I find the possibility of triggering the San Andres fault by a nuclear bomb doubtful, as well. If it can be done, the Russians are the ones for the job however, since they've built and tested the world's largest nuclear bomb. Designed capability of 100 megatons, the lead scientist supposedly dialed it back to half that out of concerns a massive amount of radioactive fallout would be released.
I would think triggering a volcano a more likely possibility.
Yep. Not likely to trigger a big earthquake, certainly not anything more than a localized quake and not likely a big one. But volcanoes are something else. There are several volcanoes in northern California, that have active steam vents indicating magma chambers deep underground. There have been swarms of earthquakes lately around these volcanoes. So there is the possibility of a nuke collapsing the roof over a magma chamber and causing a volcanic eruption. Any one of them from Mt. Lassen to Mt. Shasta could erupt. Even then we could survive the devastation outside areas in its path. These volcanoes have erupted within the last 200 years at various times without impacting nearby cities. I'd worry more about what Obama is doing to us than these things.