Measuring Earthquakes

Seismographs are machines that detect and measure seismic waves caused by earthquakes. A seismogram is the tracing of an earthquake's motion. The arrival time and magnitude of each type of seismic wave is recorded in a seismogram.

Seismologists use the P and S wave arrival times to calculate the distance between the location of the seismograph and an earthquake's epicenter. The more time that passes between the arrival of the waves, the greater the distance from the epicenter.

Readings from three different seismograms are needed to determine the epicenter of an earthquake. Circles are drawn indicating epicenter distances. The intersection of these circles is the location of the epicenter.

The strength of an earthquake is recorded on a scale of 1 through 10, where 1 is the weakest and 10 is the strongest. This is called the Richter scale and was developed in 1935.

The Richter scale is logarithmic, not linear. The amplitude of waves on a seismogram is 100 times greater in a magnitude 7 earthquake as compared to a magnitude 5 earthquake. The amount of energy released between each level on the scale increases at an even greater rate.

The Mercalli scale is used to measure an earthquake's intensity as it relates to the amount of damage the earthquake causes. Locations closer to the epicenter typically have a higher score on the Mercalli scale.