This graph was produced in 2008, showing how it will take approximately 40 seconds for a 7.8 quake at the Salton Sea to begin to have P waves hit Santa Monica and downtown Los Angeles.
After that it is another approximately 30 seconds for force 7 ground
motion waves to hit and 50 secs for force 9 ground motion to hit.
At that point it is too late be able to walk or avoid falling building
pieces. You really only have those 40 seconds between when P waves hit
and the force 7 or 9 waves hit when its too late to escape.