R4_Earthquakes
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EESC 1000, Fall ‘23
Recitation #4, Earthquakes
Name:
Tiffany Fu
Names of collaborators:
You will determine the location of an earthquake epicenter using seismograms from Carrier,
Oklahoma, Smith Ranch in Marlow, Oklahoma, and Bolivar, Missouri. These are actual
seismograms that you will be reading, from an actual earthquake event. For each seismogram,
three different readouts are given, because the seismograph measured movement along three
different axes simultaneously. You can use any of the three readouts for each station in your
calculations, because each of the readouts will have the same arrival time for the different
components of each wave.
First, determine the time when the P and S waves first arrived for each station. To identify the P
and S waves, refer to the illustrations in the overview section of this chapter. Look for a pattern
change as the amplitude of the lines gets bigger; this indicates the arrival of each of the waves.
Mark both the arrival of the P-wave and S-wave on the seismogram, then using the time scale in
seconds, determine the time difference between the P and S wave first arrivals. Write these
times in Table 3.1 below for each of the three seismograms. (12 pts).
Table 3.1
Station
P-wave arrival
time (s)
S-wave arrival
time (s)
Difference between P-
and S- wave arrival
times
Distance to
epicenter from
station (km)
Carrier, OK
19
43
24
240km
Smith Ranch,
Marlow, OK
15.5
21.5
6
60km
Bolivar, MO
25
83
58
580km
EESC 1000, Fall ‘23
Recitation #4, Earthquakes
Figure 3-E1 |
Seismogram readings from Carrier, OK.
Source: USGS (2015) public domain, source webpage: https://earthquake.usgs.gov/
EESC 1000, Fall ‘23
Recitation #4, Earthquakes
Figure 3-E2 |
Seismogram readings from Smith Ranch, Marlow, Oklahoma.
Source: USGS (2015) public domain, source webpage: https://earthquake.usgs.gov/
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EESC 1000, Fall ‘23
Recitation #4, Earthquakes
Figure 3-E3 |
Seismogram readings from Bolivar, Missouri.
Source: USGS (2015) public domain, source webpage: https://earthquake.usgs.gov/
The difference between the P and S wave first arrivals will be used to determine the distances to
the epicenter from each station in Figure 3-E4. Use Figure 3.11 below to determine the distance
of the epicenter from each station. Make sure that you use the curve for the difference between
the S and P wave first arrival times (S-P). Find the difference between the S and P first arrival
times in seconds on the y-axis, draw a line over to the S-P curve at the same time, then draw a
line down to the x-axis to determine the distance. Add the values to Table 3.1 above.
EESC 1000, Fall ‘23
Recitation #4, Earthquakes
Figure 3.11
| A travel-time graph that includes the arrival of P-waves and S-waves. Note that
these curves plot distance versus time, and are calculated based on the fact that the Earth is a
sphere. Curves vary with the depth of earthquake because waves behave differently (i.e. their
velocities change) with depth and change in material. This particular curve is used for shallow
earthquakes (<20 km deep) with stations within 800 km. The S-P curve refers to the difference in
time between the arrival of the P-wave and S-wave. If you noted on your seismogram that the
P-wave arrived at 10 seconds, and the S-wave arrived at 30 seconds, the difference between
arrival times would be 20 seconds. You would read the 20 seconds off the y-axis above to the S-P
line, then drop down to determine the distance to the epicenter. In this case, it would be
approximately 200 kilometers. Source: Randa Harris (2015) CC BY-SA 3.0
(https://ung.edu/university-press/books/laboratory-manual-for-introductory-geology.php)
Figure 3-E4 is a map showing the three stations on it. Next, you will draw circles on this map to
represent the distance of the earthquake epicenter from each station. Use a drafting compass
to draw the circles. This map includes a legend in kilometers. For each station, note the distance
to the epicenter. Measure the scale on the map in Figure 3-E4 in centimeters and convert your
distances in kilometers to centimeters (e.g., if the map’s scale of 100 km = 2.1 cm on your ruler,
and you had a measured distance from one station of 400 km, that would equal 8.4 cm on your
ruler). For this fictional example, you would use a drafting compass to make a circle around the
station that is 8.4 cm in radius (from the center to the edge). Create a circle for each of the
EESC 1000, Fall ‘23
Recitation #4, Earthquakes
three stations, using their different distances to the epicenter. They should overlap (or nearly
overlap) in one location. The location where they overlap is the approximate epicenter of the
earthquake. (7 pts total: 2 points for each circle and 1 pt for location of epicenter)
Tip
: To plot the distances, you can use Microsoft PowerPoint’s (PPT) “insert shape” features. For
example, you can copy the below image into PPT, insert a line identical in length to the scale bar
on this image and see how long (in cm or inches) the line is. You can then use this length to
insert a circle shape with length and width or the same length. If for example you measure the
600-kilometer scale bar to be 2 inches long on your monitor, you could get a 300-km long line
such as:
2in * 300km / 600km = 1in.
You can then create a 1in long line or circle. Pay attention to
the distinction of radius and diameter when plotting!
Figure 3-E4 |
Map depicting three seismograph stations (Carrier, Smith Ranch, and Bolivar)
located in Oklahoma and Missouri, USA. Source: Joyce McBeth (2018) CC BY 4.0, after Randa
Harris (2015)
The plot: where the blue star depicts the location of the earthquake
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EESC 1000, Fall ‘23
Recitation #4, Earthquakes