Earthquake Magnitudes are Logarithmic!
Earthquakes occur when two pieces of ground move relative to each other. In an earthquake the
two pieces can move up, down, side-to-side, or a combination of both. Earthquakes often occur
along faults where pieces of the Earth crust have been moving (and causing earthquakes) over
hundreds, thousand or even millions of years. Earthquakes produce a series of ground waves
(just like waves on water) that can be recorded on machines called seismographs. The size of the
waves is measured by a pen that records the wave passing through the machine on a piece of
paper called the seismogram. The magnitude of an earthquake is a measure of the size of the
ground waves and increases logarithmically (exponentially).
1. Set up your semi-log graph by labeling your x axis Magnitude and your y axis size of
earthquake wave. The graph paper is on the back of this page;
2. Scale and number your y axis logarithmically you have already learned. The origin is 1; the
next lines are 2,3,4,5,6,7,8,9 and 10. The next line is 10, then the next lines are 20, 30,
40,50,60,70, 80, 90,100. Continue labeling numbers (remember to label the 100’s and then
1000’s correctly!) to the top of the y axis.
3. Scale and number your x arithmetically axis as follows: Label the origin 1, then count ten
lines and label it 2, count ten lines and label it 3, count ten lines and label it 4, and count ten lines
and label it 5.
4. Now draw a line from the lower left corner of the graph to the upper right corner of the graph
and use this line to answer questions a. through d. below.
a. What is the size of the wave in a magnitude 2 earthquake compared to a magnitude 1?
b. What is the size of the wave in a magnitude 3 earthquake compared to a magnitude 1?
c. What is the size of the wave in a magnitude 4.5 earthquake compared to a 1?
d. What is the size of the wave in a magnitude 5 earthquake compared to a magnitude 4?
Earthquake energy released increases by thirty (30) times from one magnitude to the next.
e. How much more energy is released in a magnitude 4 earthquake than a magnitude 3?
f. How much more energy is released in a magnitude 5 earthquake than a magnitude 3?
g. How much more energy is released in a magnitude 5 earthquake than a magnitude 1?
h. In October 2011 the Hayward fault experienced a swarm of earthquakes in the 4
magnitude range. Mark and label them on the line. The Hayward Fault is one of the most
dangerous faults in the United States because in the past it has generated large magnitude
earthquakes on a regular periodic interval and because so many people live on or near the
fault. The Hayward Fault runs along the base of the hills in Fremont approximately along
Mission Boulevard. What plans have your and your family made to prepare for a large
earthquake?