Lab # 12 Earthquakes (1)
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GEOS 130 | Lab 12 Earthquakes
General Geology
Name______Kaitlyn Stallbaumer__________________________________________
Goal: Explain the phenomenon known as an earthquake, describe how geologists quantify the energy
released and the destruction produced by an earthquake, and interpret seismic data.
Directions: Use Chapter 11 in your textbook to help develop your responses the following questions.
1.
Listen to and analyze Robert Pate’s recording of the 1964 Alaska Earthquake (9.2 magnitude). This is
the largest known earthquake to occur in North America. Refer to your textbook to help develop your
responses .The recording is available in the Canvas Lab 12 Assignment.
a.
As you listen to the recording, take notes describing background noises that could be related
to ground shaking as well as noting Mr. Pate’s description of the experience.
Pate, began recording his thoughts and reactions within 5 seconds of feeling the earthquake,
indicating his quick response to the event. About 1 minute into the recording Pate mentioned
the earthquake had not shown signs of stopping, indicating a prolonged shaking event. Pate
then mentioned that the shaking was so intense that he couldn’t walk and had to crawl to put
the television set on the floor. This suggests that the ground shaking was severe.
Approximately 1 minute and 10 seconds into the recording, Pate expressed concern about
the continued shaking and wondered if he should go outside. This reflects his apprehension
about the ongoing seismic activity. One and one-half minutes after the recording began, Pate
noted that the noise level substantially decreased. This may indicate that the initial intense
shaking had started to subside. Pate seemed to sense that the worst was over at this point,
suggesting that the seismic activity had diminished. This reflects a common emotional
response during earthquakes when individuals start to feel relief as the shaking subsides.
Despite the intense shaking, Pate’s dwelling remained undamaged. Only three or four dished
fell from kitchen cabinets, and the doors had swung open. This indicates that the building
may have been constructed to withstand seismic activity to some extent.
b.
Compare his account to the
Modified Mercalli Intensity Scale
.
https://www.usgs.gov/programs/earthquake-hazards/modified-mercalli-intensity-scale
. What
intensity would you report based on the recording?
Based on Robert Pate’s account of his experience during the 1964 Alaska Earthquake, we
can make a comparison to the Modified Mercalli Intensity Scale. We have the immediate
Reaction, duration of shaking, difficulty walking, consideration of going outside, decrease in
the noise level, and minimal damage. Based on these observations, it is reasonable to
estimate that the intensity of the earthquake experienced by Pate would likely fall within the
range of IV to VII on the Modified Mercalli Intensity Scale. This would indicate that the
earthquake was felt over a large area, may have caused some damage to weak or
unreinforced structures, and would have been a significant event, but not one that resulted in
widespread destruction.
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GEOS 130 | Lab 12 Earthquakes
General Geology
c.
Explain why you choose that intensity.
I chose the range of IV to VII on the Modified Mercalli Intensity Scale for Pete’s experience
during the 1964 Alaska Earthquake based on several key factors described in his recording. It
includes, quick recording and an immediate response, prolonged duration, difficulty walking
and crawling, consideration of going outside, decrease in noise level and lastly, minimal
damage. Considering these factors, the overall assessment of the intensity experienced by
Pate during the earthquake would likely fall within the range of IV to VII on the Modified
Mercalli Intensity Scale. This indicates a significant earthquake event with noticeable shaking
potential damage to weaker structures, and a range of human reactions that align with these
intensity levels.
2.
Describe how an earthquake generates seismic waves by completing the sentences below with
selections from the word bank.
energy
ground shaking
hypocenter
propagate
slips
seismic waves
When a fault ____slips______________, _________energy______________ is released in the
form of rapidly moving _______seismic waves__________________that
__________propagate_______________from the _______hypocenter________________. We
feel the energy in the form of __________ground shaking________________.
3.
Go to the USGS Earthquakes page
http://earthquake.usgs.gov/earthquakes
. Select “Interactive Map”
beneath “Latest Earthquakes.” Zoom out until you see the globe. Click on a dot to open a pop-up
window and review a summary of the earthquake. Then explore several more earthquakes. Look at
some that took place in the continents and some that occurred beneath the oceans. Did you notice
that while some earthquakes report intensity, not all of them do? Consider the distinctions between
the methods used to measure magnitude and report intensity.
a.
Explain why magnitude is always recorded while intensity is only recorded for some earthquakes.
Magnitude and intensity serve different purposes in earthquake monitoring and risk assessment.
Magnitude is a standardized measure of an earthquake’s size and energy release, recorded for
all earthquakes globally, while intensity is a subjective measure of the earthquake’s effects at
specific locations, recorded only for significant events or in areas where it is deemed necessary to
assess the impact.
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GEOS 130 | Lab 12 Earthquakes
General Geology
b.
Notice the continents are shown in white. The oceans are gray. The red lines represent tectonic
plate boundaries. Where do most earthquakes occur in relation to these features?
Most earthquakes are concentrated along the boundaries of Earth’s tectonic plates, both beneath
the oceans and on continents. These plate boundaries are often associated with the movement
and interaction of tectonic plates, resulting in the stress and release of energy that generates
earthquakes.
c.
Explain why the majority of the world’s earthquakes occur there,
The majority of the world’s earthquakes occur along tectonic plate boundaries because these
boundaries are the primary zones of interaction between Earth’s lithospheric plates. The movements
and interactions of these plates, weather converging, diverging, or sliding past each other, generate
the stress and energy release that result in earthquakes. This geological activity is a fundamental
consequence of plate tectonics, a driving force behind the dynamic nature of Earth’s surface.
d.
Look for earthquakes along the tectonic plate boundary south of Alaska. This region is the
megathrust fault that generated the 1964 Alaska Earthquake (9.2 magnitude). Click on the largest
Earthquake near Alaska’s megathrust fault.
What was the magnitude?
The 1965 Rat Island Earthquake’s magnitude was an 8.7
What was the intensity? Record them all if more than one intensity is reported.
The intensity was VI which is strong.
Explain why an earthquake can have multiple intensities but only one magnitude.
An earthquake can have multiple intensities but only one magnitude because magnitude
and intensity measure different aspects of seismic events and serve distance purposes in
earthquake assessment.
Refer back to the Modified Mercalli Intensity Scale. Describe the type of activity that occurs
during an earthquake of that intensity/those intensities.
I.
Not felt: The earthquake is not felt by people, and there is no
perceptible activity.
3
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GEOS 130 | Lab 12 Earthquakes
General Geology
II.
Weak: Weak or mild shaking is felt by a few people near the
epicenter. It is generally not felt by others.
III.
Weak to light: Weak shaking is felt by some people indoors. It
may be compared to the sensation of a heavy truck passing by.
IV.
Light: Light shaking is felt by many people indoors, similar to the
sensation of a heavy truck striking a building.
V.
Light to moderate: Moderate shaking is felt by most people
indoors, and some may experience difficulty walking. Unsecured
objects may move or be shifted.
VI.
Moderate: Moderate to strong shaking is felt by everyone
indoors, with some people feeling panic. Hanging objects swing
noticeably, and there may be minor damage to weak or
unreinforced structures.
VII.
Strong: Strong shaking is felt by all, causing people to move
unsteadily. Damage to weak or unreinforced buildings is likely,
and some chimneys may fall.
VIII.
Severe: Severe shaking occurs, causing damage to many
buildings, including those of good design. Most chimneys fall,
and some walls may crack or collapse.
IX.
Violent: Violent shaking causes damage to well-designed
buildings, and many poorly constructed structures are severely
damaged or collapse. People may be thrown to the ground.
X.
Very violets: Very violent shaking results in the widespread
damage of all types of buildings. Many structures are destroyed,
and there may be ground cracks and landslides.
XI.
Extreme: Extreme shaking causes virtually all buildings to be
heavily damaged or destroyed. Landslides and ground ruptures
are common, and bridges and dams may fail.
XII.
Total destruction: Total destruction occurs, with the ground
surface being dramatically altered. This level is exceptionally
rare.
4.
Analyze seismogram data from an earthquake that occurred in Alaska on October 11, 2021. The
magnitude of this earthquake was 6.9 and it occurred at a depth of 69.1 km. When analyzing the
data, notice the vertical green lines on the seismograms indicate the P wave and the S wave arrival
times. The time scale on this seismograms are in minutes and seconds. You can zoom in to see the
data more closely. The arrival times are also stated in each text description.
Washington State Seismogram
: This seismogram was recorded at a seismograph station in the state of
Washington, about 2,777 km away from the epicenter in Alaska. The P wave arrived about 16 minutes
after the earthquake. The S wave arrived about 20 minutes after the earthquake.
a.
What was difference in the arrival times at this location?
T
S
-T
P
=
4
GEOS 130 | Lab 12 Earthquakes
General Geology
The P wave arrived about 16 minutes after the earthquake.
The S wave arrived about 20 minutes after the earthquake.
TS-TP= 20 minutes – 16 minutes – 4 minutes
The difference in arrival times (TS – TP) at the Washington State seismograph station is 4
minutes. This means that the S wave arrived approximately 4 minutes after the P wave at this
location.
Kansas Seismogram
: This seismogram was recorded at a seismograph station at Kansas State about
4,697 km away. The P wave that originated in Alaska took about 18 minutes to arrive at K-State. The S
wave that originated in Alaska took about 24 minutes to arrive.
b.
What was difference in the arrival times at this location?
T
S
-T
P
=
The P wave took about 18 minutes to arrive after the earthquake.
The S wave took about 24 minutes to arrive after the earthquake.
TS – TP = 24 minutes – 18 minutes = 6 minutes
The difference in arrival times at the Kansas State seismograph station is 6 minutes. This
means that the S wave arrived approximately 6 minutes after the P wave at this location.
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GEOS 130 | Lab 12 Earthquakes
General Geology
Massachusetts Seismogram
: This seismogram was recorded at a seismograph station in
Massachusetts, 5,963 km away. The P wave that originated in Alaska took about 20 minutes to arrive in
Massachusetts. The S wave that originated in Alaska took about 27 minutes to arrive in Massachusetts.
c.
What was difference in the arrival times at this location?
T
S
-T
P
=
The P wave took about 20 minutes to arrive after the earthquake.
The S wave took about 27 minutes to arrive after the earthquake.
TS – TP – 27 minutes – 20 minutes = 7 minutes
The difference in arrival times at the Massachusetts seismograph station is 7 minutes. This
means that the S wave arrived approximately 7 minutes after the P wave at this location.
a.
Describe how the P-S time interval, the difference in seismic wave arrival times (
T
S
-T
P
),
changed
with distance from the epicenter.
As you move away from the earthquake’s epicenter, the P-S time interval, or the difference in
seismic wave arrival times, tends to increase. This is primarily due to the difference in the speeds
at which P-waves and S-waves propagate through the Earth. P-waves travel faster and arrive
first, while S-waves travel more slowly and arrive later. The time interval between their arrivals is
a valuable parameter for seismologists to determine the epicentral distance and study the Earth’s
interior structure.
b.
Explain how geologists use P-S time interval to determine the location of earthquake epicenters.
Geologists use the P-S time interval (the difference in seismic wave arrival times, to determine
the location of earthquake epicenters through a method called seismic triangulation. This
technique relies on the fact that seismic waves travel at different speeds through the Earth and
arrive at different times at various seismograph stations.
6
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GEOS 130 | Lab 12 Earthquakes
General Geology
5.
Analyze real earthquake data to determine the epicenter using a method called triangulation. Go to
https://www.sciencecourseware.org/VirtualEarthquake/
to get started. Once you are at the website,
click “Execute Virtual Earthquakes.” The button looks like this:
Select an earthquake
: Review the text on earthquakes, then scroll to the bottom to choose one of
the four earthquake regions. Then click on the button that says, “Submit Choice.”
How to measure S-P time intervals
:
Review the instructions to learn how to accurately read the S-P
interval, the time that passed between the arrival of the S and P waves. It is important you understand
how to do this step accurately, so read carefully. Then click the “View Seismograms” button.
Read the S-P time intervals
: Carefully estimate the S-P time interval for each of the seismic station
recordings. Enter your responses in the data boxes. Carefully read each seismogram or you may
need to repeat this step. Then click the “Convert S-P interval” button.
Next, follow the directions to read the graph and determine the distance from each seismic recording
station to the earthquake's epicenter based on the known travel times of S and P waves. Enter your
responses in the data boxes.
a.
Why is seismic data from three stations needed to triangulate the epicenter?
Seismic data from three stations is necessary for triangulating the epicenter because it provides
enough constraints to uniquely determine the earthquake’s location in two-dimensional space. It
reduces ambiguity and ensures an accurate determination of the epicenter’s latitude and
longitude.
b.
Did you successfully determine the epicenter location? Take a screenshot and upload it to
Canvas as part of your lab submission or insert your screenshot at the end of this document
before you submit it. Congratulations. You are done. If you were unsuccessful, proceed to c.
c.
Were you unsuccessful in determining the epicenter?
Try again
OR click “View True Epicenter.” Take a screenshot and upload it to Canvas or insert your
screenshot in this document, AND analyze your data versus the actual data. Where did your
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GEOS 130 | Lab 12 Earthquakes
General Geology
triangulation go wrong?
8