Like all transform plate boundaries, the San Andreas is a strike-slip fault, movement along which is dominantly horizontal. Specifically, the San Andreas fault zone separates the Pacific and North American Plates, which are slowly grinding past each other in a roughly north-south direction. The Pacific Plate (western side of the fault) is moving horizontally in a northerly direction relative to the North American Plate (eastern side of the fault). Evidence of the sideways shift of these two landmasses can be found all along the fault zone, as seen from the differences in topography, geologic structures, and, sometimes, vegetation of the terrain from one side of the fault to the other. For example, the San Andreas runs directly along Crystal Springs Reservoir on the San Francisco Peninsula. Topographically, this reservoir fills a long, straight, narrow valley that was formed by erosion of the easily erodible rocks mashed within the fault zone.
Movement along the San Andreas can occur either in sudden jolts or in a slow, steady motion called creep. Fault segments that are actively creeping experience many small to moderate earthquakes that cause little or no damage. These creeping segments are separated by segments of infrequent earthquake activity (called seismic gaps), areas that are stuck or locked in place within the fault zone. Locked segments of the fault store a tremendous amount of energy that can build up for decades, or even centuries, before being unleashed in devastating earthquakes. For example, the Great San Francisco Earthquake (8.3-magnitude) in 1906 ruptured along a previously locked 430 km-long segment of the San Andreas, extending from Cape Men-docino south to San Juan Bautista.