Hi Chris. Your right. Here are some of the most common types of seismographs used.

Ms = based on measurements of surface waves with a period of about 20 seconds

Ml = local magnitude, based on waves with period of 0.1 to 3.0 seconds

mb = magnitude of body waves

Mw = moment magnitude. Total amount of seismic wave energy released in an earthquake, based on waves with a period of about 200 seconds. Measures the earthquake source. More often as not the reading with this one will be higher then the others. It is the preferred choice for large quakes.

Md= duration magnitude in seconds from peak to peak of trace.

The Ml type is used in California because we have so many small quakes. This seismograph is the short period type. The Mw type is also used for the occasional large quakes we get. Take Care…Don in creepy town