However,
in geologically active areas like the San Francisco Bay area, geologic
units tend to be broken up and moved along faults (it is fault movements
that cause earthquakes!). When different geologic units have been moved
next to one another after they were formed, the contact is a fault
contact, which is shown on the map by a thick line (location 5). Faults
can cut through a single geologic unit. These faults are shown with the
same thick line on the map, but have the same geologic unit on both sides.
Remember, just because the map shows a fault doesn’t mean that fault is still active and is likely to cause an earthquake. Rocks can preserve records of faults that have been inactive for many millions of years. But knowing where the faults are is the first step toward finding the ones that can move. Special geologic maps of the faults known to be still moving are constantly being upgraded here at the United States Geological Survey, as well as by State geological surveys and university researchers.
Information provided by: http://geology.wr.usgs.gov