| A | B |
|---|
| earthquake | the shaking and trembling that results from the movement of rock beneath Earth's surface |
| focus | the area beneath Earth's surface where rock that is under stress breaks, triggering an earthquake |
| epicenter | the point on the surface directly above the focus |
| P wave | primary waves; first waves to arrive at a seismograph; waves that compress and expand the ground like an accordion |
| S wave | secondary waves; second waves to arrive at a seismograph; waves that vibrate from side to side as well as up and down |
| surface wave | a type of seismic wave that forms when P waves and S waves reach Earth's surface; cause the ground to roll with a wavelike motion; most destructive of the seismic waves |
| Mercalli scale | a scale that rates earthquakes according to their intensity and how much damage they cause at a particular place |
| magnitude | the measurement of an earthquake's strength based on seismic waves and movement |
| Richter scale | a scale that rates aan earthquake's magnitude based on the size of its seismic waves |
| seismograph | a device that records ground movements caused by seismic waves as they move |
| moment magnitude scale | a scale that rates earthquakes by estimating the total energy released by an earthquake |
| where most earthquakes occur | within 35 miles of the surface |
