View from San Juan Canyon Road

Starting south from the town of San Juan Bautista, the San Andreas fault shifts from its locked condition in the northern segment to a creeping condition. The town registered some movement on the fault in 1906 but to the south there was none, although observations were not as precise as today. The boundaries of the creeping and locked segments are matters of convention. So is the location of the fault itself. The trace that ruptured in 1906 is just one strand in a whole skein of dislocations across a zone between 100 meters and a kilometer or so in width. Down at the base of the crust, the fault is presumed to be a clean plane, but in the brittle rocks above the plane variations in tectonic forces, rock types and specific earthquake events can change the rupture zone over time. Geologists have mapped various fault traces, some abandoned but still evident to skilled eyes.