Earlier this week, an expert from the Southern California Earthquake Center spoke at a conference in Long Beach and called the southern San Andreas fault "locked, loaded and ready to go" for a major 'quake. He said that people should be preparing for something around a magnitude 8.0—that's larger than the devastating San Francisco earthquake back in 1906, the LA Times notes, and that one caused about 3,000 deaths from both the shaking and the fires that followed (which LA's former earthquake czar Lucy Jones has said we should be worried about).
What would an earthquake that big even look like? How would it move and where could we expect the shaking to be felt? For that, there's a video from the SCEC that shows where the movement would occur and how far away it could be felt in the event of a 'quake that starts near San Luis Obispo and moves south along the fault.
The San Andreas fault doesn't cut through LA, but it comes close and, as the video shows, LA would feel a lot of shaking because of our "underlying soft sedimentary basins." More than a little scary, but good information to have, say, before a big earthquake hits.
Here's another video, from 2014's Great ShakeOut earthquake drill, which shows a mere 7.8 magnitude that starts near the Salton Sea and moves north. The waves of shaking are depicted again in red, orange, and yellow, but in this video that makes them look like the lit fuse of a cartoon stick of dynamite just burning its way through Southern California, ready to blow everything up.