Measuring Earthquakes
Objectives- Describe properties of the different kinds of earthquake waves and the scales used to
measure them.
When plates move against or along each other pressure builds up. Eventually the pressure gets
released, often suddenly. Energy travels rapidly outwards through the earth. The location where the
rocks first begin to move (inside the lithosphere) is called the focus. To a geologist (actually a
seismologist), this point is important, but most people would not understand it. The epicenter (epi =
top) is the location on the earth’s surface directly above the focus.
The vibrations or waves generated by the earthquake are called seismic waves. They travel
outwards from the focus in all directions. The types
of rocks and soil determine how much the ground
shakes. There are three types of seismic waves:
Primary or P waves, Secondary or S waves, and
Surface waves. The P waves are the fastest of the
three. They travel around 6km/sec. P waves make
the surface of the ground move like an accordion.
P waves can travel through both solids and liquids.
S Waves travel about 4 Km/sec and are the second
fastest of the waves. The make the ground go up
and down. S waves can only travel through solids.
Surface waves are the slowest of the waves. They
make the water roll like waves on water.
Earthquakes are detected using a seismograph. Older mechanical seismographs looked like the
one shown. The spring held the pen stationary. When the earthquake
occurred, the base moved, but not the pen. In other words, the paper
moves underneath the pen leaving a mark on the paper. The stronger
the earthquake, the higher the marks. Newer seismographs work on
the same principle, but are electronic.
There have been several scales used to measure earthquake
magnitude or the amount of energy released. Most people are
familiar with the Richter scale. It was developed in the 1930’s. The
Richter scale was limited in its effectiveness, mostly to California
earthquakes and smaller earthquakes. It was based upon the height of
the wave that was recorded.
The Moment Magnitude Scale, the currently used scale works for all earthquakes. The scale
measures the magnitude or the amount of energy released. For every whole number, ten times more
energy is released. This would be like going from a 3 to 4 on the Moment Magnitude Scale. Going from
a 3 to a 5 would be a 100 times increase in the amount of energy. The number is the product of how
much the fault move and how much force it takes to move it. Sounds complicated.
In order to determine the epicenter of an earthquake, seismologists need readings from a
minimum of three seismographs. By using the differences in the time between the P and S waves
arriving at a specific location, the can tell how far away they are from the focus. Remember that the
speed P and S waves travel are known and the farther
away the location is from the epicenter, the greater the
difference between when the P and S waves arrive. If only
one seismograph station is used, every location on a
circle drawn at the appropriate distance away from the
seismograph would be a possible location. If two
seismographs are used, then there could be 2
intersections of the circles around each seismograph.
Only with three could you get one location.
Questions:
1. View the website Seismic Waves (sunshine.chpc.utah.edu). Read the information and answer the
questions.
2. Complete the chart on seismic waves
Type of Wave
Speed
Ground motion
P
S
Surface
3. How much more energy does an earthquake with a magnitude of 7 release than a magnitude 3
earthquake?
4. Suggest a reason why information from California earthquakes cannot be applied to all earthquakes
to determine their strength.
(rest is not from website)
5. The point where a rock under stress breaks and triggers an earthquake is called the ____________.
6. The point on the surface directly above the focus is the ________________________.
7. What are seismic waves?
8. True or false- Seismic waves carry the energy of an earthquake away from the focus in all directions.
9. True or false- Surface waves move more quickly than P waves and S waves.
10. A device that records the ground movements caused by seismic waves is a(n)
_________________________.
11. True or False- The closer an earthquake, the greater the time between the arrival of P waves and the
arrival of S waves.