Name: _________________________________________ GOL 105: Physical Geology Earthquake Assignment Objective: This assignment is meant to give you practical experience on locating the epicenter of an earthquake and determining its magnitude from seismogram data. Directions: It will be easiest if you print this sheet out onto paper and work from that, but it’s also possible to do it completely digitally. We have provided you with three seismograms from a single earthquake. Each seismogram is from a different location in the vicinity of the earthquake. 1) Label the P-wave and S-wave on each of the three seismograms 2) Determine the time difference between the first arrivals of the P and S waves for each location and enter the times in the Table below. 3) Determine the distance to the epicenter of the earthquake from each of the stations using the time difference between the P and S-wave arrivals determined in the previous step, and the two provided graphs of arrival times verses distance. We have provided two graphs showing the arrival times for the P and S waves verses distance from the earthquakes epicenter. Both graphs are identical. The first is a smaller scale and covers smaller distances (thus shorter time intervals between P and S wave arrivals) from the epicenter. The second can be used for larger distances (thus longer time intervals between P and S wave arrivals) from the epicenter. Determine the length along the vertical axis representing the time interval between the P and S-wave arrivals (step 2) for each station. Then find the distance on the horizontal axis where the lines representing the P-wave arrival and S-wave arrival are exactly that time interval from each other ALONG A VERICAL LINE. For example, if the time interval between the P and S-wave arrivals at a station were 20 seconds, this would correspond to a horizontal distance of approximately 200 km (see arrows in Figure 1). 4) Locate each of the stations on the accompanying map of California. Use a compass to draw a circle centered on each city/station with a radius corresponding to the distance determined to the epicenter from that station. The horizontal scale in kilometers is provided on the map. 5) If you have measured drawn everything correctly and carefully, the three circles around the stations should intersect at only one point, which will represent the epicenter of the earthquake. (note: there are many places to make small errors in this exercise and each will add cumulatively. If the circles do not intersect in one point, you should go back and make the measurements and calculations again, but more carefully.) You can use the Table provided below to organize your data. Show all work to get full credit! Table 1 Location Los Angeles San Jose Bakersfield Time between P and S arrivals (s) Distance to Epicenter (km) Location of Earthquake (intersection of drawn circles on map): Using the seismogram from the location Bakersfield location and the Richter scale provided at the end of this exercise, determine the magnitude of the earthquake: 1) Plot the distance to the earthquake epicenter from the Bakersfield station on the vertical axis on the left. 2) Measure the amplitude (in millimeters) of the highest seismic wave on the Bakersfield seismogram from the horizontal equilibrium line (do not measure the distance from the lowest point to the highest point of a wave, that is the wave height, not the amplitude, which should be approximately half that distance) and plot it on the vertical axis on the right. Enter your measured amplitude in the space provided below. 3) Connect the two point that you plotted. This line will intersect the center vertical axis, where you can read the Richter magnitude of the earthquake. Record the Richter magnitude of the earthquake in the space below. Amplitude of largest seismic wave: Richter magnitude of earthquake: (mm) Figure 1 Figure 2 250 km total on this scale bar Figure 3
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