The San Andreas is the state's longest fault and perhaps its most famous—well-known enough to be the standalone title of a recent disaster film. And, says the LA Times, citing an earthquake expert speaking at the National Earthquake Conference, it's also the one we need to be keeping our eyes on the most right now.
The issue is that the fault line has been too quiet for too long. Scientists have found that the "earthquakes should be relieving about 16 feet of accumulated plate movement every 100 years" on the San Andreas, but the fault hasn't let off any steam in way over a century.
The southern section of the fault hasn't had a big 'quake since 1857, which "ruptured an astonishing 185 miles between Monterey County and the San Gabriel Mountains," near LA, in a 7.9 earthquake. Other nearby sections of the fault are also overdue. A segment by the Cajon Pass (where the 15 Freeway cuts between the San Bernardino and San Gabriel mountains) "have not moved substantially" since an 1812 earthquake, and over by the Salton Sea, it's been seismically silent as far back as 1680.
That indicates it's long overdue for a rupture. Or, to put it more chillingly, "The springs on the San Andreas system have been wound very, very tight. And the southern San Andreas fault, in particular, looks like it’s locked, loaded and ready to go."
The expert, Thomas Jordan, director of the Southern California Earthquake Center, went on to say that, with this information, LA should focus on being prepared for a BIG one—a magnitude 8 or so. (For perspective, a 2008 US Geological Survey study warned that a 7.8 earthquake along the southern San Andreas would send shaking far and wide from the Coachella Valley and Inland Empire to East LA and the San Gabriel Valley, and cause about 2,000 deaths. So, stronger than that.)
Jordan applauded the city's efforts to retrofit buildings vulnerable to collapse in a strong seismic event, saying, "It’s remarkable that this happened...We know politically how difficult it is to make these kinds of changes." LA's also at work making its cell phone network more resilient to serious shaking so that communication post-'quake would still be possible.
With most Angelenos less than totally prepared for a significant earthquake, there's no time like the present to get started.