Section 2: The Distribution of Earthquakes

Activity #2: WHERE DO LARGE EARTHQUAKES OCCUR?

Concept: Large earthquakes are generally associated with major faults, because a large rupture area is required to produce a large earthquake.

Materials:

Procedure:

After reading this introduction, look at the image used in Southern California Earthquakes. This map does not show all the epicenters of earthquakes greater than magnitude 4.5 (the smallest magnitude given on this map's scale) ever recorded in southern California, but it does show most every quake greater than magnitude 6 that has occurred since the year 1910 (and a few from before that time).

Locate the epicenters for the earthquakes of magnitude 6 or greater (using the color scale as your guide). Then consider the following questions:

  1. Describe your findings about this distribution. Do these epicenters seem more evenly, less evenly, or no differently distributed than those in Activity #1 (Seismicity for 1932-1996)?

  2. Again, note the locations with respect to the major fault traces shown on the background. Is there any relation apparent between the epicenters of these large quakes and the location of major faults?

  3. If a relation is apparent, how much similarity does there seem to be between the locations of the epicenters and the fault traces?

  4. How does this degree of correlation compare to that seen in Activity #1 (Seismicity for 1932-1996), and, given what you know about earthquake rupture and its relation to magnitude, can you deduce why this might be so?

Now, look over the image of the Los Angeles area shown on the Los Angeles Basin Seismicity page. This map shows the epicenters of all earthquakes greater than magnitude 4.8 which have occurred in this area since 1932. Major faults are drawn in light blue-green. Notice where the epicenters plot in relation to these faults, then consider:

  1. Does this seismicity plot support or contradict the observations and conclusions you made in answering questions 1 through 4?

  2. If there are epicenters which do not fall on plotted fault lines but should have occurred along a major fault, how might these discrepancies be explained?

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