LAB 8 EARTHOUAKE LOCATION WORKSHEET

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Geology

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Dec 6, 2023

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—' Dale:_,{flé/vzj LAB 8: EARTHQUAKE LOCATION WORKSHEET GEOLOGY 1030 S Name: < 7 - L. Adevice that records seismic waves moving through rock is called 2 aSMma 1ol 2. The printed record of a seismic eventis a_ N\ SYY)(OCU@R M 3. The location on Earth’s surface directly above the focus is calléd the \ ! (% 4. The first wave to arrive at a selsmograph station is always the wave. 5. The second wave to arrive at a seismograph station is always the wave. On June 14™ of 2015 an earthquake occurred in the map region shown in Figure 1 below. Seismograph stations located in the cities of Carrier, OK, Marlow, OK, and Bolivar MO (blue dots on the map) recorded ground movements caused by this earthquake. Figure 1: Map depicting three seismograph stations (Carrier, Marlow, and Bolivar) located in Oklahoma and Missouri, USA. Source: Joyce McBeth (2018) CC BY 4.0, after Randa Harris (2015). ~ « Bolivar / \\ Kansas Oklahoma | M5V Catrier ¢ Arkansas ) ahorna City )i t Mprlow | o & e miles 400 A km 600 L10 um Figures 2 through 4 below show the actual seismograms recorded at each of these three seismograph stations following this earthquake. Each figure has three different seismograms because each station recorded ground movement along three different axes simultaneously.
Figure 2: Seismogram readings from Carrier, Oklahoma. Source: USGS (2015) public domain, source webpage: hnps://eanhquake,usgs gov/ ols egin Time: 204 V4180702 GMT - " pit w Time (s) 9 600 9 12,00 15 . ¥ ORI oS Al el o/ WSl WAt e WARR B s e W Tre0s & ] = e~ po—| Y vl E1E03 r . O 1E-03+ < PP i 9 U103 = 0E00— [} 1 Ly 1 < { | I I T T =y 1 0 0 600 900 1200 2400 X Time (s) Figure 3: Seismogram readings from Marlow, Oklahoma. Source: USGS (2015) public domain, source webpage: https://earthquake.usgs.gov/ Seismogram Begin Time: 2015-06-14 18:17:21 GMT Smith Ranch, Marlow, OK, USA Station L Latitade 34.60 N, Longitude 97.83 W Time (s) 00 500 1000, 1500 2000 2500 3000 3500 4000 4500 1 Lis sanelaeeal iy N | I | 1 o o & =, oE00— Mo 2 e 05 Clae05] W WAL A A i . @ 4E-05— = —t ey rean 4\ o I I I I I | I 1 ~
Figure 4; Seismogram readings from Bolivar, Missouri. Source: USGS (2015) public domain, source webpage: bttps://earthquake. u S8s.R0V/ - RIRIRIE . . . > M Time (s) : i b : ha 1 1 1 1 1 1 1 L 1 3 e = - -»—~.w~~ .... ~__"..._..—_.u~p-.—»\i4‘,fl‘ &l e ] T 3 10 | 2 .1 A;)‘ | E | 2 o 3 1ed = o4 - TOrew— ¥ ;J‘” & .ol | = 4 a 5 { 1 1 T T I T | M | T ) 10 20 30 40 S0 60 70 0 Study Figures 2 through 4 to determine the time when the Primary (P) and Secondary (S) waves first arrived for each station. To identify the P and S waves, look for a pattern change as the amplitude of the lines gets bigger; this indicates the arrival of each of the waves. It doesn’t matter which of the three seismograms you choose to read for each figure because the arrival times will be the same on each axis. Write the arrival times for the P-Waves and S- Waves at each station in the first two columns of the Table 1 below. Then use the time scale in seconds on the seismographs to determine the time difference between the P and S wave first arrivals at each station. Write the differences in the third column of the table below. Table 1: Wave arrivals and distance to epicenter data Station P-Wave Arrival S-Wave Arrival Difference Between P-and | Distance to Epicenter Time (S) Time (S) S- Wave Arrival Time (S) From Station (Km) Carrier, OK |6 m Q\'m V) \% !\ Marlow, OK \% . 00 41: (X) g M \ SO \J\YY\) Bolivar, MO m A (x_j wfi— 3*\ /? : ;;Cl() ¥\m e Next, use the difference between the P- and S- wave first arrivals and Figure 5 to determine the distance to 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
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90 80 70 60 Time (seconds) 20 Seismic Wave Arrival Time Chart " S-wave ] P-wave 100 200 300 400 500 600 Distance (kilometers) 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 distance value to the fourth column of Table 1. Figure 5: 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: Bunds e 20 SR BY-SA 3.0 view source Next, you will draw circles on the map in Figure 1 to represent the distance of the earthquake epicenter from each station. Use a drafting compass to draw the circles. If you don’t have a compass, you can draw the circles with a string tied to a pencil. You can also draw circles with software if you are working on a digital document. Figure 1 includes a legend in kilometers. For each station, note the distance to the epicenter. Measure the scale on the map 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 centre to the edge). Create a circle for each of the 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. Draw an arrow pointing to the position of the epicenter and label it EC. Table 2 contains data showing the number of fracking wells in the state of Oklahoma and the number of significant earthquakes (magnitude 3 or greater) that have occurred between 2000 and 2015. Plot the information in Table 2 on the graph provided in Figure 6. Note the graph has two y-axes, one for the number of fracking wells and the other for the number of earthquakes. Plot a line for each set of data on the graph, using the appropriate y axis for each set of data. Table 2: Numbers of fracking wells and earthquakes ocurring in selected years Year | Number of Fracking Wells in Oklahoma Number of Earthquakes Greater Than M3 in Oklahoma 2000 0 0 2001 0 0 2002 0 3 2003 0 0 2004 0 2 2005 0 1 2006 0 2 2007 0 1 2008 1 2 2009 4 20 2010 1 43 2011 637 63 2012 1568 34 2013 1939 109 2014 3296 585 2015 1749 850
Figure 6: Seismcity vs Fracked Wells in Oklahoma Seismicity vs Fracked Wells in Oklahoma # of fracked wells # of earthquakes M3 or greater Referring to Figure 6: 6. What year does the number of magnitude 3 or greater earthquakes begin to rise significantly? 30\ 7. What year does the number of fracking wells rise significantly? 80 |\ 8. Based on the graph that you constructed, do significant earthquakes and the number of fracking wells appear to be related? \} QS This exercise was adapted from Deline B, Harris R, & Tefend K. (2015) “Laboratory Manual for Introductory Geology”. First Edition. Chapter 13 “Earthquakes” by Randa Harris, CC BY-SA 4.0. View Source. o
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