The San Andreas Fault is a zone several miles wide with multiple strands. The main active strand runs both on and off-shore between Cape Mendocino and the Sea of Cortez. From Cape Mendocino, it runs offshore to Tomales Bay. Then it goes southward through Bolinas Lagoon, just west of the San Francisco Peninsula, to come onshore again at Daly City, through the hills of the Peninsula. Crystal Springs reservoir is formed by the fault itself. 