ABSTRACT
But because California is the only state cut by three different types of plate boundaries, it is one
of the most seismically active region in the country. A glance at the fault map and seismicity map of California (Figure 11.1a-c) shows that the known and mapped faults occur almost everywhere, and it’s virtually impossible to avoid them. Most of the faults are parallel to the plate boundary and the San Andreas transform system. Through their motion, part of the strain of the sliding between the two plates is taken up in subsidiary faults, reducing the amount of energy that builds on the San Andreas itself. Since the Pacific plate is moving northwest with respect to the North American plate, the sense of shear on these strike-slip faults is always right lateral (i.e., the block on the opposite side of the fault from where you stand appears to move to the right). More than 95% of the active strike-slip
faults in California are right lateral, with one glaring exception: the Garlock fault, which runs along the Tehachapi and Sierra Nevada mountains and out into the Mojave Desert, which is left lateral. As discussed in Chapter 7, the Garlock fault is the exception because it takes up the westward motion and rotation of the southern tip of the Sierras (Figure 7.13).
