There are many earthquake faults in the San Francisco Bay Area. A fault is a break in the Earth’s crust, caused by movement of rigid blocks. The Hayward Fault dissects Fremont creating offset features. The In 1836 there was a large earthquake but the larges was felt on October 21, 1868. 

The Hayward Fault is an offset of the San Andreas Fault system that dominates the landforms of eastern San Francisco Bay.  The relative motion between the North American Plate  (southeast) and the Pacific Plate  (northwest) create stress that releases energy as earthquakes. This slip slide motion is called a transform fault.  The Hayward Fault is within the San Andreas Fault  Boundary Zone between the North American Plate and the Pacific Plate.  The Hayward fault is one of only a few dozen faults in the world that “creeps,” or slowly moves.  Evidence of creep can be found from Pt. Pinole in the north to south Fremont.

The Hayward Fault has a strike-slip motion which is when one land mass moves, nearly horizontally in the opposite direction of the other on the surface. This movement causes stress, which results in earthquakes. The Hayward Fault is a strike-slip fault on the surface but changes to a low angle thrust fault as it descends under the Easy Bay Hills. This creates an uplift of the East Bay hills in the Fremont area exposing the rocks of the Briones Formation, which is a fossilerous rock made of marine shells.  It is Miocene in age and can be correlated to other similar sediments throughout coastal California.

In the big picture, scientists know where the fault is because of curbs and streets being offset from fault creep, pre-development aerial photos showing creeks shifted and other markers of fault movement, and from previous trench exposures showing the fault below ground.   Scientists can take sonar pictures of the earth below.

strike slip fault motion	Hayward Fault going under the East Bay Hills in Fremont area

What is a fault creep?