Question

Earthquake Lab: Locating the Epicenter, Determining the Magnitude, and Calculating the Time of Earthquake The goal of this lab is to learn how seismograms are used to locate the epicenter of earthquakes, determine earthquake magnitudes, and to calculate the origin time of earthquakes. A seismogram measures the amount of shaking caused by earthquake waves. The seismogram first records the arrival of P-Waves (Primary), followed by the arrival of the S-Waves (Secondary). The S-P interval is the time difference between the arrival of the P- and S- waves and is used to calculate the distance of the seismograph station from the earthquake epicenter. This material is discussed in the earthquakes chapter of your textbook. In addition, check out the example already completed in Blackboard. Be sure to review both the textbook and the example before attempting this lab. A. Locating the Earthquake Epicenter (50 pts) 1. Estimate to the nearest half-second the S-P interval from the three seismograms shown in Figures 1, 2 and 3. After, use the S-P travel time curve (Fig. 4) to determine the distance between the epicenter and the city where the seismogram was generated. Enter the data in the table below. (20 points) City (Seismograph Station) Distance (km) S-P (seconds) Boise, Idaho Klamath Falls, Oregon Reno, Nevada 2. Use the method of triangulation to determine the location of the earthquake's epicenter on the map (Fig. 5). Use a drafting compass to draw a circle around each city (Boise, Klamath Falls, and Reno), with the radius of each circle equal to the distance from the epicenter determined in the table above. The three circles you construct should ideally intersect at one point then find a point that is equidistant from the three edges of the circles, and use this as the epicenter). Record the location of the earthquake epicenter with a star on the map (if the three circles do not intersect single point, at a ( ) on the map. 3. What geologic feature might be responsible for the earthquake at this location? Of the cities shown on the map, which one is at greatest risk of damage? Explain why. Include any references used. 16 Boise, ID 12 mml 12 16 10 20 30 40 50 60 80 70 Time (Seconds) Figure 1. Seismogram recording earthquake in Boise, Idaho. Each tick mark along horizontal axis represents 2.5 seconds of time. TUDE 370 Klamath Falls, OR 240 160 A M 80 P L O mm T U 80 E 160 240 320 60 80 30 40 50 20 70 10 Time (Seconds) Figure 2. Seismogram recording earthquake in Klamath Falls, Oregon. Each tick mark along horizontal axis represents 2.5 seconds of time. 40 IReno, Nvi 30 20 10 P L O mm S T U 10 20 30 40 30 40 50 60 80 10 20 70 Time (Seconds) Figure 3. Seismogram recording earthquake in Reno, Nevada. Each tick mark along horizontal axis represents 2.5 seconds of time. S-Wave 80 P-Wave 70 60 S-P 50 40 30 20 10 100 200 300 400 500 600 700 800 Distance (Kilometers) Figure 4. Travel-time curve displaying the amount of time (seconds) that it takes P- and S- Waves to travel certain distances (kilometers). In addition, S- P interval is given as a function of distance. Time (Seconds) OREG ON Klama th Boise Falls IDARO EurekaA Ogden Elo Reno Salt Lake City Provo Sacramento San NEVADA Francisc Toopah UTAH Fresno Las e gas Baterstle ld ARIZ ONA Los Ange les San Bernardno Phoenix San Diego Yuma Map Scale www.ea 100 200 300 400 500 600 700 Kilometors Figure 5. Map of the western United States. Seismograms used in this assignment were from the cities of Boise, Klamath Falls, and Reno. Please include circles on the map, and make sure to label the epicenter of the earthquake. gathered :ALIFORNIA B. Determining the Magnitude of the Earthquake (20 pts) So far, you have worked on locating the epicenter of the earthquake, now you will try to determine the strength (amount of shaking) of the earthquake. We will use a nomograph to determine the Richter magnitude of the earthquake. This is a unitless number, derived from the amplitude of the ground motion created and the distance from the epicenter of the earthquake. 1. Measure the maximum amplitude of the seismic waves from each seismogram. Be careful as the amplitude scale for each seismogram changes. Add the distances from the epicenter you obtained from the last section in the table below. 2. Use the nomograph (Fig. 6) on the next page to determine the Richter magnitude of the earthquake as indicated by each of the three seismograms. Ideally, they should all intersect at a single magnitude. Must include work on nomograph for full credit. Distance City Maximum Magnitude Amplitude (mm) (km) Boise, Idaho Klamath Fall, Oregon Reno, Nevada C. Calculating the Origin Time of the Earthquake (10 pts) The first seismic waves felt in Reno, Nevada occurred at 9:31:45 am (hrs:minutes:seconds). Using this time, the information gathered in Part A, and the travel time curve, to calculate the exact time (to the nearest second), when the earthquake originated. Explain how you arrived at your answer. Time of Earthquake: . (hours: minutes : seconds) 500 800 8.0 200 700 600 100 7.0 F50 500 6.0 400 20 300 10 5.0 5 200 4.0 2 100 -9:0 1 60 0.5 2.0 40 30 0.2 1.0 0.1 20 Distance Amplitude (millimeters) Magnitude (kilometers) Figure 6. A nomograph used to determine Richter magnitude by using the distance (km) from the epicenter of the carthquake and the maximum amplitude (mm) of the seismic waves recorded.

Answer 1

From the graphs and the nomograph and the graphs provided the following table can be completed. The first waves at any site are always P waves, which are followed by S-waves.

City	S-P	Distance	Maximum Amplitude	Magnitude
Boise	65	640	12	6.6
Klamath Falls	7.5	80	300	5.5
Reno	42.5	420	31	6

Nomographs are used in the following way:

Join the maximum amplitude with the distance by a straight line. The intersection with the magnitude scale is the magnitude of the earthquake in that region. Note that the readings in this case may be off since the photograph provided is tilted.

500 800 8.0 200 700 600 100 7.0 F50 500 400 20 300 50 10 5 200 4.0 2 100 -9:0 1 60 0.5 2.0 40 30 0.2 1.0 0.1 20 Distance Amplitude (millimeters) Magnitude (kilometers) Figure 6. A nomograph used to determine Richter magnitude by using the distance (km) from the epicenter of the carthquake and the maximum amplitude (mm) of the seismic waves recorded.

Since I cannot draw accurate circles on the image, it is difficult for me to demonstrate how I found the epicenter, but I can explain the procedure. Using the scale provided, we use the distance from the epicenter calculated as the radius and draw three circles. The intersection of these circles gives us the epicenter, which in this case is Eureka, California.

The structural feature causing the earthquake is probably the Sand Andreas fault.

C. Since the earthquake was felt at 9:31:45 am in Reno, we can calculate the time it originated, since we know the distance of Reno from the epicenter (420 km), as well as the average P wave velocity (6.7 km/s).

It took a total of 420/6.7 = 62.6 s for the earthquake to reach Reno. Thus, it originated at 9:30:42.4 am.