ground's motion. Seismograms show different types of waves from earthquakes, including P-waves, which are compressional waves, S-waves, which are shear waves, and surface waves. The P-wave arrives first, followed by the S-wave, and then the surface waves.
S-waves are slower than P-waves, but they are also larger and cause the most damage. The distance of an earthquake is pinpointed using the time difference of P-waves and S-waves at various seismographs. The total strength, or absolute magnitude, of an earthquake is reported through readings and recordings on a moment magnitude scale, formerly known as the Richter scale, while the felt magnitude, or shake of an earthquake, is recorded through the Mercalli intensity scale.