GEOSCI 106 Lab 4_ Earthquakes- MARCH
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Dec 6, 2023
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GEOSCI/ENVIR ST 106: Environmental Geology
Lab 4: Earthquake detection and seismic hazards
Assignment Overview:
Plate tectonics can have major impacts on human life in the form of earthquakes.
Geologists collect a variety of data about historical earthquakes to understand future risks. In this lab, you
will use data from maps and seismic stations, along with simple computations, to explore these hazards.
You will be acting in the role of a seismologist, using data from seismic sensors to determine the location
and time of an earthquake that has already taken place. You will then use this information to determine
which nearby sites can be effectively warned in time, and how much advance warning you can give them
before damaging surface waves hit them.
Instructions:
1.
Download the “Pacific NW seismicity.kmz” dataset associated with this lab in Canvas.
2.
If you do not have Google Earth Pro, download and install it. It is freely available here:
https://www.google.com/earth/versions/
. Note that you will need Google Earth Pro, not the
version accessible in a web browser. The web browser version does not have all the
functionalities that you will need to complete this lab.
3.
Fill out each red highlighted field (_________) according to each question’s instructions.
Submission:
To submit the assignment on Canvas, use the following steps:
1.
In Google Docs, generate a PDF: File → Download as → PDF Document
2.
In Google Docs, use Share → Get Shareable Link, and copy the link address
3.
In Canvas, upload your PDF to the assignment.
4.
In Canvas, paste the link address to your Google Doc in the assignment comments.
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Background
The energy released by an earthquake is transmitted through the Earth as vibrations in several different
ways. Body waves – much like sound waves through air – travel through the Earth itself, and are
relatively fast. These include pressure (or primary) P-waves and shear (or secondary) S-waves. Surface
waves – much like ripples moving across a pond – travel primarily along the surface of the Earth, and are
somewhat slower. Surface waves are especially important because they are the waves that are almost
entirely responsible for damage to structures. The propagation and speed of these waves is shown below
in Figure 1.
Figure 1:
Cartoon of energy movement away from an earthquake and toward a seismic observation
station, including approximate propagation velocities for P-waves, S-waves, and surface waves.
An example of a record that might be recorded by a seismometer is shown in Figure 2, along with the
different wave arrivals highlighted on the record.
Figure 2:
Example of an seismograph record (seismogram) with arrows showing the arrivals of the P-
and S-waves. Notice that the surface waves arrive after both the P- and S-waves. This implies that the P-
and S-waves provide a brief warning before the damaging surface waves arrive.
Seismologists and, increasingly, computers, can detect and differentiate these different types of vibrations
to determine the arrival time of each wave, i.e., the time at which they arrive at the seismometer. As
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described in the textbook, the difference in the arrival times of seismic waves of different speeds (e.g., a
P-wave and an S-wave) tells us how far away the earthquake was. Just like anything moving at a set
speed, its arrival time at a location is determined by when it started, how fast it was moving, and how far
it had to go. Said more mathematically:
Arrival time = start time + distance / velocity
For an earthquake, we are able to measure when each wave arrives (arrival time) and we can roughly
know its velocity, based on prior knowledge of how different types of waves travel through different
types of rock. We don’t know in advance how far away the earthquake was from the seismometer that’s
measuring the waves (distance), or the time the earthquake started. In this lab, you will use information
from multiple seismometers to determine where and when an earthquake happened.
Based on the equation above and some algebra (see the appendix at the end of this lab), we can use the
following two equations to estimate an earthquake’s distance from a seismometer and the earthquake’s
start time.
Distance = (S arrival time - P arrival time) / (1 / S velocity - 1 / P velocity)
Start time = S arrival time - distance / S velocity
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Seismic hazards: The Pacific Northwest subduction zone
You have been hired by Amazon to assess the danger of a typical earthquake to their operational facilities
in the Pacific Northwest. Amazon has chosen a particular historical seismic event as an example case to
assess what its impact on operations would be. The data from this historical event is provided in terms of
the arrival times of P- and S-waves at seismograph stations S1, S2, and S3 in the table below. The
locations of these seismograph stations are included in your Google Earth .kmz file.
1. Seismic wave travel times.
(a) To start your analysis, fill in the table below by calculating the
differences in arrival times between the P- and S-waves. (1 point)
Station
P-wave arrival time
HH:MM:SS.SSS
S-wave arrival time
HH:MM:SS.SSS
Arrival time difference (s)
S1
20:12:35.761
20:12:59.938
0:0:24.177_________
S2
20:12:17.034
20:12:27.750
0:0:10.716_________
S3
20:12:29.761
20:12:49.625
0:0:19.864_________
(b) Using the seismic wave velocities given in Figure 1, compute the reciprocals of the P-wave and
S-wave velocities and their difference. This will be useful for calculating each seismometer’s distance
from the earthquake. (1 point)
1 / V
s
(s / km)
1 / V
p
(s / km)
1/V
s
- 1/V
P
(s / km)
_1/3.2________
_1/5.5________
_0.13________
2. Estimating EQ location and timing.
(a) Using the values you calculated in the previous question and
the seismic wave velocities given in Figure 1, determine how far away from each seismometer was from
the earthquake, and use that distance to estimate when the earthquake occurred. First estimate the
earthquake’s distance from each station, and then use that information to estimate when the earthquake
started. (Note: To simplify your estimates of the earthquake start times, report all answers in seconds after
20:12. Report these to three figures after the decimal point: e.g., 7.456 seconds.) (3 points)
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Station #
Distance to EQ
(km)
Estimated EQ start time
(seconds after 20:12)
S1
186_________
1.813__________
S2
82.4_________
2.000__________
S3
152.8_________
1.875__________
(b) We know that this was one earthquake, so it only had one start time. Do you see differences in the
estimated earthquake start times among the stations? If so, what could be causing these differences? (2
points)
It could be a difference in equipment quality _________
3. Mapping the event.
(a) In Google Earth, draw a circle centered on seismic station S1 that corresponds
to the distance from that station to the earthquake epicenter. (This can be accomplished by going to Tools
> Ruler > Circle, generating a circle with a radius equal to the distance you calculated in the previous
question, and clicking “Save”.) Each point on this circle represents a possible location where the
earthquake occurred. Now draw a second circle centered on S2 that corresponds to the distance from S2
to the earthquake epicenter. Using the two circles around S1 and S2, can you say where the earthquake
occurred? Why or why not? (2 points)
Yes because the two circles have two places where the lines cross and that could be a possible earthquake
location._________
(b) In Google Earth, draw a third circle centered on S3 that corresponds to the distance from S3 to the
earthquake epicenter. Can you now say where the epicenter was? Why or why not? (2 points)
Yes because all three circles cross at one point in the ocean._________
(c) Based on these results, add a placemark to Google Earth showing your best estimate of the location of
the earthquake. (Use the Add Placemark tool for this.) Label the placemark with your best estimate of
the start time, e.g., “EQ at xxxx seconds after 20:12”. Report the latitude and longitude of the epicenter
here in decimal degrees to four decimal places (e.g., as 43.0707°N, 89.4062°W). (2 points)
45°40’58.78’’N, 124°29’40.27’’_________
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(d) Save an image of the study area that includes all elements that went into answering these questions:
stations S1-S3, the Amazon sites, the circles you drew, and the placemark with your estimate of the
epicenter location. (Do this in Google Earth by zooming to the appropriate scale, and then applying File
> Save > Save Image.) Paste this image here. (1 point)
_
________
4. Predicting surface waves and providing warnings.
(a) Using the surface wave velocity given in
Figure 1, estimate when surface waves from the earthquake will arrive at each of Amazon’s holdings. To
do this, measure the distances between these holdings and the earthquake (in Google Earth, use Tools >
Ruler > Line). Assume that a warning can be sent to Amazon’s holdings immediately after the
earthquake location was calculated, and assume that the earthquake location can be calculated
immediately after both the P- and S-waves have arrived at all three stations S1, S2, and S3. (In other
words, the earliest a warning can be sent out is the moment the S-wave arrived at the seismic station
farthest away from the earthquake.) How much warning can you give each of Amazon’s holdings before
the damaging surface waves arrive? (6 points)
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Time of last S-wave
arrival at S1-S3:
(seconds after 20:12)
24.177_________
Location
Distance to EQ
(km)
Expected surface wave
arrival time
(seconds after 20:12)
Warning time
(seconds)
Amazon HQ
276.65________
_
98.8_________
74.623_________
Amazon Portland
Distribution
Center
149.6_________
53.42_________
29.243_________
Columbia River
Shipment Center
101.5_________
36.25_________
12.073_________
(b) For a similar seismic event, if you wanted to provide better warnings to Amazon regarding its
facilities, what actions might you take? Write a brief suggestion to Amazon of 2-3 sentences suggesting a
plan of action to protect against a similar seismic event. (2 points)
If I were in Amazon’s shoes I would buy the best graphs that I could because they definitely have the
money to do that. I would also have people watching for earthquakes on those graphs at all
times._________
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Appendix: Using seismic data to compute EQ distance and time
The following provides a brief description of how the equations presented earlier were developed. For
each of the seismic waves, we can write the following equations:
P arrival time =
EQ start time
+
EQ distance
/ P velocity
(1)
S arrival time =
EQ start time
+
EQ distance
/ S velocity
(2)
Here, note that our unknowns are the (italicized) EQ start time and the EQ distance. Thus, this is a set of 2
equations with 2 unknowns, so we should be able to determine both values.
Since we know the S wave will arrive later and want to deal with positive numbers,
Subtracting (1) from (2) gives:
(S arrival time – P arrival time) =
EQ distance
x (1/S velocity – 1/P velocity)
And solving for the EQ distance gives the first equation we used in lab:
EQ distance
= Arrival time difference / (1/S velocity – 1/P velocity)
(3)
Rearranging equation (2), we arrive at the second equation used in lab:
EQ start time
= S arrival time –
EQ distance
/ S velocity (4)
Since EQ distance has already been estimated in equation 3, we can now solve for the EQ start time.
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