Main Menu >  Plate Tectonics and Volcanoes  >  San Andreas Fault >  Transforming California What's happening under California? Some rocks are moving and some are stuck. Below depths of 12-15 km, the rocks are hot and ductile, enabling the San Andreas Fault to move steadily. Closer to the surface, the rocks are cold and brittle and the Fault locks...until the plates on either side lurch past each other in an earthquake.