S waves (originally called secondary waves) are transverse waves, like light and other electromagnetic radiation. S waves cause the particles in the materials they pass through to vibrate at right angles to the direction the waves move. You can see how S waves move in Figure 36. Unlike P waves, S waves cannot travel through liquids.
Surface waves are waves that develop when seismic waves reach Earth's surface. Surface waves move more slowly than P waves and S waves, but usually produce larger ground movements and greater damage. Some surface waves are transverse waves, and others have a rolling motion at Earth's surface that is similar to ocean waves.
Measuring Earthquakes
When an earthquake occurs, the first things people want to know are how big it was and where it was centered. 
To measure earthquakes and pinpoint their epicenters, geologists record seismic waves using seismographs. A device that can detect and record seismic waves is called a seismograph. The How It Works box on page 688 illustrates a seismograph. The record of an earthquake on a seismograph is called a seismogram. Earthquakes can be located using the seismic waves recorded by many different seismographs.
The range in recorded earthquake sizes is enormous. Most earthquakes are too small to be felt by humans. The largest earthquakes release more energy than the United States consumes in a year.
Richter Scale
The most well-known scale is the Richter scale. The Richter scale rates earthquakes based on measurements of the times and amplitudes of seismic waves by certain seismographs. However, the Richter scale is no longer used by geologists.
Moment Magnitude Scale
The most useful scale for geologists is the moment magnitude scale (Mw). This scale gives a measure of the amount of energy released by an earthquake. Each unit increase on this scale represents about a 32-times increase in the energy released. The largest earthquake ever recorded was a Mw 9.5 earthquake that occurred in southern Chile in 1960.
Modified Mercalli Scale
The effects of earthquakes can also be rated using the modified Mercalli scale. This scale, which ranges from 1 to 12, is based on observations of the intensity of ground shaking and damage in the areas affected by an earthquake. For example, an earthquake rated 7 on the Mercalli scale would knock over some chimneys. A rating of 12 applies to earthquakes that cause total destruction of nearby towns or cities.
What is the moment magnitude scale?
Quick Lab
Modeling a Seismograph
Procedure 
Obtain cash register paper from your teacher. Roll it up and place a pencil through the center of the roll so the roll turns freely around the pencil.
Hold the pencil and paper roll horizontally on a flat surface so that a classmate can slowly pull the free end of the paper across the surface. Have a second classmate hold a felt-tip pen so that it marks a line on the paper as it is pulled across the surface.
While the paper is being pulled and marked, have a third classmate do each of the following for 5 seconds: shake the desk gently, shake the desk more vigorously, and hold the desk still.
Analyze and Conclude
Observing How did shaking affect the ink line?
Using Models What did the shaking represent?
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