GLS100 Lab – Earthquakes: Location and Hazards
Physical Geology—Dr. Lindley S. Hanson
Objectives
Upon completion of this lab you will be able to:
 Read a seismogram
 Determine distance travel by seismic waves from the lag time between seismic waves
 Locate an epicenter using seismic data
 Determine the timing of a tsunami
 Interpret shaking vulnerability from a geologic map
 Identify the conditions, both natural and man induced that can increase loss of life during
an earthquake
Required Materials and Resources:

PENCIL

Drawing compass

Scrap paper for measurements

Calculator (if desired) or scrap paper for calculations

Appendix containing:

o
Two sets of seismograms (pp. 1-2 and 6)
o
Travel time curves for p- and s-waves (p.3)
o
Tectonic map of the world (p. 5)
o
Maps 1 (p. 4) and 2 (p. 7) for plotting the earthquake epicenter
Internet access
Section I: The 2004 Sumatra Earthquake
Introduction: The incredible damage and tragic loss of life resulting from the 9.0
magnitude earthquake and ensuing tsunami was shocking and almost beyond belief. The
event marked the most devastating natural disaster to hit the world in the last century.
While earthquakes are somewhat unpredictable, and always beyond our control,
earthquake related tsunamis can be measured and predicted in time to provide some
warning to residents of susceptible coastal areas, and shoreline structures can be built to
withstand the force of a tsunami. And there are natural warning signs of impending
tsunamis, too, that properly understood and heeded can give individuals along the shore
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time to get to higher ground. Unfortunately for the tens of thousands of victims of the
tsunami, a warning system did not exist in the Indian Ocean Basin, most shoreline
structures were not built to reduce the destruction from the force of a tsunami, and many
people on the shores did not recognize or understand the warnings nature provided. In the
affected areas proximity, population Studies in Southeastern India following the
earthquake revealed that clearing of coastal mangrove forests destroyed an important
buffer, severely increasing damage to coastal communities. Thirty trees per 100 square
meters can potentially reduced the maximum flow of a tsunami by 90%. Geologists will
learn from this tragedy, and hopefully work to provide better warning systems, better
construction, and better natural disaster preparedness education in the future.
In this lab you’ll study seismograms from 6 different seismic stations recording the
magnitude 9.0 Sumatra earthquake of December 26th, 2004. By comparing the arrival
times of the P and S waves on each seismogram, you’ll be able to determine the distance
from the epicenter to each station. Using that data, you can accurately map the location
of the epicenter of the earthquake. Once you’ve located the epicenter, you’ll calculate the
position of the tsunami generated by the earthquake at one-hour intervals. From those
determinations, you will be able to predict how much time people had before the tsunami
crashed onto their shores. Finally, you will investigate some of the ways people can
lessen the impact of the next great tsunami.
PART Ia: Finding the Epicenter
Procedure
1. Using the seismograms on pages 1 and 2 (appendix), read the arrival times the P and S
waves reach a station. Note: The first vertical line marks the P-wave arrival and the
second vertical line marks the S-wave arrival time.
2. Devise a way to determine the amount of time that elapsed (lag time) between the
arrival of the P and S waves at each station and record it in Table 1 (page 2) in minutes
and seconds. One way is to subtract the P-wave arrival time from the S-wave arrival
time (S-P), though there is a more direct way to get that information off the seismogram.
Double-check before recording your answer.
3. Use the P and S wave travel-time curves (appendix, page 11) to find the distance from
each station to the earthquake epicenter. Do this by finding the unique epicenter
distance where the difference in the P and S wave travel times is exactly equal to the
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difference you calculated from the seismogram. Record that distance in the last column
of the data table.
4. On the “Indonesian Earthquake” map (appendix page 4), use the map scale and your
compass to draw circles around each station of a radius equal to the epicenter distances
that you just determined using the travel time curves.
5. The intersection of the 3 circles marks the epicenter of the earthquake. Label it
“Epicenter” on your map.
6. Why do you need at least three seismic stations to find the epicenter?
DATA TABLE 1:
Seismograph
Time Difference (S – P )
Epicenter Distance
Station
min:sec
(km)
IC.LSA
KMBO
GUMO
Questions/Activities: (Use whatever resources you need)
1. Which seismic station is located farthest from the epicenter? _________________
a. How could you have determined which was farthest by simply looking the seismograms?
2. The quake occurred on a tectonic plate boundary. Refer to the tectonic map (p. 5) to
answer the following questions:
a. Between what two tectonic plates did this ‘quake occur? ______________________ and
_____________________________
b. State how the plates are moving relative to each other in the area of the ‘quake?
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c. Classify and describe the plate boundary.
3. The focus of the quake occurred about 150 km (90 mi) northeast of the surface
expression of the plate boundary, (see fig. 1) and at a depth of about 30 km (18 mi).
Explain how this information helps you to determine which plate is being subducted.
b. On the grid (fig. 2), draw a properly scaled profile (geologic cross-section) of the plate boundary
region along line A-B. Draw the plate boundary, and mark the location of the earthquake focus
with a small star.
Figure 1. Profile line location
across plate boundary along the
Sumatra Coast.
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Figure 2. Locate the plate boundary and focus, and a tectonic cross-section
illustrating the plate boundary configuration.
Part Ib: Using more data to find the epicenter
Now that you have an idea where the earthquake originated you will use three closer seismograph
stations to more accurately pinpoint the location of the epicenter.
1. Label the following Countries on MAP 2 (appendix p. 7): (Use whatever resources you need)
India, Sri Lanka, Somalia, Sumatra (Indonesia), Myanmar, and Thailand
2. Follow the same procedures used in Part la to find the epicenter on Map 2.
*You will need to use the seismograms from stations: PALK, DGAR, and COCO (appendix p. 6).
DATA TABLE 2:
Seismograph Station
Time Difference (S –P)
Epicenter Distance
min:sec
(km)
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PALK
DGAR
COCO
Part Ic: Predicting the Arrival of Tsunami Waves
A sudden displacement of the ocean floor (fig. 3)
over the epicenter created a large tsunami. The
speed [V=(gh)1/2] at which a tsunami moves through
the ocean is dependant largely on the depth of the
ocean. The tsunami generated by this quake moved
at an average speed of about 600 kilometers per
hour. Though tsunamis travel fast, their wave heights
are at most only a meter or two, and the wavelengths
are over 100 km long (!), so they are often unnoticed
Figure 3. Mechanism of Sumatra
as they pass beneath ships at sea.
tsunami formation.
As a tsunami approaches shallow water near the coast it will slow down. The wavelength from crest
to crest decreases and the wave height increase. The Sumatra tsunami produced wave heights
ranging from 20-to-30 meters (60-100 ft).
1. Assume the tsunami generated by the Great Sumatra Quake traveled 600 km/hr in the open
ocean. On Map 2, draw and label circles around the epicenter showing the distance the tsunami
had traveled in 1 hour, 2 hours, 3 hours, and 4 hours.
(You should have four labeled circles surrounding your epicenter representing the position of the
leading edge of the tsunami as it traveled through the ocean after the earthquake occurred)
Part Id Speed of Seismic Waves
2. On the data table below, list the 6 seismic stations you’ve used in order of increasing distance
from the epicenter.
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3. Fill in the P-wave travel time (convert minutes and seconds to seconds) and distance data.
Calculate and record the average speed of the P-waves arriving at each station. The P-wave
travel time is calculated by dividing the distance divided to each station by P-wave velocity
obtained from the travel-time curve (p. 4 appendix).
DATA TABLE 3:
Station
Epicenter
P-Wave Travel
Average Speed of
Distance (km)
Time (s)
Recorded P-Waves (km/s)
1. At first glimpse, the results of your calculations may be surprising. How can the differences in
average speed be explained? Read in your text about the way the seismic waves travel through
the earth. Explain why waves arriving at more distant places travel at a higher average speed.
2. Listed below is the number of people killed by the tsunami in the following countries:

India 18,045

Sri Lanka 35,322

Thailand 8,212

Myanmar 400-600

Somalia 289

Indonesia: 167,736
a. Why do you suppose Indonesia’s casualty rate was so high?
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b. Had a warning system been in effect, would there have been time to warn the residents
of Aceh at the northern tip of Sumatra?
c.
How might the residents of Aceh have known that there was some danger of a tsunami
before it actually arrived?
3. Using MAP 2 and the tsunami circles that you drew, estimate the amount of time that these
countries had before the tsunami crashed onto their shores.

India ______________________________

Sri Lanka __________________________

Thailand ___________________________

Indonesia __________________________

Myanmar __________________________

Somalia ___________________________
PART II: Variations in earthquake intensity and loss of life
The following factors will contribute to the death toll resulting from an earthquake.
The following factors will contribute to the death toll resulting from an earthquake.
1. Magnitude – The energy released
2. Size of fault and distribution of energy
3, Duration of shaking
4.. Distance from epicenter and Depth of focus - proximity
5.. Population density
6.. Tsunami - flooding
7.. Ground Conditions (geology and geometry): Along with magnitude and depth of focus these
affect intensity; the amount of shaking.
8.. Infrastructure: building construction, land use, sewerage and water distribution (Typically
relate to the economic standing and political stability of a country.)
Part IIa: 1989 Loma Prieta Earthquake, California
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On October 17, 1989, just prior to game I of the World Series between the San Francisco Giants
and the Oakland Athletics, a magnitude 7.1 Earthquake struck northern California (This
earthquake is known as the Loma Prieta Earthquake). The earthquake occurred in the
mountains west of San Jose California on the San Andreas Fault. As a result of the earthquake,
41 people were killed when a double-decked section of the Nimitz Freeway in Oakland
collapsed, crushing people in the cars on the lower deck. A geologic map showing the various
rock types present in the Oakland area and the location of the collapsed portion of the freeway
(shown as a short dashed line) is depicted below in Figure 1. Between the thick bars is the area
where the freeway was double-decked.
On the map, the areas marked as Holocene mud are areas that were formerly occupied by San
Francisco Bay, but have been filled with loose sediment in the last 100 years, the mud contains
lots of water in the pore spaces between the grains. The areas marked Quaternary Alluvium are
areas underlain by unconsolidated sediment deposited by streams over the last 2 million years.
Hard sedimentary, igneous, and metamorphic rocks with a thin cover of soil underlie the area
marked Franciscan Formation.
Several days after the 7.1-magnitude earthquake, small aftershocks shook the area. All had
epicenters near the main shock of October 17. Seismologists placed several portable
seismometers at stations S1, S3, and S4 (as shown on the map) and recorded these
aftershocks. Seismographic recordings for one of these aftershocks, a 4.1-magnitude
earthquake, for each of the three stations are shown in Figure 1b, below. Note that the epicenter
of the aftershock was far enough away that all of the recording stations could be considered to
be about the same distance from the earthquake.
1. What observations can you make about the seismic response (degree of shaking) on
the three types of materials underlying the area?
2. What conditions were likely responsible for the double-decked Nimitz Freeway to
collapse where it did?
3. Considering that New Orleans is built on water-saturated river muds, how do you think
New Orleans would fare if there were a major earthquake nearby?
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Figure 1a (left) Map of earth materials underlying Oakland, CA. Figure 1b (right) seismograms recording
aftershocks of the 1989 Loma Prieta Earthquake.
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Part IIb: 2010 Earthquakes in Haiti and Chili
On January 12, 2010 an earthquake of magnitude 7 struck Haiti killing 230,000. Soon after, on February
27 a magnitude 8.8 earthquake struck Chili. To date the death toll is under 1000 people.
1. Locate Haiti and Chili on the tectonic map (p. 5 appendix). Explain why these two countries are
prone to earthquakes. State the plates involved, boundary types and relative motion along each.
Haiti:
Chili:
2. Considering that for each point in the magnitude scale the about of energy release from an
earthquake increases approximately 30X, approximately how much larger was the Chilean
earthquake? ___________________
3.
Research these quakes on the Internet. Discuss the factors the conditions that contributed to
the loss of life in each area, and explain why the death toll in Haiti was so much greater than that
in Chili.
4. List the reasons and explain why the loss of life (<100 people) in the 7.1-magnitude Loma Prieta
Earthquake was far less than that (230,000 people) in the 7.0 magnitude earthquake in Haiti.
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Credits
This lab was added to and modified from the Sumatra and Tsunami lab (referenced below) written by
Dave Robison (drobison@wilson.wnyric.org); Steve Kluge (skluge@bedford.k12.ny.us)
Dave Robison and Steve Kluge (2004) the Sumatra Earthquake and Tsunami, December 26, 2004,
URL: http://www.regentsearthscience.com/tsunami.htm
The section relating to the Loma Prieta Earthquake is from a seismology exercise written by Stephen
Nelson, Tulane University. URL: http://www.tulane.edu/~sanelson/geol204/eqexercises.htm
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