The odds of an 8.9 in southern California are actually much lower than they are in Japan. The entire plate boundary is mechanically completely different. Even if it does happen, Cali also has the advantage of being part of a solid plate structure, and the plate boundary itself (part of which is the San Andreas Fault System) is in consolidated rock. The shallower quakes will dissipate faster and cause much less surface shaking (ironically) than a deep subduction zone quake like the one in Japan. The fact that your part of the world is largely solid rock means less shaking damage than Japan experiences. Think of hitting Jello vs. solid rock with a hammer. In the event of a tsunami, California's coast is much steeper than Japan's, meaning less inward transgression of water.
It would still be a terrible thing, mind you, but in my professional opinion, not nearly as bad as Japan.
Added at: 15:48
In fact, while I'm on the topic, I had a serious "calm down" day with 200 geology students here the other day, who were surprisingly freaking out a little about Japan and Haiti and NZ. 8.9 is large. There is no arguing that. I showed a graph of volcano fatalities vs. year, dating back to the 1500s. On first inspection, it's tempting to say that the world will end in lava rich fire, because the graph rises exponentially. But the truth is that population is also rising exponentially. People live in places they did not before. More people die in a cataclysm. Combine that with the free flow of information in 2010-2011, and it looks like Hell is coming to Earth. A mere 15 years ago, a disaster of this magnitude might not be so well known. Fear certainly wouldn't virally spread through social media the way it has.
If you step back and look at the observational data, quake activity (I'm not counting tsunamis because as tragic as this one is, it's a side effect of an earthquake) is normal right now. The dice roll just came up on "big", and "near inhabited areas".