4. Recording Sensitivity of Your Seismograph

Objective:

To develop a method of determining whether an earthquake of a given magnitude and given distance from your school’s seismograph is likely to be recorded by your seismograph.

Activity:

This activity involves determining the sensitivity of your school seismograph for recording earthquakes of given magnitudes and distances away from your school. Students will search the Internet for earthquakes that have occurred and will plot magnitude as a function of the distance from the school seismograph to the earthquake epicenter. In these plots, the students will use one symbol to represent that the earthquake was “recorded” and another symbol to represent “not recorded.”  The earthquake information necessary to conduct this exercise can be obtained using the same Internet resources that were given in the previous Earthquake Tracking exercise. An Excel spreadsheet file that can be used to plot these data is included with this exercise.

Begin by plotting the examples of earthquakes that were analyzed for the Exploring What Your Seismograph Records exercise. This should provide a preliminary sense of what the graph will eventually look like as the students continue plotting points on the graph each week for the rest of the semester. Once the students have a sense of how these examples plot on the graph, continue adding points to the graph on a weekly basis. Exactly how the class will divide up the task of plotting these data on a weekly basis will depend on the size of the class and how the classroom is set up. An example of how the tasks can be divided would be to have a different group of students chosen each week to investigate about a dozen of the earthquakes reported that week on the National Earthquake Information Center web site. The task for the group is to analyze the seismograph screen to see if any of those earthquakes were recorded. Then the students will plot these earthquakes on their graph of magnitude versus distance, using the appropriate symbol for “recorded” or “not recorded.” Since the EQ1 and AS1 only record earthquakes about once or twice a month (on average), most of these points will be plotted as “not recorded.” The students should, of course, also plot those earthquakes that are clearly recorded.

(It is helpful to start by plotting the BC examples analyzed for the Exploring What Your Seismograph Records exercise, and then check that your results are consistent with what we obtained for BC.)

Distances between the seismograph and the earthquake epicenters (measured in degrees, see Figure 1) can be determined using the following web-based program:

neic.usgs.gov/neis/travel_times/compute_tt.html.

Alternatively, you can use a world globe to estimate the distance by stretching a string between the epicenter and the location of your school. Then measure the length of the string and convert the length to distance using the distance scale on the globe.

Over time, students should notice a pattern representing how large an earthquake needs to be in order to be recorded at given distance away from their school seismograph. As the students become more confident that they can discern this pattern, they should draw a curve separating the “recorded” versus “not recorded” symbols to represent the magnitude threshold for recording an earthquake at a given distance. Some classes have found it useful to add a category for “barely recorded” using a third symbol.


Figure 1: Distance measured in degrees (Δ) from the epicenter of an earthquake.

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