In our lifetime, and for many generations to come, San Diego and Southern California will reside near the San Andreas Fault system, which divides the North American tectonic plate from the Pacific plate.
There are roughly 12 major tectonic plates that cover the surface of the earth. Movement at these plate boundaries has occurred for millions of years, and will continue for millions of years. In general, these plates move past each other at about the same rate that your fingernails grow. Friction at these boundaries causes stress to build, and it is the sudden release of this stored energy that produces an earthquake.
Motion at the San Andreas boundary is referred to as “strike-slip,” which means that the two plates are moving horizontally past each other. The motion between the two plates in California is further classified as “right-lateral,” meaning that any “object” on the opposite side of a fault appears to move to the right during or after an earthquake event (see how the river is deflected in the picture below). Though San Diego is not directly over the San Andreas Fault, there are many smaller right-lateral faults within the wider San Andreas Fault system, such as the Rose Canyon Fault that runs directly through the city of San Diego.
San Diego and the Pacific plate will keep moving northward with respect to the North American plate. This means in about 2 million years, San Diego and San Francisco will be neighbors!
-- Jeff Babcock, project scientist, Institute of Geophysics and Planetary Physics
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