How to Use a Richter Scale

Lay out the seismogram on a table. Study the seismogram from left to right. The very gentle wiggles are microseisms. These are recordings of ground movements due to heavy traffic or wind that the seismograph records continuously but that are too gentle for humans to notice. Identify the first sharp wiggle that rises clearly higher than the microseisms. This is the primary, or P-wave. Note its arrival time on the horizontal scale of the seismogram. Identify the next sharp wiggle that rises clearly higher than the P-wave. This is the secondary, or S-wave. Note its arrival time.

Subtract the P-wave arrival time from the S-wave arrival time. This is the S-P time difference. Download the S-P time versus distance graph from a university, such as Concord, that teach the basics of earthquakes within a geology or geophysics course, or from the United States Geological Survey (USGS) website. Note where the S-P time difference intersects this graph and read the distance on its horizontal scale. This is the distance between the earthquake epicenter and the seismograph that recorded it.

Note the maximum amplitude of any wave recorded on the seismogram. This is the vertical distance of the trace from the zero horizontal axis. Download the Richter nomogram from Concord University or another university geology or geophysics department's website, or from USGS. It is a graphical device that shows the relationship between earthquake distance, maximum amplitude and earthquake magnitude according to the Richter scale.

Note the value for earthquake distance on one scale and the maximum amplitude on the second scale. Draw a line between these two points and read where they intersect the third scale. That intersection point is the magnitude of the earthquake on the Richter scale. The Richter scale is logarithmic, meaning that for each whole number increase in magnitude, the wave amplitude increases 10 times.