Section 2: The Distribution of Earthquakes

Activity #1: WHERE DO EARTHQUAKES HAPPEN?

Concept: Earthquakes can happen almost anywhere in southern California, but are typically located in zones associated with major faults.

Materials:

Procedure:

Study the image entitled Seismicity for 1932-1996. Over the background map of southern California are plotted, in red, the epicenters of all earthquakes recorded instrumentally in this area since 1932. Every quake, regardless of magnitude, is represented by a single pixel (many of these pixels overlap). To avoid interfering with the data, a minimum of labelling was used on this image, but several major highways are shown in yellow. Thus, you may wish to compare it to a well-labelled road map of the area to get acquainted with the geography.

Once you have had a chance to look the image over, consider the following questions:

  1. How would you describe the general distribution of epicenters shown in this image? Does there seem to be any sort of pattern?

  2. Compare this seismicity distribution to the set of major fault traces plotted (in light green-blue) in the image. (Keep in mind the fact that the seismicity was plotted atop the background, so faults may be obscured in places.) Is any relation between these two features apparent?

  3. If some relation is apparent, how much similarity is there in the distribution of these two sets?

  4. Remember what is required for an earthquake to occur, no matter how small. What does the distribution you've seen suggest about the number of faults (of all sizes) in southern California?

  5. By now you should be familiar with the location of the San Andreas fault zone, the most infamous of all the faults in California. Are you surprised by the seismicity level associated with it? What might this say about the nature of faulting along this giant fault zone?

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