The `Big Bend' and the Plate Boundary

As noted before, the "Big Bend" of the San Andreas fault is responsible for much of the complexity of faulting in southern California. This bend is a convergent (restraining) bend, creating a localized collision of tectonic plates, and a tremendous amount of compressional stress. To release this stress, additional faults have formed over time. A typical response to large-scale compression is crustal shortening. This allows compression to continue by "squeezing" up the rocks in the compressional zone. This is accomplished by thrust faults -- low-angle reverse faults that drive sections of crust over one another to create a thicker pile of crust with a shorter (horizontal) length. The surface traces of such faults are shown in pale yellow on the map view at right. The 1994 Northridge earthquake (magnitude 6.7) occurred on one of these numerous thrust faults.

Not all the compressional force generated by the "Big Bend" of the San Andreas fault goes into thrust faults. The collision boundary is not square with the plate motion, but at an angle, in such a way that some of the material "caught in the middle" has a chance to move laterally out of the way. This is exactly what happens. Large zones of left-lateral faulting, shown here in green, have formed in an effort to relieve some of the stress created by the fault bend. The most obvious example of this left-lateral faulting is the Garlock fault zone, which intersects with the San Andreas near the northern end of the "Big Bend" and continues eastward for several hundred kilometers.

In addition, several right-lateral strike-slip faults south of the Big Bend, and west of the southern San Andreas fault zone, seem to be managing some of the overall slip between the two tectonic plates. These fault zones, shown here in orange, are quite lengthy and roughly parallel the plate boundary.