Three rating scales can be used to measure an earthquake.

**Mercalli:** rates earthquakes by describing their effects on people, building, and the land surface
in a given location.

**Richter:** rates
earthquakes according to the size of seismic waves as measured by a seismograph.

**Moment magnitude:** rates earthquakes by estimating the total
energy they release and can be used to rate earthquakes of all sizes, near or far.

EVERY
earthquake…

originates from a specific point beneath Earth’s surface where rock that is under stress
breaks. **(FOCUS**

has a point on the surface directly above the focus called the **epicenter**.

creates vibrations that
travel through Earth carrying the energy released during the earthquake. **(SEISMIC WAVES)**

**Seismic waves carry
the energy of an earthquake away form the focus**, through Earth’s
interior, and across the surface.

Three types
of seismic waves

**Primary (P) waves**: compress and expand the ground like an accordion. Travel fastest.

**Secondary (S) waves**: vibrate from side to side and up and down.
Travel second fastest.

**Surface waves**: caused by some P and S waves that reach the
surface. Slowest wave.

Scientists measure and record
the vibrations of seismic waves with instruments called **seismographs**.

An earthquakes__ __**magnitude**
is a measurement of its strength based on seismic waves and movement along faults.

Finding the epicenter

When an earthquake strikes, P waves arrive at a seismograph
first and s waves next.

The farther away the epicenter is, the greater the difference between the
two arrival times.

This time difference between P and S waves tells scientists how far the
seismograph is from the epicenter.

The scientists then use the information from three different seismograph
stations to plot circles on a map.

Each circle shows the distance from one seismograph station to all the points
where the epicenter could be located.

The single point where the three circles intersect is
the location of the earthquakes epicenter.