# Earthquakes and Seismic Waves (2.2)

## 15 terms

### what is an earthquake?

the shaking and trembling that results from the movement of rock beneath earth's surface

### what is the focus?

the area beneath earth's surface where rock that is under stress breaks, triggering an earthquake

### what is the epicenter?

the point on the surface directly above the focus

### what is the motion of seismic waves?

they carry energy from an earthquake away from the focus, through earth's interior, and across the surface

### what are the 3 main categories of seismic waves?

P waves, S waves, surface waves

### what are P waves?

primary waves are the first ones to arrive; they compress and expand the ground like an accordion; they travel through solids and liquids

### what are S waves?

secondary waves come after P waves; these waves shake the ground from side to side and up and down; S waves only move through solids

### what are surface waves?

when P and S waves reach the surface they become surface waves; surface waves are the slowest waves but they are very dangerous; they make the ground roll like ocean waves

### what are 3 commonly used methods of measuring earthquakes?

mercalli scale, richter scale, moment magnitude scale

### what is the mercalli scale?

rate earthquakes according to the level of damage at a given place

### what is the richter scale?

rating of an earthquake's magnitude based on the size of the earthquake's seismic waves; works best for small and nearby earthquakes

### what is magnitude?

a number based on the earthquake's size

### what is a seismograph?

an instrument that records and measures seismic waves

### what is a moment magnitude scale?

a rating system that estimates the total energy released by an earthquake

### how do geologists locate the epicenter?

by using seismic waves; they measure the difference between the arrival times of the P and S waves. the farther away an earthquake is, the greater the time between the arrival of the waves