Lab 8 Wilkinson copy

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Apr 3, 2024

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5 2 Earth Sciences 1023/2123 Lab #8* Earthquake Seismology NAME: Lab Section : 1. Method In this laboratory, you will use a simple step-by-step approach to locate the epicentre and determine the magnitude of an earthquake based on seismograms from seismic stations in east-central area of the United States. The map of the region is illustrated below. Fig. 2. Map of east-central area of the United States with the seismic stations in blue circles. You will need to follow the steps below for 3 of the stations shown in Fig. 2. The recorded earthquake seismograms from the seismic stations are given in Fig. 3 on the next page. 1. Measure the S-P times and the maximum amplitude of the wave on the seismogram for 3 stations and record the data in the table below. 2. Use the Travel-Time tool on page 5 to find distances (km) for the 3 stations and record the data in the table at the end of the document. 3. Find the location of the epicenter using the triangulation method and the Latitude/Longitude tool. Record the Latitude and Longitude in degrees ( o ) and minutes (‘) of the epicenter below. 4. Find the magnitude as measured at each station using the Nomogram tool on page 5. *after Department of Earth and Atmospheric Sciences, St. Louis University

3 2 mm 0.5 mm 0.15 mm Fig. 3. Seismograms from seismic stations in Fig. 2. 385 255 105

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4 Distance tool: Move up and down the linear trend in Fig. 4 to find what distance corresponds to the S-P time of each of the seismograms. For example, a S-P time of 5 sec corresponds to a distance of 40 km between that station and the epicenter. Fig. 4. Theoretical S-P time versus distance from the epicenter Latitude/Longitude Map For each station, use a pencil/pen attached to a string ( or use the Format picture tool in Word ) to draw a circle on Fig. 2 with a radius that corresponds to the distance recorded in the table and centered on the relevant seismic station. The intersection of all 3 circles is the location of the epicenter. Record the coordinates of the epicenter on page 6. Nomogram Tool to Determine Magnitude From the table on page 6, select a station and mark the distance to the epicenter on the left scale in the nomogram tool at the right. For the same station, mark the appropriate amplitude on the right most scale. Draw a line between the mark on the left column and the mark on the right column. The Richter magnitude (a measure of the earthquake strength) of the earthquake corresponds to the value where the line intersects the center scale. Enter this value in the table on page 6. Repeat this procedure for the other 2 stations. For the example already given on the nomogram, for a distance of 22 km and an amplitude of 0.2 300 200 80

5 mm, the magnitude of the earthquake is 1.2.

6 Show your procedures and calculations on the seismograms, maps and nomogram tool for each of the 3 stations studied. Record your values in the table below. Station S-P (sec) Distance (km) Amplitude (mm) Magnitude Mph 10 80 1 2.9 Ualr 25 200 1.5 3.6 slm 38 300 0.525 3.7 Average magnitude: Epicenter location: Latitude : 90° 30‘w Longitude: 36° 59‘N According to Fig.1. what is the peak gravitational acceleration at the earthquake’s location ? 0.4 x 9.8m/s^2 = 3.92 m/s^2 Therefore the peak gravitational acceleration at the earthquake’s location is 3.92 m/s^2 Total /20

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