The Hayward Fault is part of the network of geological flaws running through the San Francisco Bay Area that together make up the San Andreas Fault System.
The Hayward is considered to be the most dangerous urban fault in America and of all those in the Bay area, it has the highest probability of breaking in the coming years.
This animation illustrates the likely shaking of an earthquake on a similar scale to the last major event in 1868.
Animation courtesy of US Geological Survey.