Section 1: What is an Earthquake?

Activity #13: REGIONAL DISTIBUTION OF FAULT SLIP

Concept: The distribution of slip -- both in terms of sense and rate -- along faults in southern California follows definite patterns and is most strongly influenced by the characteristics of the plate boundary.

Materials:

Procedure:

In this activity, you will be studying two different maps of the same area (at the same scale) of southern California. Each will show the surface traces of major faults of southern California, color-coded according to a scale. On one map, the scale will be in terms of sense of slip. One the other, the scale will represent slip rate.

For each map, you will be asked a short series of questions. These questions are intended to gauge how well you understand the material about faults, fault slip, and regional tectonics that you have covered in Section 1. All the information you will need to know to answer these questions is contained within the maps, or within the text material of Section 1. You will begin with a map showing faults colored according to their sense of slip.

Sense of Slip

Go to the map showing the sense of slip distribution of faults in southern California. Study this map for a while (and read the explanatory text for help in interpreting it), then use the "Back" button of your browser to return to this page and answer the questions below. While answering these questions, you are welcome to go back to the map page at any time to check on the details. (If you haven't already, go to the map now, before reading on.)

Now that you've studied the effects of tectonics on faults, and (hopefully) completed Activity #12: Find That Fault Slip!, did the distributions of the various senses of slip shown on this map fail to surprise you? (If so, the answers to the questions below should be pretty obvious, and basically a review of what you've learned thus far.)

  1. Do the right-lateral faults in southern California generally occur in a particular orientation, or do they trend in many, varied directions? How does their orientation compare to that of the plate boundary (San Andreas fault)? Does the answer to that question seem to confirm the importance of the plate boundary in determining the slip of faults in southern California?

  2. Do reverse (and thrust) faults seem equally distributed around the map, or are they concentrated in a certain region? Is their distribution similar to what you might expect, given the "bent" nature of the plate boundary here? Why?

  3. Is it fairly clear that something different is happening north of the Garlock fault zone (the longest green fault)? What sense of slip becomes more prominent in this region (as compared to the rest of the map)? Would you say this area is being pulled apart, or squeezed together?

Slip Rate

For the next set of questions, you will be studying the map of slip rate distribution in southern California. Again, study this map and read the text that accompanies it. Then use your browser's "Back" button to return to this page and answer the questions below. As before, you are welcome to go back and forth between the map and this page as you answer the questions below. (If you haven't already, go to the map now, before reading on.)

  1. Do most of the faults with the highest slip rates visible on this map seem to be strongly associated with the San Andreas fault and its Big Bend, or is the distribution of them more scattered?

  2. Note the parallel (right-lateral) faults across the bottom of the map. What generally happens to the slip rates of these faults, the further you move away from the plate boundary (the San Andreas fault, in red)? Looking at the same set of faults, what generally happens to the slip rate of each fault near the northern end of its length?

  3. Did you notice the blind thrust faults (dotted lines) in the Los Angeles area? Some people form the impression that blind faults don't reach the surface because they are not as active, or slipping as quickly, as other faults. From what you can see on the map, is this a reasonable assumption? In terms of slip rate, do the blind thrusts seem less "active", just as "active", or more "active" than nearby faults which reach the surface?

Additional Notes

Note that, although the plate boundary exerts an obvious influence on the slip rates of faults in southern California, there are some faults near the boundary that do not have high slip rates, and a few far removed from the boundary that seem to have anomalously high slip rates. Of course, this map is not by any means perfectly accurate or precise. Many of the "transitions" between or within fault zones, or at the ends of fault zones, are not accurately displayed. Many of the slip rates may be in error, and some blind faults are completely missing. Just remember to keep this basic fact in mind in Activity #14: Partitioning Slip: while we can model an area based on simple principles, the real story is usually much more complex.

Return to the Text