The original definition of fault line (from which the slang terms are derived) is basically an underground crack in
the earth's tectonic plates. It is along these lines where earthquakes most frequently occur and with the greatest impact. Fault lines may also be referred to as earthquake faults.
I know this is supposed to be a
slang dictionary, but I think that understanding the origin of this word gives greater meaning to the slang terminology.
The 1906 earthquake was in a way far more devastating in
Santa Rosa, California than in San Francisco. Both cities are located along the same fault line,
the San Andreas fault, but the epicenter was closer to Santa Rosa, where the shaking was much stronger. The reason so many died in San Francisco was due to the fires that started and not as a direct result of the motion of
the quake itself.