Understanding the Strength of Earthquakes
You may have heard of earthquakes referred to by their magnitude. For example the Great San
Francisco Earthquake of 1906 has been estimated to be somewhere between 7.7 and 8.3 on the
Richter magnitude scale which measures the amount of ground (seismic) shaking produced by the
earthquake wave that breaks along the Earths surface. The exact number is not known since
seismograms, machines designed to measure seismic waves were not in place in 1906 and the
number is estimated from the damage that occurred. Today earthquakes magnitudes or strength
are measured in two ways. The Richter scale measures the size of the seismic wave motion and
is a base 10 logarithmic scale so every step up represents a 10 times greater motion than the last
step.
For example: a Richter magnitude 5 earthquake has a seismic wave 10 times greater than a
Richter magnitude 4 earthquake and a Richter magnitude 6 earthquake has a seismic wave 100
(10 x 10) times greater than a Richter magnitude 4 earthquake.
The moment magnitude scale measures the amount of energy produced and is also a base 10
logarithmic scale and each step up represents a release of about 32 times more energy.
For example: a moment magnitude 5 earthquake releases 30 times more energy than a moment
magnitude 4 earthquake and a moment magnitude 6 earthquake releases 900 (30 x 30) times more
energy than a moment magnitude 4 earthquake.
The 1989 Loma Prieta Earthquake in the Bay Area had a Richter scale magnitude of 7.1 and a
moment magnitude of 6.9.
Try these problems below:
1. How many times greater is a seismic wave of magnitude 2 earthquake than one of magnitude
1? How many times more energy is released?
2. How many times greater is the seismic wave of a magnitude 7 earthquake than of a
magnitude 4. How many times more energy is released?
3. If a moment magnitude 6.2 earthquake releases 4 times more energy than a 6.0 earthquake and
a moment magnitude 6.4 releases eight times more energy than a 6.0, how many times more
energy will a moment magnitude 6.8 release than a 6.0?
4. How many times greater is a seismic wave of magnitude 7.9 than one of magnitude 2.9? How
much more energy is released?
5. How often do you think magnitude 1 earthquakes occur in the Bay Area? How often do you
think earthquakes of magnitude 7.0 occur in the Bay Area?
6. What is a seismogram? Why is the magnitude of the Great San Francisco Earthquake of 1906
estimated? How was this estimate made?