After the disaster my professor measured increases in radioactivity to try and determine if there was any increase in background radiation from man made sources. Unfortunately even our liquid nitrogen cooled gallium detector couldn't find anything. I think you need a mass spectrometer to really detect any change in California. (Fukushima itself is a different matter!)

Variation in radiation levels of the magnitude you are showing can be caused by many different sources, most of which are natural. Sun spots, changes in the upper atmosphere, cat sleeping on the detector, etc...

EditNote: We definitely could still detect the consequences of Chernobyl and atmospheric weapons testing. Fukushima didn't come close to touching the contamination from THAT