Name: _______________________________
Lab: Earthquake Epicenters
Adapted from Exploration in Earth Science, The Physical Setting, United Publishing Company, Inc
INTRODUCTION:
Earthquakes occur when there is movement along a fault. The friction between the rock masses
rubbing against one another generates shock waves which travel through Earth. These shock waves, called
seismic waves, are created by the earthquake and are radiated in every direction from the focus, the point
with Earth where the actual movement takes place.
An earthquake occurs, on average, every 30 seconds, day after day. Most of these are so weak
they would go unnoticed without the use of sensitive modern instruments called seismographs.
While seismologists use many seismograph stations, in this lab you will use seismograms from
three distant stations to locate the epicenter, the point on the Earth’s surface directly above the focus of the
earthquake.
OBJECTIVE: You will learn to interpret a seismogram and using differences in seismic waves, locate the
epicenter of an earthquake.
VOCABULARY:
Fault:
Epicenter:
Focus:
P-wave:
S-wave:
Seismograph:
PROCEDURE A:
The diagram below illustrates the method of using the difference in arrival times of P and S waves
to determine the distance to the epicenter. Using the three seismograms provided, and the “Earthquake
P-wave and S-wave Time Travel” graph in your Earth Science Reference Tables, calculate the following
for each city and record on your Report Sheet:
1.
The arrival Times for P and S waves
2.
The difference in arrival times between P and S
waves.
3.
The distance (in km) of the epicenter to each
city.
4.
The length of time it took for the P-wave to
travel from the epicenter to each city.
5.
Since you know when the P-wave arrived at a
city and how long it had to travel, calculate the
time at which the P-wave started (Origin Time).
PROCEDURE B:
1. To locate the epicenter on the map, for each city construct a circle whose radius is equal to the
distance from the city to the epicenter. Use the scale of distance of your map to set the compass at
the correct radius.
2.
Mark and label the epicenter on the map where all three circles intersect.
SEISMOGRAMS
REPORT SHEET
Discussion Questions:
1. How do P-and S-wave differ?
2.
What was the approximate location of the epicenter of this earthquake?
3.
Why is three the minimum number of stations necessary to locate an epicenter?
4.
Why does the time between the arrival of the P-wave and the S-wave become greater and greater as you get
farther away from the epicenter?
Conclusion: Describe, step by step, how the epicenter of an earthquake can be located (at least 5 steps!)
Reading Comprehension Read the portion of the article on measuring earthquakes below and answer the
following questions based on the reading. Use complete sentences.
How Are Earthquake Magnitudes Measured?
http://www.geo.mtu.edu/UPSeis/intensity.html
The Richter Scale
The magnitude of most earthquakes is measured on the Richter scale, invented by Charles F. Richter in
1934. The Richter magnitude is calculated from the amplitude of the largest seismic wave recorded for the
earthquake, no matter what type of wave was the strongest.
The Richter magnitudes are based on a logarithmic scale (base 10). What this means is that for each whole
number you go up on the Richter scale, the amplitude of the ground motion recorded by a seismograph
goes up ten times. Using this scale, a magnitude 5 earthquake would result in ten times the level of ground
shaking as a magnitude 4 earthquake (and 32 times as much energy would be released). To give you an
idea how these numbers can add up, think of it in terms of the energy released by explosives: a magnitude 1
seismic wave releases as much energy as blowing up 6 ounces of TNT. A magnitude 8 earthquake releases
as much energy as detonating 6 million tons of TNT. Pretty impressive, huh? Fortunately, most of the
earthquakes that occur each year are magnitude 2.5 or less, too small to be felt by most people.
The Richter magnitude scale can be used to describe earthquakes so small that they are expressed in
negative numbers. The scale also has no upper limit, so it can describe earthquakes of unimaginable and (so
far) unexperienced intensity, such as magnitude 10.0 and beyond.
Although Richter originally proposed this way of measuring an earthquake's "size," he only used a certain
type of seismograph and measured shallow earthquakes in Southern California. Scientists have now made
other "magnitude" scales, all calibrated to Richter's original method, to use a variety of seismographs and
measure the depths of earthquakes of all sizes.
Here's a table describing the magnitudes of earthquakes, their effects, and the estimated number of those
earthquakes that occur each year:
Magnitude
2.5 or less
2.5 to 5.4
5.5 to 6.0
6.1 to 6.9
7.0 to 7.9
8.0 or greater
Earthquake Magnitude Scale
Earthquake Effects
Estimated Number Each Year
Usually not felt, but can be recorded by seismograph.
900,000
Often felt, but only causes minor damage.
30,000
Slight damage to buildings and other structures.
500
May cause a lot of damage in very populated areas.
100
Major earthquake. Serious damage.
20
Great earthquake. Can totally destroy communities near the epicenter.
One every 5 to 10 years
Earthquake Magnitude Classes
Earthquakes are also classified in categories ranging from minor to great, depending on their magnitude.
Class
Magnitude
Great
8 or more
Major
7 - 7.9
Strong
6 - 6.9
Moderate
5 - 5.9
Light
4 - 4.9
Minor
3 -3.9
The Mercalli Scale
Another way to measure the strength of an earthquake is to use the Mercalli scale. Invented by Giuseppe
Mercalli in 1902, this scale uses the observations of the people who experienced the earthquake to estimate
its intensity.
Modified Mercalli Intensity Scale
Mercalli
Intensity
I
II
III
IV
V
VI
VII
VIII
IX
X
XI
XII
Equivalent Richter
Witness Observations
Magnitude
1.0 to 2.0
Felt by very few people; barely noticeable.
2.0 to 3.0
Felt by a few people, especially on upper floors.
3.0 to 4.0
Noticeable indoors, especially on upperfloors, but may not be recognized as an
earthquake.
4.0
Felt by many indoors, few outdoors. May feel like heavy truck passing by.
4.0 to 5.0
Felt by almost everyone, some people awakened. Small objects moved. trees
and poles may shake.
5.0 to 6.0
Felt by everyone. Difficult to stand. Some heavy furniture moved, some plaster
falls. Chimneys may be slightly damaged.
6.0
Slight to moderate damage in well built, ordinary structures. Considerable
damage to poorly built structures. Some walls may fall.
6.0 to 7.0
Little damage in specially built structures. Considerable damage to ordinary
buildings, severe damage to poorly built structures. Some walls collapse.
7.0
Considerable damage to specially built structures, buildings shifted off
foundations. Ground cracked noticeably. Wholesale destruction. Landslides.
7.0 to 8.0
Most masonry and frame structures and their foundations destroyed. Ground
badly cracked. Landslides. Wholesale destruction.
8.0
Total damage. Few, if any, structures standing. Bridges destroyed. Wide cracks
in ground. Waves seen on ground.
8.0 or greater
Total damage. Waves seen on ground. Objects thrown up into air.
The Mercalli scale isn't considered as scientific as the Richter scale, though. Some witnesses of the
earthquake might exaggerate just how bad things were during the earthquake and you may not find two
witnesses who agree on what happened; everybody will say something different. The amount of damage
caused by the earthquake may not accurately record how strong it was either.
Some things that affect the amount of damage that occurs are:
•
•
•
the building designs,
the distance from the epicenter,
and the type of surface material (rock or dirt) the buildings rest on.
Different building designs hold up differently in an earthquake and the further you are from the earthquake,
the less damage you'll usually see. Whether a building is built on solid rock or sand makes a big difference
in how much damage it takes. Solid rock usually shakes less than sand, so a building built on top of solid
rock shouldn't be as damaged as it might if it was sitting on a sandy lot.
1. Explain what a logarithmic scale means.
2. New York State’s largest earthquake occurred in 1944 and measured 5.8 on the Richter
scale. About how many earthquakes a year occur that are of a similar size, and what
magnitude class would they belong to?
3. Why is the Mercalli Intensity scale a better scale for estimating the strength of
earthquakes such as the Great San Francisco Earthquake in 1906?
4. Why might people in two different locations report different intensities/damage from the
same earthquake?
5. If you were about to buy a house in Southern California (near the San Andreas Fault),
what would be one thing you would want to know to make sure the house is prepared for
an earthquake or is “earthquake ready”?