The last big one was 1906 a 7.8. In geologic time that is a few seconds and the geologic record indicates the next big one on the San Andreas is overdue and as such will no doubt be larger than the 06 quake. If the 06 quake happened today hundreds of thousands would be killed and at least 20,000 buildings/structures destroyed.
Seismologist Lucy Jones from the US Geological Survey warned in a dramatic speech in Japan last month that people need to stop ignoring the threat. In 2011 there was a 9.0 earthquake off the east coast of Japan and around 10% of the population died. The Pacific ring of fire is a system of faults mirrored on either side of the Pacific.
A 9.0 quake would be 15 times bigger and 65 times stronger than the 06 quake and would sink either San Francisco bay area or the Los Angeles coast and valley three feet in a few seconds with resultant seawater flooding and tsunami. The disaster would lead to at least a million deaths and most of either city completely destroyed.
http://www.dailymail.co.uk/sciencetech/article-4540590/California-s-Big-One-happening-devastate.html
You use the Daily Mail as a source for your scientific information?
Please tell me you're kidding.
9.0 earthquakes have happened in the US, just not in California. While devastating, they simply have not caused the armageddon you describe. The most recent was 1964 in Anchorage. That's one of the largest earthquakes ever recorded anywhere. Yet, the few bridges didn't come down, and the deaths were in the hundreds, despite Anchorage having a population around 100,000 at the time. It did, however, take out the PANC control tower.
No one seriously believes a 9 point earthquake is all that likely. 8 point, sure, but the bridges are designed to withstand that. It's not AT ALL a small difference, so don't even go there, or you'll prove you don't understand logarithmic scales.
Here is a substantially more reliable source than a British tabloid known for sensationalism and questionable reality:
https://www.scec.org/ucerf
And here's a big hint: The San Andreas is not the major fault risk. It ruptures fairly regularly, and just doesn't make the largest earthquakes. Despite your claim, it has ruptured in 1989 (north) and 1994 (south), each of which killed several dozen people, no more.
Another big hint: Almost all of San Francisco and especially Los Angeles is well above 3 feet above sea level. A tsunami would cause significant damage, especially to the harbors, but fall well short of completely destroying either city.
Oh, and we had a tsunami in 2011. If you didn't own a yacht in the Santa Cruz harbor, you didn't notice. That was 5 feet of flow, not 3.