You should see a yellow circle with a radius that increases as you move your mouse away from the station. Move your mouse away from LUG until it reaches the distance you determined from question 5. It can be hard to get the exact distance, but try your best. Once you’ve drawn a circle with the correct radius click again. 5. Save the circle by clicking ‘save’ in the ruler dialog box, name the circle after the station (LUG). 8 Repeat these steps for stations CCA and ALP, and be sure to save each circle. 9. All three circles ‘should’ overlap at the same point, the epicenter of the earthquake, or close. If your circles do not overlap at the same point, hypothesize where in the methods error was introduced (i.e., reading seismograms, plotting SP separations, etc.). There are a number of nonhuman errors that can affect the speed of seismic waves, such as variation in rock type. Don’t stress (too much) if your circles don’t overlap perfectly. 10. Insert a screen shot of your plotted circles in Google Earth below. Turn on the ‘X’ pin, by clicking in the box to its left so a black check mark appears. This pin marks the actual epicenter of this earthquake. Zoom in so you can see all the 9 mapped faults in the area. 11. Did this earthquake occur along a previously mapped fault? If not, what is the name of the fault closest to the epicenter. To find the name of a fault click on it (orange line). Based on the name of this fault do you think it is an important fault? How do you know? 12. What is the name of the closest major fault? (Hint: This fault is located to the south of X and is roughly east-west trending). 13. Turn on ‘Borders and Labels’ in the ‘Layers’ panel. Zoom into the area around the location of ‘X’. What is the closest large city to ‘X’ based on the development you can see in Google Earth? Now that we’ve found the location of this earthquake let’s determine its Richter Magnitude. Those are the two basic questions we ask about earthquakes, where was 10 the earthquake located and how strong was it. There are many ways to evaluate how strong an earthquake was, we are going to discuss two, the Richter Magnitude and Moment Magnitude. The Richter Magnitude Scale (or ‘local magnitude’, ML) is the most well-known scale and was introduced by the seismologist Dr. C. F. Richter of California Institute of Technology in Pasadena in 1935. It is determined by the amplitude of the largest seismic wave in millimeters from the zero line and the distance from the epicenter. The Richter magnitude of an earthquake is a number: about 3 for earthquakes that are strong enough for people to feel and 8 or larger for the strongest earthquakes (largest measured event was a M9.5). The scale is logarithmic, meaning for every increase on the scale, the magnitude increases 10-fold. The Richter Magnitude is also known as local magnitude and is easy and quick to calculate. [the energy actually increase by a factor of 32 for a log unit, but you probably don’t want to get into that] Moment Magnitude (Mw) is based on physical properties of the earthquake analysis of all the waveforms recorded from the shaking. It considers several factors such as the rigidity or strength of the rock, the area of the fault that slipped or moved, and the distance that the fault moved. This information is used to calculate first the Moment then the Moment Magnitude that is roughly equal to the Richter Scale for earthquakes smaller than about 6. Because the Richter Magnitude is quick and easy to calculate we will use this method to determine the ML of the earthquake. We need two pieces of information: distance from the S-P time and the Maximum Amplitude of the seismic waves. We already have the distance so let’s determine the maximum amplitude. Follow the steps below to determine the ML: 1. Measure the distance in mm from the zero line up to the maximum amplitude from figure 7 and Record this value here (could be negative or positive): 2. Record the distance for station CAA from table 2 here: 3. Use figure 8 to plot the distance and amplitude values. Draw a line to connect them and record the magnitude here: 11 Figure 7: Seismogram for station CAA with millimeters on the y-axis. Figure 8: Nomogram used to determine Richter Magnitude ML by plotting the max amplitude of a seismogram and its distance from the epicenter. Connect these two points with a line and where this line intersects the Magnitude scale read the magnitude. 12 14. The data shown in this lab are from the 2019 Mw 7.1 Ridgecrest earthquake. Was the ML you determined 7.1? Based on what you know about Richter Magnitude (ML) and Moment Magnitude (Mw) hypothesize why the values are different. 15. The Los Angeles Times wrote an article about the Ridgecrest earthquake and created a GIF showing the land surface before and after the rupture or earthquake. Navigate to the below url to see the GIF https://www.latimes.com/california/story/2019-11-13/ridgecrest-earthquakeruptured-dozens-of-faults What is the sense of motion across this fault? Is it a right-lateral strike slip fault or a left-lateral strike slip fault? 13 Google Earth help: When Google Earth (GE) first opens, all the ‘Layers’ are turned on. This can be distracting and make finding what you are looking for hard. Turn off everything in ‘Layers’ to make your life easier. When a layer is on, a check mark will be in the box to its left. To turn it off click the box and the check mark will disappear. To expand a folder to see its subfolders/layers, click the black arrow head located to its left. An expanded folder has a black arrow head pointing down. A collapsed folder has a black arrow head pointing to the right. 14 The ruler tool is located in the menu bar at the top of Google Earth, to the right of the search bar and it looks like a small blue ruler. Click the ruler tool once to get the ruler tool dialog box. Make sure the circle tab is open. You can change the units by clicking the drop-down box located to the right of ‘Radius’. Once you have the units set, zoom in to one of the stations close enough that you can see the pin sticking into the Earth’s surface and click once. You do not click and drag. Now move your mouse away from the station and watch the radius distances in the ruler tool dialog box, until you reach the desired distance and click again. Be sure to save your circles by clicking ‘Save’ in the bottom right of the ruler tool dialog and name them after each station. To measure another radius, open the ruler tool again. To close the ruler tool, click the red x located in the top right corner of the ruler dialog box. 15
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