FIGURE 4.27 The San Andreas fault is a transform boundary associated with the East Pacific Rise spreading center. The San Andreas forms the boundary between the Pacific and North Amercian plates. Note that just north of San Francisco, the western edge of North America changes from a transform to a convergent boundary that includes a subduction zone and volcanic arc.
is well known because it is on land and generates powerful earthquakes in highly populated areas. From Figure 4.27 notice how the San Andreas is actually a long transform fault that connects two rather distant spreading centers, and also serves as the boundary between the North American and Pacific plates. The photo in Figure 4.28 shows a site north of Los Angeles where the San Andreas is well exposed. Here the offset stream channels prove that dramatic movement along the fault has taken place relatively recently. Interestingly, a person at this site could stand on one tectonic plate, then walk a short distance and be on a different plate that is literally moving in the opposite direction. This lateral movement along the San Andreas means that Los Angeles, which sits on the Pacific plate moving to the northwest (Figure 4.27), will eventually meet up with San Francisco as it moves to the southeast on the North American plate. Keep in mind that it will take millions of years for the cities to meet. Also, contrary to common folklore, these plate movements are not going to cause Los Angeles to suddenly slide off into the ocean in some giant landslide!
Finally, notice in Figure 4.27 that as one goes north of San Francisco, the tectonic setting changes from a transform boundary to a convergent boundary. Moreover, the presence of a subduction zone and volcanic arc (Cascade Range) tells us that the boundary along the Pacific Northwest involves the convergence of an oceanic plate with the continental plate. As a result, the residents of the Pacific Northwest face both volcanic and earthquake hazards, whereas the primary hazard in Southern California is one of earthquakes. We will explore both earthquake and volcanic hazards in considerable detail in Chapters 5 and 6.