Indeed, such complexity does seem to have an impact upon the slip rates of the major faults. Where slip is drawn off of the main plate boundary (the San Andreas fault zone) and onto other faults and shear zones with a similar sense of slip, the slip rate along the main boundary tends to decrease. The total relative motion between the two plates, however, always remains constant.
The same thing is true on a smaller scale. Suppose a certain fault has a (right-lateral) slip rate of 8 mm/yr. Now, imagine splitting that fault into two parallel faults somewhere along its length, and then rejoining them. Anywhere you cross this zone of faulting, you must cross 8 mm/yr of right-lateral slip. Thus, the combined slip rate of the "split", parallel faults must be 8 mm/yr, regardless of what each individual slip rate is. In this way, a larger amount of slip can be partitioned among several different faults to accomplish the same overall motion as a single fault of higher slip rate.
This process can be easily understood by working through some simple models, as in the activity below.
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Plate motion can be split up among multiple faults. Learn
how this can happen, how to identify it, and what it means to risk
assessment. |
After working the last exercise of the activity above (which has actually been simplified a great deal), you can probably begin to see how complicated the plate boundary in southern California really is, and how the way slip is transferred through this entire system is no simple problem to solve, especially given our data, which is limited to a very short time period.
Still, you are hopefully beginning to see a clearer picture of how the inner workings of the tectonic boundary we have in southern California create the particular faults and earthquake activity for which this region is so well-known.
Partitioning Slip