Transform Boundaries.
Did you know that when two different plates move past another plate is called Transform Boundaries. Plates move in either opposite directions or same directions but different speeds. Earthquakes occur when one plate suddenly slips past another plate. The pacific plate overtime is sliding past the American plate and that is forming a fault in California.
http://www.faculty.iu-bremen.de/course/fall02/c210101/students/PlateTectonics/boundaries.gif