1. On each of the seismograms on page 3, analyze the recorded groun...

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1. On each of the seismograms on page 3, analyze the recorded ground motion (the squiggly lines on each seismogram) to determine the signals of the first P and S waves (surface waves are not shown on those seismograms). Pages 337 and 338 in your text might help you make the picks. Keep in mind that: the P waves arrive first, but creates the lesser amount of ground motion. Don't be fooled by very small background motions that occur prior to the P waves' arrival. the S waves arrive second, they show an increase in ground motion relative to the P wave no surface waves are shown on these seismograms Mark the first arrival of each P and S wave on the seismogram using a vertical line. Extend the line upward to the "Time since earthquake" scale, and determine the time when that wave type first arrived (in seconds - note that the values appear on the seismogram in the form 00:00:00, which stands for hours:minutes:seconds) Record the time of the P and S wave in the chart below. 2. Determine the time interval (Atsp, in seconds) between the arrival of S and P waves. Record this value (in seconds) on the chart below. 3. Use the plot of travel time interval (Atsp, in seconds) versus distance shown on page 3 (top of next page) to determine the distance (in kilometers) to the earthquake epicenter from each seismograph station. Record this value on the chart below. An example is shown in red arrows for a Atsp of 90 seconds and explained on the top of page 4. 4. Repeat steps 1-3 for each seismogram. Seismograph P-wave S-wave Atsp (difference in Distance to the Station ID arrival time arrival time arrive times, in epicenter (km) (seconds) (seconds) seconds) BBR TIN W12A 2 Part 1: Seismograms Time since the earthquake 1 00:00 00:00:20 00:00:40 00:01:00 00:01:20 00:01:40 00:02:00 00:02:20 00:02:4 80 60 Station BBR 40 20 -20- -40- -60- -80- Time since the earthquake 2 00:00 00:00:20 00:00:40 00:01:00 00:01:20 00:01:40 00:02:00 00:02:20 00:02:4 I I 100- 80 Station TIN 60 40- X 20- SIL. 0- -20- -40- -60- -80- -100- Time since the earthquake 3 00:00 00:00:20 00:00:40 00:01:00 00:01:20 00:01:40 00:02:00 00:02:20 00:02:4 I 21.00- 18.00- 15.00- Station W12A 12.00- EVO 9.00 6.00 X 3.00 $ 0.00 -3.00 -6.00 -9.00 -12.00 -15.00 -18.00 Red lines show an example of how to use this graph. If you know Atsp is 45 seconds for seismogram XXX, find that value on the x-axis, project upward along the red arrow to the diagonal line, then horizontally to the y-axis to estimate distance between seismograph stationXXX and the epicenter. In red line example, that distance is - -360 kilometers. Now repeat that process for the Atsp values you put into the chart on the bottom of the prior page. - I 400 300 200 100 10 20 30 40 50 Atsp Interval Time (sec) Now you will go to Part 2, BUT If you did not pick the correct first arrivals for the P and S waves in part 1 (for any or all seismographs), you will not get a solution to the earthquakes epicenter in Part 2 and will have to go back to part 1 and re-examine the seismographs. You may have mis-identified one or more of the first P and S waves. And if so, then you have an erroneous Atsp, which also means an erroneous distance determined from the above graph. Analyzing the solution you initially get in Part 2, and deciding what might be wrong with it is a key part of this exercise (i.e., monitor & reflect on you own work as a self-regulated learner). And yes, the above paragraph says the same thing as what is on page 1, but Dr. Budd has learned over the years that not all students take the time to not only read the information provided, but also process it. 4 Part 2: Map of Seismograph Stations Locate that Epicenter! 1. Using information from only one seismograph station, you can determined the distance from the station to the earthquake epicenter - but you do not know the direction from the seismograph station to the epicenter (you know the distance, but not the direction). 2. To locate the earthquake epicenter, you will use the information from all 3 seismograph stations. To do this, for each seismograph station, draw a circle that is centered on the station and that has a radius equal to the estimated distance to the epicenter (in km) from your chart. Use S compass to draw the circle. Use the map scale below to determine how much to open your compass. Do this for each of the 3 seismograph stations. 3. Put an "X" on the spot where the three circles tell you is the location of the epicenter. Nevada Utah Califomia an Jose TINO W12A Los Angeles Arizona BBR 47 San Diego Mexico 100 200 300 400 500 kilometers 5 PART 3: Determine the earthquake magnitude. The magnitude of an earthquake can be determined from the maximum amplitude of the S-wave (your text, p. 339). 1. Determine the maximum amplitude (height) of the S-waves on each of the seimograhs. Draw a horizontal line from that maximum to the y-axis and determine the amplitude in. Record the maximum amplitudes for each seismograph in the chart below 2. Convert maximum amplitude to millimeters by dividing by 20. Record that in the chart. Seismograph Station ID Max amplitude Column 2 divided by 20 = Max amplitude in millimeters BBR TIN W12A 3. Using the nomograph below, determine the magnitude of the earthquake. Draw a line connecting the distance of each seismograph from the epicenter (left vertical line) to the amplitude you calculated for that seismograph (right vertical line). The three lines you draw (one for each seismograph) should intersect the middle vertical line at one point, which is the earthquake's magnitude. Because we are doing this by estimating values off of graphs, don't be surprised if one of the lines does not intersect the others at the same point. If two intersect, we will consider the job down. Record that magnitude in large writing in the blank line (make it easy for the grader to see) 500 800 8.0 700 200 Magnitude of the 600 7.0 100 quake is 500 50 6.0 400 20 300 5.0 10 8 6 4 200 4.0 2 100 3.0 1 60 0.5 2.0 30 0.2 1.0 0.1 20 Distance (km) Magnitude Amplitude (mm) 6 PART 4: Questions. 1. What were some of the difficulties you had in deciding what was the first P wave at each seismograph. Use sketches to illustrate those difficulties. 2. What were some of the difficulties you had in deciding what was the first S wave at each seismograph. Use sketches to illustrate those difficulties. 3. Did the circles exactly intersect on the map in one point? If not, what are some ideas that you have to explain why they did not do so exactly? 7

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